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My 2008hhr used to stall and then suddenly quit. Now electrical problems. A/C works, doesn't work. Gauges work, don't work. I've been told by mechanic I need a new alternator but I'm not sure that's the problem. Sometimes I go over bumps and things quit, like the radio will shut off. At times some issues correct when car is turned off and restarted later, like the power steering. Thoughts?

Low battery can cause a lot of problems with these cars. If the alternator isn't charging, that's a major concern.

A parts store can do a free charging system check if you want a second opinion.

You might have some bad grounds. Maybe a flood car?

As suggested, take the HHR to a parts palace to have the battery load tested and the charging system tested. At 12.4 volts the system will not work correctly! The computers will deem it a dead battery. Heat and moisture can cause these problems, my 2007 went down this road in St Pete’s beach , it had 2 dead cells, thankfully AAA tested and replaced the battery, problem solved.