[loraleewilliams2]

I have a 2018 Cruz with 97,000 miles. Auto 6sp. All highway miles. I’ve had it serviced regularly. I got new tires at Goodyear. I told them to change the oil also. They came back and told me my coolant need changed. I said ok. Left the shop and car wouldn’t run right. 3500 rpm and 20mg less. This was October. Now intermittently does the same thing. Will in the middle of driving shift down to I believe 5th gear, but could be 4th. And it won’t shift back up. Even if you floor the gas it will not shift either up or down. Out of the blue. Doesn’t matter if it’s cold or hot. Took it to a transmission shop. He was able to get it to do it, but it shows no codes and no dummy lights. Took it to Chevy since my power train warranty had expired at 90,000 miles I figured I’d start with a transmission specialist. Well chevy told me, that my car had 1 qt too much transmission fluid. It’s never been touched since factory that I’m aware of. So since I wasn’t aware that the fluid needed changed at 50,000 miles they refused to change it. Got no codes again and it wouldn’t do it for the mechanic. Got in my car next day and after 20 miles she downshifted again never to return. I got no clue and neither does Chevy or a transmission specialist. Any clues?

 

[Johnny B]

Any clues?

Well, you have provided some clues. I have a couple guesses.

It having no codes is odd, my guess is they created a problem during the coolant change. They broke a fitting or damaged a sensor, maybe created a vacuum leak. Even checking the engine air filter can create a huge vacuum leak, and that would be my first place I would look.

You need to find a competition 3rd party shop, because I can tell your aren't exactly a car person.

chevy told me, that my car had 1 qt too much transmission fluid

This is almost certainly a lie. Call them up and ask them how they determined that. Be prepared for a lot of silence.
A specialist would have looked at the OBD2 gear shift times and shift errors (and a bunch of other stuff), and not make up some BS about fluid level.

wasn’t aware that the fluid needed changed at 50,000 miles they refused to change it.

True and I don't blame them.

This is what I would do.

#1 Check that the coolant overflow bottle has the proper level of coolant in it. Keeping the coolant at level is critical.
#2 Bring it to a shop that regularly does transmission services. And have them do a drain and fill on the transmission. In draining the transmission about 4.5 quarts of ATF should come out, replace the fluid and that is it. Don't do a second drain and fill, and certainly do NOT flush the transmission.
#3 Have the oil changed again. It is hard to mess up an oil change, but it is possible if they tried hard enough. They need to use a Dexos 1 Gen 2 oil, if they don't have it go somewhere that does and have them prove it to you.

In doing those things the problem might present itself. Also be aware the ATF will be black as sin, that is from the transmission seals. If the ATF is burnt or not is what will be telling.
97k on the factory fill is not good, but it should be recoverable with a fluid change.