* A/C Pressure Switch: This switch monitors the refrigerant pressure in the system. If the pressure is too low (leak) or too high (blocked system), the switch might send a signal to the PCM that triggers the check engine light. This is a very common cause.
* A/C Clutch Relay or Circuit: The A/C compressor is engaged via a clutch. A faulty relay or a short/open circuit in the wiring leading to the clutch could cause erratic behavior and trigger a code. The added load of the compressor could reveal this intermittent fault.
* Evaporator Temperature Sensor: This sensor monitors the temperature of the air leaving the evaporator. A faulty sensor could provide inaccurate data, leading to the PCM setting a code.
* Refrigerant Leak: While a leak itself won't directly trigger a code, the resulting low pressure *will* activate the pressure switch (as mentioned above).
* Problems with the PCM itself: Although less likely, a failing PCM could misinterpret signals from the A/C system, causing the light to illuminate. This is a more serious and expensive potential issue.
What to do:
1. Get the Diagnostic Trouble Code (DTC): The most crucial step. You need to use an OBD-II scanner to read the code(s) stored in the PCM. Auto parts stores often offer free code reading services. The code will pinpoint the specific problem area.
2. Check Refrigerant Levels: Have a qualified mechanic check the refrigerant level. Low refrigerant is a common problem and often the root cause of other related issues. Do not attempt to add refrigerant yourself unless you are properly trained.
3. Inspect Wiring and Connections: Look for any damaged, corroded, or loose wiring related to the A/C compressor, clutch, pressure switch, and sensors.
4. Consider Professional Help: Unless you're experienced in automotive repair, it's best to consult a qualified mechanic, especially considering the potential for refrigerant handling. They can diagnose the problem accurately and perform necessary repairs.
Simply turning on the A/C and causing the check engine light to come on points strongly to a connection between the A/C system and the engine control system. The DTC is the key to solving the mystery.
