P0172 System Too Rich (Bank 1)
The PCM has reached the lower limit of fuel correction on Bank 1 and the mixture is still rich.
Definition
P0172 is the opposite of P0171: the PCM is pulling fuel out of the mixture as much as it can and the O2 sensor still reports a rich condition. Common causes are leaking injectors, high fuel pressure, or a contaminated MAF reading too much airflow.
Common causes
- Leaking fuel injector
- Stuck-open fuel pressure regulator (excess fuel pressure)
- MAF over-reporting airflow
- Contaminated engine oil (fuel dilution)
- Failed evaporative purge solenoid stuck open
Common misdiagnoses
- Replacing O2 sensor when the underlying cause is a leaking injector.
Troubleshooting steps
-
1. Check fuel pressure
Verify fuel pressure is within spec at idle and at WOT.
-
2. Inspect injector balance
Use a scan tool or injector balance test to find a leaking injector.
-
3. Check MAF reading
Verify MAF grams/sec at idle is within spec.
-
4. Inspect evap purge
Command the purge solenoid closed and verify it isn't leaking fuel vapor into the intake.