SES Light Due to DTCs P0171 and P0172 - Monitor Fuel Pressure - keywords 4.2 4.2L CEL controls engine LL8 MIL repeat

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

On rare occasions, a SES light may be experienced due to DTCs P0171 and P0172. Typically, there will not be any performance concerns reported and the DTCs will not set at the same time but upon inspection, both DTCs will be stored in history together.

If the SI diagnostics do not isolate a cause for the above concern, perform the following suggestions as necessary:

1. Ensure that the latest PCM calibrations are present. Some models have calibrations available to address certain P0171 and/or P0172 DTCs.
2. Inspect the oxygen sensors for proper operation and possible sensor contamination. If incorrect oxygen sensor operation is noted, replace the oxygen sensors as necessary. If the sensors are contaminated, inspect for contaminated gasoline, coolant consumption, or previous repairs where silicone sealant may have been used. Repair the cause of sensor contamination and replace the oxygen sensors as necessary. Replacement oxygen sensors contain product improvements.
3. Check for a skewed MAP sensor at idle and KOEO. The PCM uses the MAP sensor as a fuel control input since this engine does not have a MAF sensor. If the MAP sensor is skewed, the PCM may mis-fuel the engine. If necessary, compare the MAP sensor value to a known good vehicle to confirm sensor accuracy. If a skewed MAP sensor is found, replace it.
4. Inspect for obvious vacuum leaks at the throttle body, intake and PCV hose between the #3 and #4 intake runners. If there are no vacuum leaks found, remove the throttle body and inspect the throttle body gasket to make sure that it is not rolled. A rolled seal could cause an intermittent or hard to find vacuum leak. If the throttle body gasket is rolled, replace it.
5. Clean the fuel injectors using the procedure found in bulletin 03-06-04-030. If cleaning the fuel injectors corrects the concern, refer to 04-06-04-051A, which relates to maintenance cleaning of the fuel injectors to prevent a reoccurrence.
6. Check for a skewed ECT or IAT sensor by making sure that they are within a few degrees of each other prior to a cold. If one of the sensors is skewed, replace it.
7. Clean the throttle body bore and the throttle valve plate using a clean shop towel with GM top engine cleaner or an equivalent product.
8. If there is no problem found, this may be due to the fuel pressure intermittently decreasing below specification. If fuel pressure decreases, the PCM will increase the injector pulse width to compensate for the lean concern, which may eventually set a P0171 (lean) DTC. If the fuel pressure increases again so it is in specification, a false P0172 (rich) DTC may eventually set because the PCM is only allowed to decrease learned fuel control values in small increments each engine run cycle. Check fuel pressure by following the SI procedure. If a momentary drop in fuel pressure is noted or suspected, inspect the fuel pump circuits for poor connections, terminal tensions, poor ground, etc... If there is no problem found, replace the fuel pump module.

Note: Disconnect the battery for several minutes after any of the above repairs are performed. This will reset the volatile and non-volatile fuel control values that may have been learned by the PCM. Road test to verify repairs.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.