Test TC-15A - System Lean L-Idle ADAP At Rich Limit (DTC 51)

This trouble code may set for several reasons not related to Powertrain Control Module (PCM) electronics. Long-Term Idle Adaptive (L-IDLE ADAP) memory is designed to aid in air/fuel control. Its function is to adapt air/fuel mixture to correct for fuel pressure differences and mechanical changes such as engine wear and mechanical malfunctions.

This trouble code affects O2S system. PCM reaches its operating limit when one O2S indicates a continuous rich exhaust gas mixture. O2S responsible is disabled and other O2S takes over. If both sensors are indicating excessive rich readings, fuel control system goes into open loop mode.

Possible causes of SYSTEM LEAN L-IDLE ADAPT AT RICH LIMIT trouble code are:

• Faulty ignition coil and secondary wires.
• Faulty MAP sensor.
• Faulty fuel pump and filter.
• Faulty injectors.
• Faulty O2S.
• Faulty fuel pressure regulator.
• Faulty exhaust.
• Faulty wiring harness or connectors.
• Excessive engine wear.
• Mechanical malfunctions.

Perform tests listed in SYSTEM LEAN L-IDLE ADAP AT RICH LIMIT TESTS table, in sequence until problem causing message is found.