Dreaded "Reduced Engine Power" issue

6.0 Smitty · May 9, 2014

3 separate times this past week, I have had the reduced engine power kick on and have had no throttle response. I had the truck scanned and code p1516 popped up. Looked into it a good bit. I have since checked all the connectors and wires. There are no signs of loose wiring, broken wiring, or connector issues. Also while looking up the code I found that this is a Throttle Positioning Sensor (TPS) issue. I now have one on order. Question for anyone that has ever had this issue is, will this likely fix the problem or this issue be ongoing and show up again? Again, everything else (to my best knowledge) has checked out OK since I did some troubleshooting.

Thank you!

gmtech4 · May 9, 2014

might want to call a dealer and see if a reprogram is required

6.0 Smitty · May 11, 2014

Will a reprogramming cost me anything? If so how much?

gmtech4 · May 11, 2014

Will a reprogramming cost me anything? If so how much?

its part of the job ,,,,they do when they replace the newer Throttle bodies on newer trucks (its a common failure with an extended warranty(special policy)

it may not run if you just replace it yourself IDK,

it may cost an hr labour

kag56 · August 2, 2014

MY 05 2500 HD 6.0 was showing codes p0120 p0220 p2135 all related to Throttle Body Position and Pedal Positon Sensors. Fixed by cleaning ground on block, drivers side bottom of block where oil pan meets block 2 wires on connection. Was getting trouble codes several times a day.

PH-Wolf · August 17, 2014

I had that issue with my 2011 Chevrolet Silverado back in March 2013 when I had around 50,000 miles on my truck. Since I was out of town 8 hours from home, I limped it to a Chevy dealer. The tech found I had a faulty throttle position sensor. They replace the TPS sensor, reset the idle speed and reprogram the computer, as the invoice stated. I had two codes P2135 and P0121. I now have over 72,600 miles and never had the "reduced engine power" message or any throttle issues since.

PH-Wolf · August 17, 2014

I also should of mentioned, I know with my truck if you replace the TPS sensor the engine will idle really high and you need reset it with a scan tool with a capably to do idle resets. That's what I've been told, so either way I would of had the dealer fix it. total bill was $260 for everything on my trunk. (Diagnostic/labor fee, Sensor replacement, reprogramming the ecm and resetting the idle.)

chupe442 · August 18, 2014

might want to call a dealer and see if a reprogram is required

+1

ifixedit · September 22, 2014

Hello all,

For anyone reading this old post regarding the numerous years of Silverado's going into "reduced power mode" I will share my findings, hope this helps someone with this very frustrating problem which I also have dealt with on and off for the past few years and FINALLY fixed.

Just for reference, my truck is a 2003 HD1500 crew cab with the 6.0 Vortec that has just over 200,000 miles on the clock. The last few winters when it would get at or below about 10 F I would get the REP message and check engine light on the dash, could only go about 15 MPH tops when it happens, very inconvenient and dangerous to say the least. You have to stop, shut the engine off, wait a few seconds then restart and it will run fine until it does it again. At first just seemed it was temperature related but as time went on it seemed to do it whenever, @ 30 MPH, @ 60 MPH, in the middle of rush hour traffic (not fun), after it rained, when it was 90 F outside and the A/C was on, just never seemed to make any rhyme or reason and did it when it felt like it. I was also experiencing an off and on "surge" both at slower in town speeds and on the highway when holding the pedal steady @ 65MPH, when it started doing this I new the REP message on the dash was just a minute or so away and sure enough it would happen. The code I got was always the same, P1516 which is the TAC (throttle actuator control module), but if you try look for a new TAC you will find out they are not only difficult to find new, they are very spendy as well. For those that have wasted money replacing the TAC have found in 99% of the cases it has not fixed the problem, so I decided I wasn't going to replace the TAC just yet as my thinking is they either work or they don't and I find it hard to believe it would just work fine one minute and not the next.

I checked all the usual suspects and bulletins including the plug and wires going to the throttle body, cleaned the MAF sensor, cleaned the throttle body, checked the connections to the TAC and on and on to no avail.

First off, DON'T start throwing parts at it, you are wasting your time and money! I put three (yes 3) throttle bodies on my truck since this past February, one after-market unit and 2- AC Delco re-man units, it would run fine anywhere from a couple months to a couple of days and the problem was back again with the same P1516 code as I've read here about a zillion times now with others experiencing the exact same problem.

