How to troubleshoot a catalytic converter?

I have a 2007 Chevy HHR (it does not have a turbo or flex fuel option). In the past few months my mileage has dropped by about 30% and the RPM is reving way higher than normal as if it is not going into high gear (overdrive). However I had the transmission fluid changed and the transmission shop said everything checked out on the transmission and it was shifting fine.



Background - I have 64,000 miles on the car mostly highway and I run synthetic oil in the engine.



The only thing I can think of would be a clogged or clasped exhaust, But the pipes still look great.



How do I troubleshoot the catalytic converter or if you believe the problem to be elsewhere, I am open for suggestions.



Thanks.How to troubleshoot a catalytic converter?I had that same problem a couple of weeks back ,and after a lot of checking things out it was the throttle chamber what was gummed up .

I hope this helps you .How to troubleshoot a catalytic converter?well the cat will smell like rotten eggs and you won't have much pressure out the tail pipe but I would change your fuel filterHow to troubleshoot a catalytic converter?If your Cat is defective (not filtering exhaust fumes) then you'd be able to tell because your exhause would smell like rotten eggs.



Now, it sounds more like you're thinking the Cat is blocked. The easiest way is to unbolt it and physically look inside it to see if there's any obvious obstructions. I'd also check your intake for obstructions too. The HHR captures air from behind the front passenger fender (there's a flexible hose that goes from the filter to the fender). It just pops out, just verify it's not blocked either.



I'm very suspicious of the transmission shop. If the car isn't shifting right, it could be that they didn't use the correct type of transmission fluid. The HHR should use DEXRON - VI Automatic transmission fluid only.