Number of O2 sensors?

sdeeter19555 · August 10, 2021

I'm thinking my O2 sensors are starting to get long in the tooth with nearly 190k. Mileage seems down (12mpg on a trip that should have netted 16mpg+) and cold starting is rough and lopey.

I'm also seeing higher than normal alcohol contents, may have even gotten some bad fuel a month or so ago that may have poisoned them? I haven't been able to get under 35% alcohol readings even though I haven't ran E85 for well over a month...and I got a batch of E85 that may have been E100 or just bad fuel as it pegged my alcohol readout for multiple consecutive tanks of E15 and it would barely idle upon a cold start without blipping the throttle a couple times.

Anyway, I'm looking at replacing them as part of a more aggressive PM effort among other things (plugs/wires, idlers, belts, rotors/pads, shocks, stabilizer, exhaust, etc.). How many O2 sensors are there? Rockauto indicates there are four, two upstream and two downstream?

sdeeter19555 · August 10, 2021

Oh, and without saying 2012 6.0l flex fuel...

newdude · August 10, 2021

4 sensors total. Two upstreams, two downstreams.

Go OE/GM. 12622308 is the upstream, 12609457 is the downstream.

This gen truck uses "virtual" flex fuel setup. No actual fuel/alcohol content sensor (aka Flex Fuel sensor). The O2 sensors play an important part in the operation of the virtual setup, so them being in good, working condition is somewhat crucial.

Here's the description and operation of the virtual setup in case you've not read it before:

Quote

E85 compatible vehicles no longer use an alcohol sensor to determine and adjust for the alcohol content of the fuel in the tank. Instead, the vehicle calculates the alcohol content of the fuel through measured adjustments.

The ethanol calculation occurs with the engine running after a refueling event has been detected via a measured change in the fuel level sender output. The virtual flex fuel sensor (V-FFS) algorithm temporarily closes the canister purge valve for a few seconds and monitors information from the closed loop fuel trim system to calculate the ethanol content. This logic executes several times until the ethanol calculation is deemed to be stable. This may take several minutes under low fuel flow conditions such as idle, or a shorter time during higher fuel flow, off-idle conditions.

Air-fuel ratios and the corresponding ethanol percentage are updated following each purge-off sequence. The fuel alcohol content percentage value can be read on a scan tool.

When an E85 compatible vehicle is built, an ECM or PCM replaced, or if the learned alcohol content has been reset with a scan tool the fuel system will need to contain ASTM gasoline with 10 percent or less ethanol content.

A minimum of 11 liters (3 gallons) must be put in the tank in order for the vehicle to recognize a re-fueling event. It is not necessary to turn the ignition OFF in order to have the re-fueling event recognized, however local safety regulations should be followed.

After the re-fueling event, the system registers the amount of fuel that was added, relative to the amount that was in the tank. Reading fuel trim and O2 sensor activity, the system determines if the fuel added was either ASTM Gasoline or ASTM E85. Based on that determination, the system adjusts to the expected alcohol mix in the fuel tank, and then the fuel trim and O2 sensor activity fine tunes the adjustments. The system must remain in closed loop in order for this adjustment to occur. Numerous short trips after switching from gasoline to E85, or E85 to gasoline, can result in driveability symptoms due to the inability of the system to adjust for fuel composition by not attaining closed loop operation.

sdeeter19555 · August 10, 2021

I knew the alcohol was interpreted by the O2 sensors, that's why I am leaning towards them being tired at 190k miles.

Good to know about OE sensors, I was looking at the Rockauto offerings and mainly Denso or Walker. I'll go out on GM parts and order the OEs.

Thanks!

sdeeter19555 · August 10, 2021

41 minutes ago, newdude said:

4 sensors total. Two upstreams, two downstreams.

Go OE/GM. 12622308 is the upstream, 12609457 is the downstream.

This gen truck uses "virtual" flex fuel setup. No actual fuel/alcohol content sensor (aka Flex Fuel sensor). The O2 sensors play an important part in the operation of the virtual setup, so them being in good, working condition is somewhat crucial.

Here's the description and operation of the virtual setup in case you've not read it before:

Ok, these are ordered, is there a special socket to remove or install them?

I assume the ECM autolearns the new sensors?

David89GMC · August 10, 2021

With all the checks and double checks within the computerized engine management system, don't you think it would let you know when the sensors need replacing? If you want to spend the money, be sure to replace with exact OEM parts, otherwise you're just asking for trouble.

Honestly, if it ain't broke, don't fix it.

sdeeter19555 · August 11, 2021

With all due respect, the ECM is told to allow sensors to operate within a range, outside that range is a fault. That doesn't mean that range is optimal...don't know of many O2 sensors still working well with nearly 200k on them.

I proved this with a Cummins I owned...changed one (1) sensor (still within spec and no faults set) and it changed the fuel mileage significantly (for the positive).

Would you run spark plugs until a fault is set? Would you run a tensioner pulley until it coughs a belt?

I don't mind spending a little on preventative maintenance for a truck that has cost me very little in repairs.

newdude · August 11, 2021

14 hours ago, sdeeter19555 said:

Ok, these are ordered, is there a special socket to remove or install them?

I assume the ECM autolearns the new sensors?

There are "oxygen sensor" sockets if you wanted to get one for them. I've seen wrenches used though. The sockets would offer a better grip on them.

