P0171 1/1 Fuel System Lean

NAME OF CODE
Fuel System Lean

WHEN MONITORED
See DRB III

SET CONDITION
See DRB III

THEORY OF OPERATION
The catalytic converter works best when the air/fuel (A/F) ratio is at or near the optimum of 14.7 to 1. The PCM maintains this optimum air/fuel ratio by making short-term corrections in the fuel injector pulse width based on the upstream O2 sensor output. The PCM uses the programmed memory as a self-calibration tool to compensate for variations in engine specifications, sensor tolerances, and engine fatigue over the life span of the engine. Monitoring the actual air/fuel ratio with the upstream O2 sensor (short term) and comparing it to the program (long-term or adaptive) memory determines whether the fuel system is operating within the limits needed to pass an emission test. If a malfunction prevents the PCM from maintaining the optimum A/F ratio, the MIL illuminates.

POSSIBLE CAUSES
- Ignition coil
- Powertrain control module
- Engine parts tolerance
- Exhaust pipe manifold
- Fuel pump
- Fuel pump inlet filter
- Ignition secondary wires
- Low fuel level
- Injectors stuck closed
- MAP sensor
- O2 sensor (upstream)
- Fuel pressure regulator
- Fuel pump relay
- Spark plugs
- Wiring harness/connectors
- Pinched fuel line