This is a GROUNDING ISSUE plain and simple and there are a few other things to check along the way I will cover, but I'll start with the grounding problems first. Start out by disconnecting the battery, both sides, while you work on it this will also allow the code(s) to clear.

There are 3 ground connections of concern, the one at the back of the engine (near the firewall) connected to the block on the passenger side just below the cylinder head, the one on the back of the engine connected to the back side of cylinder head on the driver's side of the engine, and the grounds on the frame under the drivers side door which is easy to get at but the others are NOT. Start out by removing and cleaning the ground connection on the frame under the drivers door, if you have to use a grinder or equivalent to get good clean metal on the frame then do so. Pick up a tube of dialectic grease, it will be your new friend, apply it to the grounds and re-connect them. This MIGHT solve the problem but I doubt it will alone but good to do anyways.

Next comes the fun part. The two grounds on the back of the engine are IMPOSSIBLE to get at without removing the intake manifold unless you are some sort of contortionist with very small hands and have x-ray vision and can see through metal. You want to remove the intake to get at them and you want to do this anyways as there is more to this story. The intake gaskets are known to fail causing an intake leak and excessive pinging under acceleration, so while you in the process of getting at the 2 grounds, replace the intake gaskets and you might as well replace the knock sensors (as they may be corroded) and the knock sensor wire harness for good measure while you are in there.

After the intake ducting is removed, the intake is off, etc., etc. The ground on the passenger side of the block is held on by a 13mm bolt, the one on the drivers side is a 15mm (at least it was in my case). This requires you kneel on the upper portion of the radiator support to reach them and is still somewhat of a trick, use some padding to protect your knees. DON'T just assume by grabbing on to these grounds and finding they are tight that they are good! Trust me they are NOT! Take your time, get both out in the open because you are going to want to fix both. Once you have these grounds out in the open you will notice the original metal ring terminals seem tight and well connected, but upon closer inspection you will see they are crimped over the plastic of the wire but very little if any inside wire strand is actually touching the metal ring, THIS IS A CRAP CONNECTION! Proceed to clip the rings off, strip the wire back and put on a new ring terminal, use some dielectric grease on the terminal where you insert the wire, crimp it tightly and I would also recommend using some heat shrink tube slipped on the wire first before you crimp on the new ring, then slide the shrink tube over the connection and warm it up with a hair dryer or equivalent, you can also solder this connection for good measure if you feel like it, but a good crimp with the dielectric grease all covered with shrink tube should do the job, in any case it will be way better than what you had in the first place.

After you have replaced both ground rings it's time for reassembly. Using a piece of sandpaper, first clean the contact area where the grounds connect, wipe them clean and apply more dielectric grease, use it liberally, re-connect the grounds, tighten the bolts and insure they are tight. Now on to the intake, install your new gaskets which clip on to the intake, install your new knock sensors and tighten them to 15 lb. ft. DO NOT over tighten the knock sensors, they are VERY sensitive! To give you an idea of how tight, they are basically hand tight and then just a hair more, if you have a torque wrench, use it. Plug in your new knock sensor wires, re-install the intake with the new gaskets, plug in all the connectors, re-attach the fuel line to the injector rail on the passenger side, reconnect the air ducting and so on, reconnect the battery, you are now ready for your first start up.

Per what I've read, you should perform a throttle body "relearn" after the battery has been disconnected for a length of time. If you look up "throttle body relearn" for your make and model you should find the process for your vehicle. Mine was basically turn the key on, turn the key off 5 seconds, turn the key on for 5 seconds, turn the key off for 15 seconds, turn it on for 15 seconds, turn it off and start the truck. Let it warm up to full operating temperature (185 F+), do not touch the throttle while it is warming up, just let it idle. After it has warmed up, turn on the A/C (if so equipped) in park for 10 seconds, leave the A/C on and put it in drive for 10 seconds, leave it in drive and turn the A/C off for 10 seconds, then put it in park and turn the vehicle off and you're done! Restart and take it out for a drive to see how it runs.

A few things to check if the problem persists and you've done all of the above: Again all this is done with the battery DISCONNECTED. Open up the power box under the hood on the drivers side inner fender, the top that holds all the relays and fuses flips open so you can look underneath it. Check for a mouse nest or debris (quite common) underneath, also check for any wires that may have been chewed on by mice, look carefully moving the plugs and wires around, you may find bare areas on the wires (also a very common problem), clean out the box and repair any wires that are damaged as this could have also been your problem. While you are in there, disconnect and clean all the positive main terminals, relays and so on and apply dielectric grease to their sockets, re-tighten terminals, for good measure unplug the plugs, apply dielectric grease to them as well and re-connect.

