Code 66, A/C Pressure Sensor Circuit (3.3L N Body): Notes

The A/C pressure sensor is a variable resistor which sends a signal to the ECM relative to A/C system pressure. This information is used by the ECM to determine A/C load on the engine. Low pressure will cause a low return signal voltage of about .5 volt. High pressure will cause a high return signal voltage of about 4.5 volts. Code 66 will set if the voltage signal falls outside of this range for 25 seconds. The A/C compressor will be disabled if Code 66 is set.

1. This tests voltage signal being received by the ECM.
2. This tests if high voltage signal is from a shorted sensor or a short to voltage in the circuit. Normally, disconnecting the sensor would make the circuit go to near zero volts.
3. This tests if the low voltage signal is from the sensor or the circuit. Jumpering the sensor signal to 5 volts checks the circuit, connections and ECM.
4. This tests if the low voltage signal was due to an open in the sensor or circuit or the 5-volt reference circuit since the prior step eliminated the pressure switch.