DTC 51 - Switch Condition Signal
ECM uses Park/Neutral (P/N) position switch signal to determine whether transmission is in Park or Neutral, or in some other gear.
ECM uses the output from A/C switch to determine whether or not A/C is operating, so that it can increase idle speed if necessary.
ECM detects idle condition through "IDL" terminal of throttle position sensor.
DTC 51 is set when throttle position switch is off for 3 seconds or more after engine starts. P/N position switch is off with shift position in either Reverse, Drive or Low (A/T). A/C switch on. Check throttle position sensor "IDL" circuit, accelerator pedal and cable adjustment, P/N position switch circuit, A/C switch circuit or for faulty ECM.