Your PCM may have grounding issues: The PCM is located at the front of the drivers side inner fender. Get it out in the open and removed the blue plug, look for the black / white wire in the #1 pin position which is the ground. A few inches from the plug, carefully strip back some of the plastic covering the wire, do not cut the wire! Solder on another wire to this ground, re-cover it well after you have made this connection and run this lead as an additional ground up to the lug on the firewall where the main ground strap from the engine is connected, again clean everything, use dielectric grease and be sure the nut is tight.

A few other possibilities. With both of the large plugs (blue and red) disconnected from the PCM inspect the pins on the PCM itself, if any are burned, corroded or otherwise it may be time for a new PCM.

If you replaced the sensor on the gas pedal (the one inside the vehicle on the gas pedal assembly) it may be out of sync with the TAC module which is on the drivers side firewall under the hood and the pedal assembly is plugged into it. I have read that the original gas pedal sensor is calibrated to work with the original TAC and if you replace either, things will not work or will result in the "reduced power message" and I tend to agree with this. If none of the above has helped and you decide to attempt replacing either it's best off to buy a TAC and a pedal assembly complete together from the same truck at a junk yard. If you have replaced any of these parts, do not throw them away as you may now need them!

I actually just performed all this recently and my truck runs so smooth now in comparison to how it did when I was having the problems it's like night and day, so far no trouble codes and it continues to run perfectly.

I hope all this can help some of you experiencing this very irritating problem.

Good luck!

Marc

Edited September 22, 2014 by ifixedit

gp2394 · September 27, 2014

That happened to me 3 years ago and it needed a new pedal assembly.

ifixedit · September 29, 2014

That happened to me 3 years ago and it needed a new pedal assembly.

If that's all you needed to do, you're one of the lucky few. After all the other checks and my second throttle body, I then moved to the sensor on the pedal, and no luck.

When you say it needed a new pedal assembly, what part did you replace? The entire pedal with the sensor or just the sensor? What about the TAC? Did you also replace that or did you leave that part alone?

gp2394 · October 2, 2014

brother I don't think I can answer that. My Uncle worked on it and I know he told me it needed a new pedal assembly which he installed. I don't know what else he may have done. I called him and he does not remember.

Wintersun · October 13, 2014

With my truck there is a setting for fast idle at startup which is probably for people living in very cold climates. It is something that takes me a minute to change should I wish to do so. The only time I have had a TPS sensor fail it actually was showing evidence of problems with reduced throttle response only I did not realize what was happening. I was lucky that it did not happen on the freeway.

Paul Pruett · September 19, 2016

Hello all,

For anyone reading this old post regarding the numerous years of Silverado's going into "reduced power mode" I will share my findings, hope this helps someone with this very frustrating problem which I also have dealt with on and off for the past few years and FINALLY fixed.

Just for reference, my truck is a 2003 HD1500 crew cab with the 6.0 Vortec that has just over 200,000 miles on the clock. The last few winters when it would get at or below about 10 F I would get the REP message and check engine light on the dash, could only go about 15 MPH tops when it happens, very inconvenient and dangerous to say the least. You have to stop, shut the engine off, wait a few seconds then restart and it will run fine until it does it again. At first just seemed it was temperature related but as time went on it seemed to do it whenever, @ 30 MPH, @ 60 MPH, in the middle of rush hour traffic (not fun), after it rained, when it was 90 F outside and the A/C was on, just never seemed to make any rhyme or reason and did it when it felt like it. I was also experiencing an off and on "surge" both at slower in town speeds and on the highway when holding the pedal steady @ 65MPH, when it started doing this I new the REP message on the dash was just a minute or so away and sure enough it would happen. The code I got was always the same, P1516 which is the TAC (throttle actuator control module), but if you try look for a new TAC you will find out they are not only difficult to find new, they are very spendy as well. For those that have wasted money replacing the TAC have found in 99% of the cases it has not fixed the problem, so I decided I wasn't going to replace the TAC just yet as my thinking is they either work or they don't and I find it hard to believe it would just work fine one minute and not the next.

I checked all the usual suspects and bulletins including the plug and wires going to the throttle body, cleaned the MAF sensor, cleaned the throttle body, checked the connections to the TAC and on and on to no avail.

