HHR 2LT 209000 mIles. Had trans rebuilt in Jan. Ran fine till 2 weeks ago. Engine lite came on and codes P0700 and P2534 showed up. Took it back to trans shop and they replaced the ignition switch module. Ran fine for 2 days then codes returned. It's back in the shop now and they are scratching their heads. Another HHR came in to their shop today with the same problem. I'm old and stubborn and I love this car. If anyone has any HELPFUL information i would truly appreciate it if you would pass it along.
Thanks Folks

 

Ignition switch module ? What’s that got to do with a transmission problem ?

 

Supposedly it has something to do with monitoring or sending voltage to the trans. F found references to it so it seems to be true. I also read somewhere that the fuel pump module can cause the same P2534 code

 

But you said that didn’t fix it and the code came back. I’ve been on this forum a long time. Never heard of the ignition causing transmission issues on a HHR.
My guess. They disconnected the battery to swap the module. That cleared the codes. It usually takes several key cycles till codes will return and light the CEL.

 

P2534 is "ignition switch CIRCUIT low". The TCM monitors that voltage.
The fuse that is involved is #8 on the BCM. I would be looking at the ECM and TCM connectors.
The P0700 is just incidental telling you there is something causing a problem.

 

Have them check the ground wires to the transmission and TCM .
you can also tighten the 4 bolts in the under hood fuse box