2010 HHR LS 2.2 138k, auto, the last few days rpm would run to 3000 in 1st so I read somewhere possibly replacing spark plugs could fix so I did, originals were bad! Ran good for 1 day then problem came back again, 3000 in 1st….sounding horrible earlier yesterday! Engine light came on today and plugged in my simple scanner and 2 codes P0700 P0842 came on. Tried going in reverse but was using a lot of power, I was in parking lot! I’ve read so many comments about battery bolt tightening or pressure sensors or low trans fluid I’m just confused. I’m a backyard mechanic, limited knowledge limited tools 🧰 and space just needs some guidance. Thanks everyone!

 

My first guess is the transfer tube in the trans has cracked. If you drop the trans pan you can see a pair if tubes the frequent fail point is the braze at the fittings.
https://www.transmissiondigest.com/a...from-the-past/

 

Or a loose wire or bad sensor?

P0842 = Transmission Fluid Pressure (TFP) Sensor Circuit Low Voltage

 

If the transfer pipe is broken, the sensor is working to tell you pressure is low.

 

@Oldblue is correct. The pipe cracks and pressure is lost and the code is set.