I have a 2010 HHR and my engine light just came on. I went to Auto Zone and 2 codes came up. P0700 Transmission Control System (malfunction indicator lamp request) and P0974 Shift Solenoid A Control Circuit High. Does anyone know what these mean and how serious they are?

 

It means that you should start by looking for wires that have been chafed or burnt. unplug and replug the connectors at the TCM and the transmission.

If you can't find any bad wires go to a transmission shop; if they suggest a new solenoid or valve as the first thing to do go to another shop immediately.

The very last thing it could be is an internal fault. However, not out of the question.

P0974 ? Shift solenoid (SS) A -control circuit high ? OBD2 Trouble Codes
https://www.yourmechanic.com/article...by-parker-hill
https://www.chevyhhr.net/forums/prob...0-p0974-56294/

 

Yes the TCM connection is most likely the area you'll find a loose wire, also check the ground wires , you'll find them on the stud on the transmission case just down to the right of the engine oil filter housing
This is an extremely rare picture of rodents dining on your wire harness looks like