Code 66 - A/C Pressure Sensor: Notes

A/C pressure sensor responds to changes in A/C refrigerant system high side pressure. ECM uses A/C compressor load input to determine engine idle speed. Sensor uses a 5-volt reference signal from ECM and returns an input signal to 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 circuit. Normally, disconnecting sensor would make a normal circuit go to nearly zero volts.
3. Checks to see if low voltage signal is from sensor or circuit. Jumpering sensor signal circuit to 5-volt reference checks circuit, connections and ECM.
4. Checks to see if low voltage signal was due to an open in sensor circuit; previous step eliminated pressure sensor as a possible cause.