First off, DON'T start throwing parts at it, you are wasting your time and money! I put three (yes 3) throttle bodies on my truck since this past February, one after-market unit and 2- AC Delco re-man units, it would run fine anywhere from a couple months to a couple of days and the problem was back again with the same P1516 code as I've read here about a zillion times now with others experiencing the exact same problem.

This is a GROUNDING ISSUE plain and simple and there are a few other things to check along the way I will cover, but I'll start with the grounding problems first. Start out by disconnecting the battery, both sides, while you work on it this will also allow the code(s) to clear.

There are 3 ground connections of concern, the one at the back of the engine (near the firewall) connected to the block on the passenger side just below the cylinder head, the one on the back of the engine connected to the back side of cylinder head on the driver's side of the engine, and the grounds on the frame under the drivers side door which is easy to get at but the others are NOT. Start out by removing and cleaning the ground connection on the frame under the drivers door, if you have to use a grinder or equivalent to get good clean metal on the frame then do so. Pick up a tube of dialectic grease, it will be your new friend, apply it to the grounds and re-connect them. This MIGHT solve the problem but I doubt it will alone but good to do anyways.

Next comes the fun part. The two grounds on the back of the engine are IMPOSSIBLE to get at without removing the intake manifold unless you are some sort of contortionist with very small hands and have x-ray vision and can see through metal. You want to remove the intake to get at them and you want to do this anyways as there is more to this story. The intake gaskets are known to fail causing an intake leak and excessive pinging under acceleration, so while you in the process of getting at the 2 grounds, replace the intake gaskets and you might as well replace the knock sensors (as they may be corroded) and the knock sensor wire harness for good measure while you are in there.

After the intake ducting is removed, the intake is off, etc., etc. The ground on the passenger side of the block is held on by a 13mm bolt, the one on the drivers side is a 15mm (at least it was in my case). This requires you kneel on the upper portion of the radiator support to reach them and is still somewhat of a trick, use some padding to protect your knees. DON'T just assume by grabbing on to these grounds and finding they are tight that they are good! Trust me they are NOT! Take your time, get both out in the open because you are going to want to fix both. Once you have these grounds out in the open you will notice the original metal ring terminals seem tight and well connected, but upon closer inspection you will see they are crimped over the plastic of the wire but very little if any inside wire strand is actually touching the metal ring, THIS IS A CRAP CONNECTION! Proceed to clip the rings off, strip the wire back and put on a new ring terminal, use some dielectric grease on the terminal where you insert the wire, crimp it tightly and I would also recommend using some heat shrink tube slipped on the wire first before you crimp on the new ring, then slide the shrink tube over the connection and warm it up with a hair dryer or equivalent, you can also solder this connection for good measure if you feel like it, but a good crimp with the dielectric grease all covered with shrink tube should do the job, in any case it will be way better than what you had in the first place.

After you have replaced both ground rings it's time for reassembly. Using a piece of sandpaper, first clean the contact area where the grounds connect, wipe them clean and apply more dielectric grease, use it liberally, re-connect the grounds, tighten the bolts and insure they are tight. Now on to the intake, install your new gaskets which clip on to the intake, install your new knock sensors and tighten them to 15 lb. ft. DO NOT over tighten the knock sensors, they are VERY sensitive! To give you an idea of how tight, they are basically hand tight and then just a hair more, if you have a torque wrench, use it. Plug in your new knock sensor wires, re-install the intake with the new gaskets, plug in all the connectors, re-attach the fuel line to the injector rail on the passenger side, reconnect the air ducting and so on, reconnect the battery, you are now ready for your first start up.

Per what I've read, you should perform a throttle body "relearn" after the battery has been disconnected for a length of time. If you look up "throttle body relearn" for your make and model you should find the process for your vehicle. Mine was basically turn the key on, turn the key off 5 seconds, turn the key on for 5 seconds, turn the key off for 15 seconds, turn it on for 15 seconds, turn it off and start the truck. Let it warm up to full operating temperature (185 F+), do not touch the throttle while it is warming up, just let it idle. After it has warmed up, turn on the A/C (if so equipped) in park for 10 seconds, leave the A/C on and put it in drive for 10 seconds, leave it in drive and turn the A/C off for 10 seconds, then put it in park and turn the vehicle off and you're done! Restart and take it out for a drive to see how it runs.

