Service Engine Soon light, codes 172 & 175

[stebob]

Hello all!  New guy on the block.  I'm having a problem I hope you guys can help me with.  I have a '97 Chevy Z71, 5.7 Vortec, reg cab, short bed.  My "Service Engine Soon" light comes on and off.  It will be on for a few days, go off for a few days so on and so forth.  The light is solid, not flashing.  I scanned it and everytime get the same codse. 172 and 175, rich bank 1 and rich bank 2.  I've put a new fuel filter on, air cleaner is new, cleaned the throttle body really well and just don't know what's wrong.  The truck seems to run fine.  The guy I bought it from admitted that it has done that since he's owned it, over 2 years.  He bought it off his brother who bought it new and stated that it always did that to him, too.  He claims to have had it checked out and they couldn't locate the problem.  I called the dealer today and spoke with a tech.  I was thinking O2 sensors.  He disagreed with those codes.  Not to mention the O2 sensors are $100ea, and there are 4 of them.  I don't have that kind of cash to just throw at it.  Any of you guys ever experience this?  Any ideas on what I can do to find out or correct it?  Thanks for any and all input.

[CMNTMXR81]

Actually, I think they're about $75 a piece.

 

P0172

DTC P0172 Fuel Trim System Rich Bank 1

 

Circuit Description

 

In order to provide the best possible combination of driveability, fuel economy, and emission control, the vehicle uses a Closed Loop air and fuel metering system. While in a Closed Loop, the VCM monitors the oxygen sensor signal voltage. The VCM adjusts the fuel delivery based on a signal voltage. The long and short term fuel values, which a scan tool can monitor, indicates a change made to the fuel delivery. Ideal fuel trim values are around 128; if the oxygen sensor signal indicates a lean condition, the VCM adds fuel. This results in fuel trim values above 128. If the oxygen sensor detects a rich condition, the fuel trim values will read below 128. This indicates that the VCM is reducing the amount of fuel delivered. If an excessively rich condition is detected, the VCM sets this DTC which is a type B DTC.

 

Conditions for Setting the DTC

 

- No IAC or idle DTCs

- No HO2S sensor DTCs

- No TP sensor DTCs

- No MAP sensor DTCs

- No EGR sensor DTCs

- No EVAP sensor DTCs

- No ECT sensor DTCs

- No MAF sensor DTCs

- No IAT sensor DTCs

- No VS sensor DTCs

- System voltage is greater than 10V but less than 17.1V

- No Misfire DTCs

- Throttle position less than 69.9%

- Engine speed greater than 525 RPM but less than 4500 RPM

- The BARO greater than 70 kPa

- The ECT greater than 0°C but less than 99.7°C for the 5.0L, 5.7L

- The ECT greater than 60°C but less than 99.7°C for the 7.4L

- The MAP is greater than 20 kPa but less than 98.9 kPa.

- The IAT greater than -20°C but less than 69.7°C for the 5.0L, 5.7L

- The IAT greater than -20°C but less than 80°C for the 7.4L

- The air flow greater than 3 grams per second but less than 150 grams per second for the 5.0L, 5.7L

- The air flow greater than 3 grams per second but less than 200 grams per second for the 7.4L

- The vehicle speed less than 85 mph

 

Action Taken When the DTC Sets

 

Important

To complete a trip cycle for the fuel trim diagnostic, the fuel trim diagnostic test must enable and run according to the current DTC Will Set When criteria above. A trip is not considered complete until the fuel trim diagnostic has run by the VCM.

 

 

When the current DTC initially sets, a software flag indicating an emissions related fault sets for 3 trips. The flag will clear provided the lean condition is no longer present. However, if the VCM detects the conditions for setting the current DTC during a trip while the flag is still set, the VCM turns on the MIL (Malfunction Indicator Lamp). The MIL remains on during all of the subsequent trips until the DTC has been cleared by service or the lean condition has not been detected and the conditions for checking the fuel trim have been met for 3 consecutive trips.

 

Conditions for Clearing the MIL/DTC

 

The VCM turns OFF the MIL after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (the coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the engine coolant temperature exceeds 71°C (160°F) during that same ignition cycle) or the scan tool clearing feature has been used.

 

Diagnostic Aids

 

If using the diagnostic tables does not isolate the problem, try monitoring the L. T. fuel trim and fuel trim cell while operating the vehicle under various loads. This may isolate

the condition which caused the setting of the current DTC.

 

A condition that causes the DTC P0300 Engine Misfire Detected may also set the current DTC; conversely an extremely rich condition which sets the current DTC can cause a misfire at idle and the DTC P0300 to set. If the cause of the current DTC cannot be determined and the DTC P0300 is also set, refer to DTC P0300 Engine Misfire Detected .

 

On vehicles equipped with a 7.4L (California emissions), an AIR Pump that is stuck ON may also cause a DTC P0172, or DTC P0175 to set.

 

Test Description

 

The numbers below refer to the step numbers on the diagnostic table.

 

2) Visually and physically checking items which may cause a rich condition may determine the cause of the DTC P0172 being set.

 

3) This step checks the fuel system for problems.

 

5) A silicon contamination of the Heated Oxygen Sensor (HO2S Bank 1, Sensor 1) can cause a false rich indication to be seen by the VCM. The VCM then reduces the amount of fuel delivered, possibly causing severe driveability symptoms.

 

Step 1

 

Important

Before clearing DTCs use the scan tool to record freeze frame and failure records for reference, as data will be lost when Clear Info function is used.

