I finally got the car up on the hoist I now have and was able to diagnose and fix the problem. I can't stand threads that don't have a successful resolution at the end, so I'm coming back to share what the result was. The car had been sitting for a while until I could work on it again.

I scanned the car again and got two codes: P0172 - Fuel System Too Rich (Bank 1), and P0300 - Random Misfire.

While scanning the PCM, I noticed the same issue I mentioned earlier in this thread: the fuel tank pressure sensor was constantly reading the same pressure. It never changed—opening the fuel cap or driving the car didn’t affect it. After talking to my father-in-law, who actually owns the car, he said his mechanic had it up on a 2-point hoist, and the car was hanging off the four frame connection points. The HHR community mentioned that the suspension needs to be compressed to remove the tank. So this "mechanic" beat the crap out of the tank trying to get it back in. That got me thinking the tank pressure sensor was probably damaged.

I put the car up on my 4-post hoist and was able to slide the gas tank toward the driver’s side and down quite easily. Sure enough, the fuel tank pressure sensor was broken off and just lying on top of the pump that the "mechanic" had originally replaced. He broke it off trying to reinstall the tank.

After replacing the sensor, the fuel trims dropped dramatically and the car ran MUCH smoother. Both long-term and short-term trims had been maxed out originally. I also had to do an oil change since the oil was saturated with fuel and still slightly elevating the trims. Now the fuel trims are between 0 and 5, which is great! I also reset the fuel alcohol content, it was at 75%, which is crazy. Now it purrs like a kitten.

 

That’s why I change parts myself still, accidentally breaking something is my fault and I know about it , if it happens.
Happy to see you fixed the problem.

 

Good to see it’s fixed,

More evidence that P0172 (in a FlexFuel car) could be caused by an evap system problem (among other things.)

With P0172 on a FlexFuel car, starting by checking the stored ethanol percent would always be a good bet.

This TSB does not include HHR but does include other FlexFuel cars of that vintage.