stirlingtrayle

I've got an 01 S4 Avant, 125K miles. I used to love the car, but the maintenance costs have been such that I've let some of that stuff slip over the past couple years. I used to do a lot of that stuff myself, but I got lazy. My bad. I was thinking about selling the costly pig.

On my code reader, I was getting multiple fault codes, which my local Audi guy specifically identified as exhaust temp sensors 1 and 2, coolant temp sensor, and bank 1 side 1 O2 sensor. I was also getting cylinder 2 misfires. The car was prone to starting and then immediately dying after being driven - really any number of miles - 100 miles or just to the store. Sometimes, it would start, rev to about 2,500 RPM for about 30 seconds and then settle down to normal idle, and then I could drive it. It was acting as if it were a carbureted engine that was flooded, and I could even smell gas. It would eventually start, usually after letting it sit for anywhere from 20 min to a couple of hours, depending on how badly it was "flooded". I drove it like that for over a year...how stupid. From what I was able to read on forums like AudiWorld (thank you everybody), I guessed the culprit was the coolant temp sensor as on this year vehicle, that sensor is tied to some portion of regulating the fuel system. I think the sensor thought it was like -40F and the computer took the info and dumped fuel into the engine to compensate - or something like that. So that needed to be replaced and it was like $72 to have that done, which I did. But my service guy also found these faults for the exhaust and O2 sensors being bad as well. The exhaust sensors are expensive parts and bit difficult to get changed - meaning expensive. The O2 sensor not so bad, but still some bucks.

Which brings me to why I writing this rather long winded letter. Replacing the coolant temp sensor cleared up both the exhaust and O2 sensor faults, and no more cylinder 2 misfires. It's been over two weeks and no fault codes. The car is running so well, it just makes me want to get back into it and treat it like an S4 Avant should be. So next up is for ME to replace the tie rod ends and control arms, which I'm probably going use the Eurospec kit from ECS (unless someone out there has a much better suggestion...). And there's a little matter of that valve cover leaking too...
Anyway, thank you to everybody who contributed to this forum. It helped me a lot, got me enthusiastic about my S4 again, and I hope this post can shed a little light for someone experiencing what I did. $72...what a dummy.

SnyperBob

Thanks for sharing your experience! You replaced the coolant temp sensor. Is that the sensor that plugs into the bottom radiator inlet hose? I just had to drain my coolant and came across that sensor during the process.

My car has similar miles, wondering if I should just replace that sensor too. Did the replacement sensor come with a new O-ring?

I'm trying to track down a coolant leak to no avail. I'm going to post a new thread about it now...

stirlingtrayle

Hi SnyperBob,
I'm not sure which sensor it was as the shop did the R&R. He used part number 059 919 501A, which was described Water Temperature Sensor, 4 Pin, Green, 20mm Dual, if that helps. He did replace the O-ring as well, but it was an extra $1.60...