Sonic A/C Compressor Won't Engage, Diagnosis Help

Had a similar issue with my 2017 Chevy Cruze last summer, these models share a lot of HVAC system components with the Sonic. The A/C compressor wouldn't engage and I got zero cold air. After some diagnostics, the shop found undervoltage in the system was preventing the compressor from activating properly. They traced it to a faulty AC relay and pressure sensor combo that wasn't sending proper signals to the A/C system. Total fix came to about 240 for parts and labor. The repair solved it completely, A/C has been blowing ice cold since. What error code are you getting specifically? That would help narrow down if you're dealing with the same relay/sensor issue. Also, can you hear any clicking from the compressor area when you turn the A/C on? This info would help determine if your problem matches what I experienced. These Chevy A/C systems are pretty similar across models, so once we know more details about your specific symptoms, I can share more about what fixed mine.

Just had my last service at 36,953 KM and started noticing this A/C issue shortly after. Thanks for sharing about the relay and sensor combo, that's helpful to know, especially since the symptoms match up perfectly with what I'm experiencing. The error code I'm getting is P0645, and there's absolutely no clicking sound when engaging the A/C. I can't seem to find any obvious refrigerant leak either. Definitely sounds like we might be dealing with the same electrical problem rather than a mechanical A/C compressor failure. That repair cost seems reasonable if it sorts everything out. Did they need to recharge the system as part of the fix in your case?

Yeah, sounds exactly like what I dealt with on my Cruze. The P0645 code is specifically for the A/C compressor clutch control circuit, which totally lines up with the relay/sensor issue I had. When there's no clicking sound, it's usually because the electrical signal isn't making it to the compressor clutch. In my case, they didn't need to add refrigerant since my pressure switch was just sending incorrect signals, the refrigerant level was actually fine. But they did test the system pressure as part of diagnosing the problem. The pressure sensor was reporting false low pressure readings, which was preventing the relay from engaging the compressor clutch. Quick tip, make sure your shop checks both components (relay switch and pressure sensor) since they work together. Some shops might just replace the relay without testing the pressure sensor, but in my experience, they often fail as a pair in these models. After getting both parts replaced, my system started working perfectly again. The compressor engaged right away with that satisfying click, and cold air was blowing within seconds.

Thanks for the detailed explanation about your experience. That definitely makes me feel better about the potential fix, especially since we're seeing the same P0645 code and identical symptoms with the lack of compressor engagement. Going to schedule it in for repair this week and make sure they check both the relay switch and pressure sensor like you suggested. Don't want them to replace one part only to find out the other was also bad. The belt tensioner and A/C compressor itself seem fine from visual inspection, so I'm hopeful it's just the electrical components causing the issue. Really appreciate the guidance on what to look for and what to expect cost-wise. Will make sure they do a full pressure test even though I'm not seeing any obvious refrigerant leak. Better to be thorough and get it fixed right the first time.