Dexron VI. as per TSB 04-07-30-037E
I'll write that down. Love this forum, good stuff. Sorry about Mike's trans, though.

 

Same thing with the hard/delayed/banging up-shifting 1-2, 2-3, and also Park or Neutral to Drive started about 2 years ago around 90,000 miles on the odometer on my 2010 one-owner (me) HHR-SS with the 4T45E 4-speed automatic transmission. Felt like I was being rear-ended hard. Turning off the engine and ignition, then restarting the car after 5-10 seconds and problem was gone (until the next time, often after stepping on the gas hard). 3-4 up-shift was always OK, however. Replaced intake cam actuator when trouble code appeared---little better. Re-flashed the ECM, little better---for a while. Still not cured by a long shot. I lived with the problem until last month when the transmission started making a slight whirring noise in Park, followed 3 days later by no Reverse plus balky forward gears. To the transmission shop on a flat-bed tow truck. Dropped the transmission pan and out came metal shavings and chunks of metal. Gear teeth were all chewed up. Had the transmission completely rebuilt including gear sets, clutches, seals, solenoids---everything plus a new torque converter and a new radiator (I didn't want to take the chance of the rebuilt transmission overheating due to metal pieces in the old radiator's transmission cooling passages)---besides, it was car's original radiator and it had 103,000 miles on it. Replaced all the factory coolant hoses, too, most importantly the ones that were easier to reach with the transmission out. HHR-SS runs like new now. And no more hard, banging up-shifts. Ultimately, there was no cure to be had for the hard shifts other than pulling the transmission and having it rebuilt (including having the internal electronics/solenoids replaced).

 

I have a 2011 HHR LT in which I also experienced the shifting problem described , but b4 it began I had 2 other codes come up that involved camshaft actuator intake and exhaust after replacing the 2 trans went back to smooth shifting and hasn’t reaccurred , do u think a timing misinterpretation sent to ecm caused trans to shift late at higher than normal rpms I could tell the engine was in loop mode very sluggish and took alo

 

Hard shifting is part of "limp mode"...