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Catalytic converter

Can a bad catalytic converter make a transmission go bad?

I would say it’s possible in a very indirect way (and that is really grasping) but you might provide some detail about symptoms and why this is claimed to be before I’d consider weighing in on something that’s a bit farfetched.

Loss of fluid due to a leak or what?

The car was sluggish when starting from a complete stop. I thought it needed a tune up and was getting ready to do that when it started having trouble shifting. I took it to a transmission shop and they said it had a bad solenoid, clutch plate and band. There were also emmisions codes in it so they checked that out and said the catalitic converter was bad and that it made the transmission go bad. No loss of fluid that I’m aware of. Other than being a little sluggish it ran fine. The technician said the car was starting up in 2nd gear which would account for the sluggishiness. The car is a 2004 Trailblazer.

Was the check engine light coming on before it started acting up? How many miles on it?

My first guess is it has multiple unrelated problems.

Please post the problem codes. They look like P0123.

The codes really need to be known but the odds of the converter causing a transmission failure are very very slim.
The only way I could see that happening would be if an overheated converter cooked a transmission seal due to proximity and which led to a leak which then led to the trans suffering due to low fluid or if a clogged converter was causing the engine to run in a chronically overheated condition. The latter, over time, could kill off the transmission fluid which is cooled inside the radiator trans fluid cooler.

Converters are also frequently misdiagnosed and it could be that your vehicle has more than one problem but they’re not related.