Fuel System Lean

Name of Code:
Fuel System Lean

When Monitored:
With the engine running in closed loop mode and the coolant temperature above 170°F.

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 using short-term memory to make corrections in the fuel injector pulse width based on the upstream O2 sensor output. The PCM uses the programmed long-term 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 combined short/ long term correction exceeds 40% ratio (greater than 1.4V), then this malfunction prevents the PCM from maintaining the optimum A/F ratio and the MIL illuminates.

Possible Causes:
- Fuel pressure regulator
- Fuel pump
- Fuel filter
- Contaminated fuel
- O2 sensor (upstream)
- Injectors
- MAP sensor
- Fuel pump relay
- Wiring harness/connectors
- Misfire
- Ignition coil
- Ignition secondary wires
- Spark plugs
- Powertrain control module
- Engine parts tolerance
- Catalyst