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 4.9 volts. The ECM will disable the compressor clutch if Code 66 is current.

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 or 5-volt reference circuit; previous step eliminated pressure sensor as a possible cause.
5. The 5-volt reference for the TPS is the same 5 volts used by the A/C pressure sensor. If the 5-volt reference has a short to ground, the MAP sensor, A/C pressure sensor and other related sensors will also have a short to ground. This short to ground will not damage the ECM. When the shorted 5-volt reference circuit is repaired, the ECM 5-volt reference will return to normal.