yakavish

Hey everyone, newbie here, Boston area. Have a nice 2001 S4 in pretty good shape, except that my CEL has been on since last summer (2011). Went to autozone for a quick diag, scanner says error codes P0421/P0431, short for "cats don't work well". So I resigned myself to cat replacement, an expensive proposition, it seems

Alas, the other day, I filled up at a Mobile gas station at the other end of town where I never go, and - surprise - minutes after that the CEL went off, and is still off days after. Umm, really!? I always buy brand name quality 93 gas, so is this just a teaser, or a coincidence? Can gas quality or possibly ethanol confuse my cats and keep the CEL on??

Confused here ... please help ...

YV

ezveedub

The those codes mean the efficiency of the precats are not as good as they should be. Possibly replacing the O2 sensors may resolve this issue, IF they are falsely reading the exact O2 levels in the exhaust. My Allroad was throwing the same codes, but now, only Bank 1 has the code now, but there is also a Bank 1 O2 sensor fault too. The recorded MVBs for my cats show Bank 2 is within specs, but Bank 1 is still out.

yakavish

Thanks! Is there any way to tell whether the o2 sensors are faulty, or the cats are broken? At $100/each (x4 + labor) it's not a quick-and-cheap experiment ...

YV

ezveedub

Yeah, they are not cheap. For me, I have an intermittent DTC fault for bank 1 front O2 sensor. That's also the same bank the catalyst is showing intermittent efficiency DTC faults too when I last checked. The bank 2 DTC code did not return, yet. but was persistent a few weeks ago. If you have never seen any O2 sensor faults, but always get catalyst faults, I would say its the catalyst that are having the actual issue. There are some people trying lacquer thinner mixed with gasoline to clean the cats, but I've never tried that and if you do, you're own your own. You can Google it and find more info about it.

ncotter

I've seen a lot of guys who buy cats cheap from non-Audi dealership parts depts and since they're lower spec/quality it doesn't fix their cel issue and below threshold codes keep popping up. The they try to fix it again by removing the 02 sensor and installing the L shaped bung spacer for 02 sensors to move it out of the direct line of the exhaust stream and it still doesnt fix the cel issue - so your idea of it being a $100 fix is low and probably won't fix your prob. I'm experiencing the same now with my new 99 avant that the prior owner told me had new cats/exhaust pipe combo - yes, they're clearly new but they were probably dirt cheap and not passing muster with what the ecu is looking for from the downstream 02 sensors. Your idea of 93 has might help me. My other 01 a6 has same cat prob and it disappears at higher elevation mountain altitudes but pops right back up after driving at sea level for a day.

yakavish

You seem to imply that scanners can differentiate between cat problems and o2 sensor failures? So far, I have only tried a generic scanner at autozone ... would a real VAG-COM scan give a conclusive answer? (never done that)

ezveedub

Well, VAG-COM will help with running actual MVBs data and some diagnosis test, but you have to know the actual data your looking at and what it means.

joeffrS4

Just wanted to throw share some experience here. Diagnosing 02 sensors really requires looking at their performance on a graph. An under performing upstream sensor could trigger a CEL on the downstream sensor. Using a VAG is the only way to diagnose 02 sensors, you need to look at their performance to determine which, if any need to be replaced.

With the engine warm, graph the O2 sensor output. Rev the engine to approximately 2000 RPM. A good sensor will oscillate from below 0.2 volts to above 0.8 volts, looking approximately like a sine wave. A bad sensor will be flat-lined and show no response to snapping the throttle. Test for O2 sensors behind the catalytic converter: Graph the rear and the front O2 sensor output. Rev the engine to approximately 2000 RPM. The front sensor will be oscillating actively. The rear sensor should show little reaction and stay at a mid-range value (approximately 0.5 volts). If the sensor is flat-lined at zero volts, it has failed. If the rear sensor follows the oscillations of the front sensor, the sensor is good but the catalytic converter has failed.

I replaced all 4 during my turbo swap with Bosch sensors from Rock Auto. They were $57.00 a piece and performed flawlessly. Hope this helps, good luck

05SPLIFIN

http://www.ebay.com/itm/EuroCustoms-...#ht_500wt_1205


would these down pipes be what you guys consider as "CHEAP" and will trip the CEL ? that is if the O2 sensors are in good condition, (im in the same boat) my o2 sensors came up a while back, been tryin to get e new exhaust with new O2 sensors

yakavish

Helps a lot, thank you Joe!
Best, Y