A few things to check if the problem persists and you've done all of the above: Again all this is done with the battery DISCONNECTED. Open up the power box under the hood on the drivers side inner fender, the top that holds all the relays and fuses flips open so you can look underneath it. Check for a mouse nest or debris (quite common) underneath, also check for any wires that may have been chewed on by mice, look carefully moving the plugs and wires around, you may find bare areas on the wires (also a very common problem), clean out the box and repair any wires that are damaged as this could have also been your problem. While you are in there, disconnect and clean all the positive main terminals, relays and so on and apply dielectric grease to their sockets, re-tighten terminals, for good measure unplug the plugs, apply dielectric grease to them as well and re-connect.

Your PCM may have grounding issues: The PCM is located at the front of the drivers side inner fender. Get it out in the open and removed the blue plug, look for the black / white wire in the #1 pin position which is the ground. A few inches from the plug, carefully strip back some of the plastic covering the wire, do not cut the wire! Solder on another wire to this ground, re-cover it well after you have made this connection and run this lead as an additional ground up to the lug on the firewall where the main ground strap from the engine is connected, again clean everything, use dielectric grease and be sure the nut is tight.

A few other possibilities. With both of the large plugs (blue and red) disconnected from the PCM inspect the pins on the PCM itself, if any are burned, corroded or otherwise it may be time for a new PCM.

If you replaced the sensor on the gas pedal (the one inside the vehicle on the gas pedal assembly) it may be out of sync with the TAC module which is on the drivers side firewall under the hood and the pedal assembly is plugged into it. I have read that the original gas pedal sensor is calibrated to work with the original TAC and if you replace either, things will not work or will result in the "reduced power message" and I tend to agree with this. If none of the above has helped and you decide to attempt replacing either it's best off to buy a TAC and a pedal assembly complete together from the same truck at a junk yard. If you have replaced any of these parts, do not throw them away as you may now need them!

I actually just performed all this recently and my truck runs so smooth now in comparison to how it did when I was having the problems it's like night and day, so far no trouble codes and it continues to run perfectly.

I hope all this can help some of you experiencing this very irritating problem.

Good luck!

Marc

After "throwing a few parts" at my truck. I followed this brilliant write up. My truck runs perfect now. I didn't do the extra pcm ground, but both the grounds on the back of my engine were pretty nasty. Thanks a ton for the fantastic information. No telling how much money it saved me. 2003 5.3 Yukon, bolt sizes and everything were the same as yours.

ifixedit · September 19, 2016

Hello Paul,

Glad to hear your truck is running good again! Just a little add on here as I've once again experienced some of the "reduced power" nonsense again. In another post here I reported after all I did which is noted above, I ended up finding that much of the original GM wiring (along with the grounding issues) can also be suspect. I ended up going out to a local wrecking yard and purchased an entire accelerator pedal assembly including the wiring harness that attaches to it and goes through the firewall to the TAC module. Apparently GM decided at some point in 2003 that they were using a bit too wimpy a gauge of wiring and that the plugs could also be suspect. I purchased the entire unit with harness for the paltry sum of $30 which appeared to have a much better looking harness than the one I took out. After replacing the pedal and harness I had no trouble at all for a year, then the problem came back earlier this summer when it warmed up! AAARRRGGG! The last codes I pulled showed a myriad of potential problems, codes were P0120, 0220, 1516, 1518 and 2135, all pointing to issues with the accelerator pedal out of sync with the throttle body and issues with the throttle position sensor in the throttle body. I got to scratching my head thinking this just isn't possible. How could it run perfectly for a year and then it's back to it's old tricks again?

Possible solution........

I had also reported earlier that I had used dielectric grease on all the connections, including the plug at the throttle body. Curious but the truck would mostly do it's "reduced power" thing either when it was very humid out or after it rained. I also reported earlier that I could just about predict when it was going to do it because I had a very pesky ignition static noise on the radio when it was on most any AM station, especially when I was near power lines, this static would also get worse when it was humid or after it rained and it was just about guaranteed the truck would go into reduced power mode. Well, as it turns out dielectric grease happens to have SILICONE in it which is notorious for ATTRACTING moisture! Go figure! I came to the conclusion that all these codes were more likely the result of either moisture OR the grease itself allowing the signals / power to possibly jump around from one pin to the next at the throttle body connection and that possibly just a good dry connection would be better. I proceeded to unplug the connection at the throttle body, got out a can of parts cleaner, sprayed it on all the plug terminals, including on the TB itself, then thoroughly blew both connections out with my air gun to completely remove the dielectric grease. What happened? Truck runs like a million bucks again and no static on the radio! I've come to the conclusion, the dielectric grease is fine for the grounds, but DON'T use it anywhere else!

Time will tell!