A/C Works Only After ECM Reset

Car: 2013 Chevy Sonic 1.8L, M/T, LUW engine code
Issue: A/C only works after cycling power to ECM

After some amount of time (clock time), engine time, or miles (haven't determined that yet), the A/C will stop working. The compressor will not engage when requested at the HVAC panel, even though all of the prerequisites are satisfied for the ECM to command on the compressor clutch. The A/C works great when it is working, blows cold and all that. The only way to get the A/C to work again is to cycle power to the ECM. I've done this both by disconnecting the battery and by disconnecting the X1 connector at the ECM, same results each time. I haven't experimented with the unpowered time, but it's usually a few minutes. I haven't pinned down what's driving the problem to crop up again, but it seems like the A/C works for about 12-24 hours. Again, I haven't played around with different amounts of driving time, engine-on time, etc.

Does anyone have any ideas about this? Anyone with inside knowledge of GM engine controls maybe? Thanks in advance!

Do you have a scan tool to look at Codes/Data pids?  What is the status of AC request/permission when it stops working?
 

Could you post a screenshot of the ECM A/C data while the compressor clutch doesn't engage? Add engine coolant temperature and ambient temperature, if possible.

I have a TopDon TopScan Lite. The ECM codes are P0597 (engine coolant thermostat heater control circuit) and P0621 (generator L-terminal circuit). The Body Control Module (BCM) also has two codes, B252C (child security lock switch circuit) and U0155 (lost comm w/ instrument cluster); just mentioning those for completeness.

I've attached a zip with screenshots of all of the ECM live data when the problem is present, before resetting the ECM, with engine on and idling, blower on, and A/C button lit.

Thanks for the screenshots! I'm wondering if the thermostat fault has something to do with it? Specifically the heater low voltage test showing 'MALFUNCTION'.

I had a chance to play with a 13 Sonic A/T today and found the A/C keeps working with the thermostat disconnected. I'd be curious to know if yours would do the same. 

Also, is the engine overheating? Several of the A/C Disengage History entries would suggest it is. 

I also was wondering if the thermostat code was blocking the compressor command.

That P0597 code has been there for a while (at least 6 months). I figured the thermostat heater wasn't affecting drivability, so I put it off. The car did have a coolant leak that caused it to overheat last year sometime, but no issues with that in a long time.

The car had been sitting for 5 days after I reset the ECM, and today we drove it for about 15 minutes. There were no problems from the A/C during that time. I was watching the live data from the passenger seat, and everything looked good. I measured the thermostat heater resistance at around 400 kΩ, so it's definitely failed open. I'm going to go ahead and replace the thermostat to take care of that code, and hopefully it will take care of the A/C problem too. If the issue comes back before I replace the thermostat, I'll disconnect the heater and bridge the cable contacts with a test light to see if that makes the A/C work. Will update either way!

Just updating this in case anyone else has the same problem. I replaced the thermostat, and the A/C has been working great. I had found that the A/C would stop working after 4 engine start cycles, and it's been working for a lot more than that, over a week now. The P0597 code also cleared on its own. So there was definitely some connection between the thermostat heater fault and the A/C not working. Thanks for all the help!

Appreciate the update, good work.

Congrats on the fix! We always appreciate the follow up.

I had found that the A/C would stop working after 4 engine start cycles, and it's been working for a lot more than that, over a week now.

Now isn't that interesting.   Because I looked up GM service info for P0597 - it didn't say anything about taking the compressor away.

 

GM dropped the ball on that one, IMO.