Diagnostic Aids

Code 66 sets when signal voltage falls outside normal sensor range and is not due to a A/C system problem. If problem is intermittent, check for opens or shorts in harness and poor connections. If wiring is okay, replace pressure sensor. If code resets, replace ECM.

Fig 1: Code 66 Schematic (5.7L "F" Body) A/C Pressure Sensor

Fig 2: Code 66 Flow Chart (5.7L "F" Body) A/C Pressure Sensor