P0171 - System Too Lean (Bank 1)

The HO2S-11 and the Mass Air Flow (MAF) sensor circuits are monitored by the Powertrain Control Module (PCM). The PCM then compares the input data to the theoretical mixture ratio. The PCM then tries to compensate for the difference in the actual and theoretical. The test fails if the amount of compensation is to large (the actual mixture is too lean).

Possible Causes:
- Intake air leaks
- Damaged or disconnected front HO2S or circuit
- Plugged injectors
- Fuel pressure regulator stuck open
- Exhaust gas leaks
- Incorrect fuel pressure
- Lack of fuel
- MAF sensor
- PCV system
- Oil overfill
- Cam timing
- Compression
- Coil and secondary side of the ignition system

Diagnostic Aids:
With the key ON and the engine running at idle the fuel pressure should be 34 PSI (235 kPa). The O2S-11 PID should vary between 0-0.3 Volts and 0.6-1 Volts with the engine running at 2000 rpm. The MAF PID reading with the engine running at idle should be 1.0-1.7 Volts. The MAF PID reading with the engine running at 2,500 rpm should be 1.7-2.3 Volts.