HHR Overheating Bypass Valve Code

Overheating and a bypass valve code on an HHR is definitely concerning. Before throwing parts at it, have you checked for any obvious coolant leaks? A vacuum leak could also cause both issues. I would start by inspecting all the vacuum lines, especially around the intake manifold and brake booster. A leak there can cause the engine to run lean, leading to overheating and potentially triggering that bypass valve code as a side effect. What exactly is the code you are getting?

I’ll check the coolant level and look for leaks first thing in the morning, and give all the vacuum lines a good once-over. I did think about a vacuum issue but wasn't sure if it could manifest like this! If there is a vacuum leak, could that also lead to any power loss or misfires? The code I got was P0068.

P0068 - that's a MAP/MAF Throttle correlation error. Yes, a vacuum leak throwing off the fuel trim can absolutely cause power loss and even trigger misfires! Given the overheating on top, I'd seriously advise taking it to a reputable workshop. They'll have smoke machines and diagnostic tools that will pinpoint the leak. Driving it while overheating could cause catastrophic damage.

There was a small crack in the hose to the thermostat housing. It was leaking coolant when the engine was warm and causing both the overheating and the faulty reading, which then lead to the bypass valve error being triggered. Replaced the hose, topped up the coolant, cleared the code and everything’s running perfectly now. Cost me 0€ to fix!