 

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Yes: Go to Step 2

No: Go to Powertrain On Board Diagnostic (OBD) System Check

 

Step 2

 

Visually and physically check the following items:

 

- The air intake duct for being collapsed or restricted

- The air filter for being plugged

- IAC: If a low or unsteady idle is being experienced.

- Check for throttle body coking or objects blocking the IAC passages.

- Check the fuel pressure regulator for proper operation. Refer to Fuel System Diagnosis .

- Check the Throttle Position sensor for loose or missing mounting bolts. Monitor the TP Sensor with the scan tool while slowly opening throttle - TP Sensor. The throttle position will increase steadily and evenly from closed throttle TP specified voltage to over the wide open throttle specified value.

- If not, replace the TP sensor.

 

Did any of the above checks isolate a condition requiring repair?

 

Specified Value =  0.2-0.74V - 4.0V

--

Yes: Go to Step 8

No: Go to Step 3

 

Step 3

 

1) Connect fuel pressure gage to fitting at fuel rail. Refer to Fuel System Pressure Check.

2) Turn off the ignition for 10 seconds.

3) Turn off the A/C.

4) Turn ON the ignition.

5) The fuel pump will run for about 2 seconds.

6) It may be necessary to cycle the ignition ON more than once in order to obtain maximum pressure.

7) Monitor the fuel pressure with the pump running. The pressure should be between the specified value.

8) When the pump stops, the pressure may vary slightly then should hold steady.

 

Is pressure correct and does it hold?

 

Pressure = 415-455 kPa (60-66 psi)

--

Yes: Go to Step 4

No: Go to Step 11

 

Step 4

 

Perform the Injector Balance Test. Refer to Fuel Injector Balance Test .

 

Was an injector problem found?

--

Yes: Go to Step 9

No: Go to Step 5

 

Step 5

 

Remove and visually and physically inspect the Heated Oxygen Sensor for a silicon contamination. This will be indicated by a powdery white deposit on the portion of the sensor exposed to the exhaust stream.

 

Is the silicon contamination evident?

--

Yes: Go to Step 12

No: Go to Step 6

 

Step 6

 

Perform the Evaporative Emission (EVAP) Control System Diagnosis .

 

Was a problem found?

--

Yes: Go to Step 7

No: Go to Diagnostic Aids

 

Step 7

 

1) Repair the EVAP System.

2) Operate the vehicle in a Closed Loop while monitoring the L.T. and the S.T. fuel trim values.

 

Does the L.T. fuel trim value increase above the specified value and the S.T. fuel trim value increase above the second specified value?

 

First Specified Value = 100

Second Specified Value = 94

--

Yes: Go to Step 10

No: Go to Diagnostic Aids

 

Step 8

 

1) Repair the items found in Step 2.

2) Operate the vehicle in a Closed Loop while monitoring the L.T. and the S.T. fuel trim values.

 

Does the L.T. fuel trim value increase above the specified value and the S.T. fuel trim value increase above the second specified value?

 

First Specified Value = 100

Second Specified Value = 94

--

Yes: Go to Step 10

No: Go to Step 3

 

Step 9

 

1) Replace the fuel injector. Refer to Fuel Injector Replacement (Central SFI) for the 5.0L, 5.7L. Refer to Fuel Injector Replacement for the 7.4L.

 

2) Operate the vehicle in a Closed Loop while monitoring the L.T. and the S.T. fuel trim values.

 

Does the L.T. fuel trim value increase above the specified value and the S.T. fuel trim value increase above the second specified value?

 

First Specified Value = 100

Second Specified Value = 94

--

Yes: Go to Step 10

No: Go to Step 5

 

Step 10

 

Rich condition not present. If a driveability symptom still exists, refer to Driveability Symptoms.

--

--

--

 

Step 11

 

Refer to Fuel System Diagnosis for diagnosis.

--

--

--

 

Step 12

 

Replace the HO2S (Bank 2, Sensor1). Refer to Heated Oxygen Sensor (HO2S) Replacement .

 

Is the action complete?

--

Yes: Go to Step 13

--

 

Step 13

 

1) Using the scan tool, select the DTC and the Clear Info.

2) Start the engine

3) Idle at the normal operating temperature.

4) Select the DTC and the Specific.

5) Enter the DTC number which was set.

6) Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.

 

Does the scan tool indicate that this diagnostic ran and passed?

--

Yes: Go to Step 14

No: Go to Step 2

 

Step 14

 

Using the scan tool, select the Capture Info and the Review Info.

 

Are any DTCs displayed that have not been diagnosed?

--

Yes: Go to The Applicable DTC Table

No: System OK

 

P0175 is the same as above, just for bank 2.

[stebob]

WOW!  Looks this could be complicated!  Thanks for the info.  I have checked the simple stuff, except the IAC.  What is that? Idle Air Control or something?  Where are the passages?

[CMNTMXR81]

That's right from GM's Service Info system.  IAC is Idle Air Control.  Do you have a scan tool that can monitor their counts?  If so, there is a cheap and quick way to test it.

 

As to passages, that would be the cast tubing that the IAC sensor is mounted in.  Usually this is after the MAF and in some cases throttle body.  So if this is somehow clogged, you may have other serious issues.  But GM has to put that out as a step to check.  One of those obvious things that may get overlooked.  

[jryerson]

Great! Thanks for the information. I'll be sure to go through and check everything. Unfortunately I do not have a scan tool. Are the tools really expensive. I saw that Autozone had a code reader for something like $90.00. Is this the same thing or are you talking about a diagnostic tool?

[kowalcs]

If you have a Murray's by you, they rent them out. You just give them $150 and when you return the tool, you get all your money back. However, I think their tool just gives you codes. I don't think you can monitor specifics with it.