Hello i have a 02 A4 1.8t and my CEL came on and the code says it was for my cat i put some kind of cat cleaner that you put in your gas tank i was woundering if anyone had any luck with that kind of stuff and what could of made this happen i dont smell that bad smell when i drive so i really never knew anything was going bad but my CEL has been on for awhile.. also woundering if putting a test pipe with a stock exhaust would be somthing to look into... thanks for the help...i also have about 85,000 on it

 

The only way the car can monitor the cat is by using the secondary O2 sensor (that's what it's there for). I'd check the O2 sensor first (the one after the cat, not the one before it) and see if that's gone bad on you. Chances are if the ECU thinks the cat is bad, it's likely that it's the O2 sensor. And given your mileage, if it's the original O2 sensor it could easily be ready to die.

 

what should i be looking for if looking for a bad 02 sen. is there anything that should raise a flag when looking at it.. thanks for the help and reply

 

I'm not sure if there's a measurement you can make on it or not, between any of the pins, but if you PM member Auditech79 he may be able to answer it for you (he's an Audi Technician by trade). When you remove the sensor though, you may see that the end inside the exhaust piping is beat up pretty badly. It's normal for it to be discolored, as it's heated to 900 degrees in order to function, and it has all that hot exhaust air baking it. At the mileage your car has, you wouldn't be wasting money to replace both sensors though. The primary one (before the cat) is used to determine the air/fuel mixture for your engine, and as O2 sensors age, they can lose sensitivity and you may end up running richer than usual. Since you're approaching 90k miles, chances are you could use new ones. If I were you I would just replace both of them, and you'll most likely see both your CEL go away and a bump in fuel mileage as well. You can get a good price on them at www.purems.com - I think they're listed around $107 each. Most vendors seem to price them around $150 or so.