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Most DTC's wont show under the ECM as it is not what controls the AC system, and will only show faults for the compressor and relay circuit its controlling. The HVAC module, which is part of your infotainment system, is what controls it, and will have the stored codes for it, if there are any. The HVAC module monitors ambient temp, cabin temp, evaporator temp, pressure sensor and probably a few other things as well. The ECM only receives a command to turn it on through the CAN buss, and then sends a ground to the clutch relay to energize the clutch coil, when requested by the HVAC module.
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