Powertrain Controls - ECM/PCM

DTC 55 - Fuel Lean Monitor:

Fuel System Routing:

Circuit Description:
The PCM's internal circuitry can identify if the vehicle fuel system is capable of supplying adequate amounts of fuel during heavy acceleration (power enrichment). When a power enrichment mode of operation is requested during "Closed Loop" operation (by heavy acceleration), the PCM will provide more fuel to the engine. Under these conditions, the PCM should detect a "rich" condition. If this "rich" exhaust is not detected at this time, a DTC 55 will set. A plugged fuel filter or restricted fuel line can prevent adequate amounts of fuel from being supplied during power enrichment mode.

DTC 55 Will Set When:
^ O2S voltage is less than .3 volt.
^ O2S is in "Closed Loop."
^ Engine is in power enrichment mode.
^ Above conditions are met for greater than 5 seconds.

Action Taken (PCM will default to): The cooling fan will enable.

DTC Chart Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

1. When the engine is idling or at steady cruise, the O2S voltage should vary, between approximately 100 mV to 900 mV. During power enrichment mode, more fuel is needed and O2S voltage should rise above 447 mV. This step checks to see if the O2S is operating properly.
2. Low fuel pressure or restricted fuel flow may cause a lean condition. Further diagnosis of the fuel system can be found on CHART A-7. Related Tests, Information and Procedures

Diagnostic Aids: A restricted fuel filter can supply adequate amounts of fuel at idle, but may not be able to supply enough fuel during heavy acceleration. Water or alcohol in fuel may cause low O2S voltage during acceleration. Check for adequate amount of fuel in the fuel tank.
Check for:
^ Plugged injectors(s).
^ Low fuel.

A DTC 55 may set as a result of vehicle running out of fuel.