Code 66, A/C Pressure Sensor: Notes

The A/C pressure sensor responds to changes in A/C refrigerant system high side pressure. The ECM uses A/C compressor load input to determine engine idle speed. Sensor uses a 5-volt reference signal from the ECM and returns an input signal to the ECM on a separate line. Low pressure (zero psi) will return a signal of about .1 volt. High pressure will return a signal of about 4.6 volts.

1. Checks voltage signal from A/C pressure sensor to ECM.
2. Checks to see if high signal is from a shorted sensor or a short to voltage in the circuit. Normally, disconnecting the sensor would make a normal circuit go to nearly zero volts.
3. Checks to see if low voltage signal is from the sensor or circuit. Jumpering the sensor signal circuit to 5-volt reference checks the circuit, connections and ECM.
4. Checks to see if the low voltage signal was due to an open in the sensor circuit since the prior step eliminated the pressure sensor.