I recently got my cat-con re-welded back in place of an aftermarket resonator. since then my fuel economy is suffering, could the cat or O2 sensor be damage?

I do not have surges during drive and the exhaust sounds quiet though.

Wat type of symtoms are associated with O2 sensor failures?

I hope the shop took proper precautions to isolate the ECU during the welding process.

If they used flame (not electrical) - no problem.

MAT

Resonators flow significantly better then OEM cats. So you should notice a little decrease in fuel efficiency (1-3mpg) and performance (especially in turbo lag).

Your cat could very well be damaged. The interior is kinda honeycomb like. These plates could be clogged from improper storage, broken plates etc. When you get a chance, get under the car (cold), remove the cat cover and knock the cat with a non-metal implement (i.e. your knuckle, or anything that won't make a pinging sound). If you here stuff rattling around, you either have junk in your cat or (a) broken plate(s).

If your O2 sensor(s) have/has failed, you should see the lambda warning light on by now. And yes, when it fails, fuel efficiency is affected as well as performance.

So if you don't have any warning lights on and your cat sounds solid, you could simply be feeling the affects of decreased flow in your exhaust system from going from a resonator to a cat.

Thanks Sin, I'll get my car lifted at the exhaust shop to double check the cat just in case. O2 sensor failure seems unlikely.

btw, I returned to the cat because the resonator is causing the car to run rich and also leaves a yellowish stain on my bumper above the tailpipe.

When you do check your cat, don't be surprised that it's in working condition. Like I said, you could simply be feeling the loss of power due to more restriction now.

How do you know the resonator was making th car run rich? What other mods do you have?

BTW, do you know what that yellow stuff is? It's sulphur! Say YES to acid rain!