I've got a '97 Z28 Camaro with 172,000+ miles on it. For the past three weeks, about once a week I'm getting an SES code being thrown for the drivers side catalytic converter efficiency being bad (P0420). So I've been grudgingly acknowledging the code and clearing it until I can get enough cash together for a replacement. (the passenger side cat was replaced last year)

For about the past two months, my gas mileage has dropped from an avg of 20mpg to 18 over the last two months. Would the cat going bad cause this or would this be some other problem?

The engine is running fine otherwise and no other codes are being thrown.

 

it definately could be the cat but you wont know for shure untill after you replace it. there are a million things that can adversely affect mileage. good luck.

 

Only if its not making efficiency because it is plugging up and reducing exhaust flow. If the cat is still free flowing, it isn't going to affect gas mileage.

 

Also remember that (depending on where you live) we have changed over to winter blend gas, which gets lower mpg then summer. So, it's possible your bad cat is causing a lower mpg average, but not for sure.

 

I have the same code, comes and goes, but my car also burns real rich--What i have read, you have to find the cause of why the cat is going bad or you have a real good chance of your new one going bad also. I think this will help--I think-- Dwayne http://aa1car.com/library/converter.htm

 

Red89GTA: Thanks for reminding me... I had a similar drop last year and had forgotten about the change. (Though not this bad so its probably a combination)

hmm... I'm burning oil for sure. Had a screwed up ignition system last year (opti) that clobbered the passenger cat and may still have a problem with the spark plug wires/distributor (going too fast through a water puddle makes the engine hiccup...)

It's probably faster to figure out what ISN'T the problem Thanks for that, though!