If the battery is less than 12.4 volts static charge, that’s not enough to keep the computers happy, the alternator isn’t strong enough to run the computers and recharge the battery.
I write from experience in this problem, a new battery resolved the issue.

 

Ok, I removed and cleaned the crap out of the throttle body and replaced the gasket. No change. Still hesitates, still dies at stops and the speedometer still jumps. I checked the battery, disconnected it showed 12.4V and connected was 12.3V. Can a 10th of a volt make that kind of difference? I'm not very familiar with DC but whether it's in drive or neutral coming to a stop, it still dies. Not every stop but enough to be frustrating.

 

That unfortunately is a dead battery.

 

Over a 10th of a volt difference? The tester at autozone showed it as completely fine.

 

Ok, they say it’s good, but they don’t have the experience and expertise we do here on the site! I’ve owned and driven an HHR since Dec 16, 2009, put 380,000 miles on that 2007 LS and 80,000 miles on my 2011 I purchased in August 2019. Over those years I had to replace the battery 3 times , each time the static voltage was only 12.4 volts, the CCA were at 380 to 400, that should be 600.
I can only recommend solutions to the problem, but here the problem is a battery issue!

 

So you have to ask yourself, "why is undercharged?" Possible reasons are, the battery is bad, or there's something wrong with charging system, or both. Autozone didn't test the charging system while you were there?

 

In my experience, the parts stores are rarely correct in their conclusion about my batteries. I guess if they tell you good ones are bad and bad ones are good, they sell more batteries and parts.

I have them test anyway, using their data and ignoring their conclusions and recommendations. Also, their charger is faster than mine so appreciate their willingness to charge my good battery for free when I’m in a hurry.

The battery store will sell you a new battery if “load test” shows insufficient cranking amps but for what? They don’t even ask what car the battery is for. The Ecotecs need little power to start. (Sometimes I think a little 9V battery would do it.) But starting and running properly are two different things. If the battery won’t hold 12.6V for a couple of weeks, I consider it dying, and 12.4-12.5V, done - the HHR will complain. Just my experience.

I’ve had parts stores tell me I had a good battery base upon the load test, even though it had a shorted cell and would drop to 10.5V or so in less than two days.

 

The computers need 9.5V to stay running, cranking takes 2-3 volts from the battery. So, a battery at 12.3 v when the starter is turned suddenly goes to less than 9.5 v and the computers shut down
I know that is simplified, it is really amperage not voltage that the starter draws but the end result is a momentary reduction in supplied voltage.