12 hours ago, David89GMC said:

With all the checks and double checks within the computerized engine management system, don't you think it would let you know when the sensors need replacing? If you want to spend the money, be sure to replace with exact OEM parts, otherwise you're just asking for trouble.

Honestly, if it ain't broke, don't fix it.

If they were reading out of their allotted range, yes you'd get a code/CEL. Fuel lean, rich, or O2 heater failure code. But with 200K on original sensors, there could be loads of carbon build up on the little holes in there or on any of the element that is used to read. They can be degraded yet still in spec, just as sdeeter said. This can apply to many sensors on modern vehicles, especially in the diesel world where coking/carboning is much more prevalent.

vucelick · August 11, 2021

15 hours ago, sdeeter19555 said:

I'm thinking my O2 sensors are starting to get long in the tooth with nearly 190k. Mileage seems down (12mpg on a trip that should have netted 16mpg+) and cold starting is rough and lopey.

I'm also seeing higher than normal alcohol contents, may have even gotten some bad fuel a month or so ago that may have poisoned them? I haven't been able to get under 35% alcohol readings even though I haven't ran E85 for well over a month...and I got a batch of E85 that may have been E100 or just bad fuel as it pegged my alcohol readout for multiple consecutive tanks of E15 and it would barely idle upon a cold start without blipping the throttle a couple times.

Anyway, I'm looking at replacing them as part of a more aggressive PM effort among other things (plugs/wires, idlers, belts, rotors/pads, shocks, stabilizer, exhaust, etc.). How many O2 sensors are there? Rockauto indicates there are four, two upstream and two downstream?

16mpg? that is unbelievably fantastic for a 6.0 HD truck. my understanding is that the factory sensors were manufactured by DENSO for GM. I would look at the plugs. My 8.1L started running exactly like you describe and i had overlooked changing the plugs. It smoothed right out and ran normally after changing them. I did change the O2 sensors as a matter of maintenance.

sdeeter19555 · August 11, 2021

1 hour ago, vucelick said:

16mpg? that is unbelievably fantastic for a 6.0 HD truck. my understanding is that the factory sensors were manufactured by DENSO for GM. I would look at the plugs. My 8.1L started running exactly like you describe and i had overlooked changing the plugs. It smoothed right out and ran normally after changing them. I did change the O2 sensors as a matter of maintenance.

When younger (around 100k), I could squeak 18mpg out of this one (3.73s) out in the flat country out west. It's always been fairly good on fuel when "commuting"...now put 5k behind it and it is below 10mpg.

I just did plugs and wires last week (they had about 110k), and had no change...wish it had been that easy. They did show the 462(?) as the new plug over the original 410(?).

sdeeter19555 · August 11, 2021

On 6/17/2021 at 10:49 PM, CamGTP said:

To be honest with you, there just isn't a ton of power to unlock without actually doing mods to the engine. No magic tune is going to give you 30+ horsepower type of thing. Most of the tuners can help you get what is already there, just faster because the factory calibrations are very emissions friendly. It'll feel like a bump in power though.

Diablo and HyperTech are what many people use, I have never used them though. I suggest running 89+ octane with whatever you end up picking.

Your biggest performance increase would come from a gear change. Going to 4.10's or 4.56's would make that truck pull so much better than 3.73's. But that is a very costly change.

1 hour ago, newdude said:

There are "oxygen sensor" sockets if you wanted to get one for them. I've seen wrenches used though. The sockets would offer a better grip on them.

Thanks, I will look one up once the sensors get here.

sdeeter19555 · August 25, 2021

Finally got the sensors, was told two were backordered and that was the delay. Genuine GM AC Delco.

These have antiseize compound on them, would the originals have been installed with it also? I'm wondering if I try twisting these out laying on my back or if I take it to my local guy?

sdeeter19555 · October 8, 2021

Finally got the sensors installed, I tried myself, laying on my back, and one came loose. So had the local garage swap them.

While the alcohol in fuel reading dropped some, it is still over 20%. The truck still stumbles with a cold start, so off to the GM garage once the brakes are done. Good news is this was the source of my surging and low rpm bucking I've suffered for 185k miles (since new) and power is up (definitely pulls harder, drops gears less often).

sdeeter19555 · December 2, 2022

Ok, so following up...I have more issues than O2 sensors. So my alcohol never did settle back down and I've never lost my cold start stumble.

I went for inspection and had the local guy hookup his diagnostic deal for a PO171 and PO174, the alcohol was all out of whack and the fuel trim was maxed. He reset just those and WOW the truck started so nice, drove so nice, and the alcohol was back to normal.

Fast forward a few days, and running a couple tanks of general e10 premium (93 octane), the alcohol jumped up to 30% and my stumbling is back. PO171 pops up and PO174 will trigger the CEL once in a while (not always).

So long story short, the O2 sensors were probably worn but not the underlying issue.

For the record, I drove from Harrisburg to Annapolis on Wednesday in Baltimore traffic and got 16 and 17mpg respectively (hand calculated). Not bad for 208k miles and studded snows.

CamGTP · December 2, 2022

#1 reason I hate virtual flex fuel vehicles, once they start to up act it never really goes away. Can be running perfectly fine and then all of sudden it thinks there is 40% alcohol in the fuel.

I disable that feature on all trucks I tune that don't have a actual sensor installed.