Fuel System Lean

Name of Code:
Fuel System Lean

When Monitored:
With the engine running in closed loop mode and the ambient/battery temperature above 20°F and altitude over 8,000 ft.

Set Condition:
The PCM conducts a test to determine whether the fuel system is running too lean. If this happens for 2 trips, the MIL illuminates and a trouble code is stored. The MIL remains on for more than one trip but goes out if the conditions that set the code are not found on subsequent trips.

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