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Hello all,

I've owned my 09 G5 base coupe for about a year now. 2.2 VVT ecotec automatic trans, 4T45E I believe is the designation for this. Bought it dirt cheap because it was hit in the front end. Fixed the damage as well as everything else I saw wrong with it. It's been my work car/daily for about 8 months. Car currently has about 185k miles on it. In the 10k I've put on the car it's done great. So great in fact that my wife and I are considering taking a cross country trip in it. As soon as I started to tout it's reliability, of course it develops an issue lol.

As I was pulling into my parking spot at work today, I decided I needed to straighten up a bit so I put it in reverse to back up. A co worker said something to me and we conversed briefly for 30 seconds or so while my foot was on the brake, car in reverse. I felt the car start to shake and I looked immediately at the instrument cluster. The rpm began to drop and fluctuate, and I thought the car was going to die. Just a few seconds later it was all gone and the car ran fine. No warning lights ever came on. I used our shop scan tool(fairly new launch tablet based) to scan the entire vehicle for codes. No current or history codes in any module.

I brought the car in to the shop after work and, with it running, moved harnesses around for all of the important things I could think of: injectors, coils, crank sensor, both cam sensors, Main A/T harness connector, MAF sensor. Manipulating those made no difference. I was unable to duplicate the issue.
After thinking on it awhile, I realized that I have felt very slight hesitation in reverse with this car on several occasions before. It's like it "skips a beat or two" It has always been on a cold start idling out of my parking spot in reverse after leaving work, and it was so slight that the average driver wouldn't have picked up on it. Today was a pretty severe event, and the vehicle was fully warmed up after my 25-30 minute commute. I do not assume that the issue is necessarily only in reverse, but this is the only time I have noticed it.

This is going to drive me crazy even if it never happens again because it'll be in the back of my mind as we're out in the middle of nowhere driving across the country. Keep in mind I diagnose and repair all kinds of junkers for a living, but I'm a bit stumped on this because I've never experienced this on a cobalt/g5 before. The lack of any DTCs and the inability to reproduce the issue so far is rather aggravating. I'm hoping someone has had this or a similar issue that might give me a better idea of where to start diagnostic wise. Thanks in advance!
 
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