I have parts of this documented elsewhere on this site, sorry for the duplicate, just thought that this issue needed separate attention.

So I have a 96 2.8L V6 with about 150,000. I have the ever present CEL gracing me with its lovely orange glow, so I decided to scan it and see what codes would show up. I received nine:

P0452: EVAP Emission Contr.Sys.Press.Sensor Low Input<-- not sure about this one
p0174: Fuel Trim,Bank2 System too Lean <-detecting lean fuel mix in exhaust, possibly due to vac leak or faulty sensor
p0171: Fuel Trim,Bank1 System too Lean <-- same
p1106: O2 Sensor Circ.,Bank2-Sensor1 Voltage too Low/Air Leak <--possibly broken 02 sensor
p0170: Fuel Trim,Bank1 Malfunction <--reached maximum amount of fuel adjustment
p0173: Fuel Trim,Bank2 Malfunction <--same
p0141: O2 Sensor Heater Circ.,Bank1-Sensor2 Malfunction <--also broken o2 sensor
p1250: Fuel Level Too Low<-- non CEL code, probably was running on a low tank

So I made the mistake of swapping the passenger side pre-cat O2 sensor with some piece of junk from the internet along with a coolant temperature sensor. The car started just fine, went for a drive around the block, and it would erratically lose power every 5 minutes or so.

I thought maybe the ECU was adapting to the new sensor as I had not reset it yet, so I parked it, let it sit for a few minutes, and when I went to try and start it, the car just cranked over and over (I made sure not to flood it), but with a very loud clicking/thumping coming from the relays under the steering wheel.

I checked all fuses, as far as I could tell they were all good. I checked the relays, and relay # 208 (fuel pump relay) was the one making the loud clicking. I could actually visibly see sparks coming from it!

So I replaced the relay with one I pulled from my 95 A6, changed the fuel pump fuse, put original O2 sensor and coolant temp sensor back, but still no go. I think the car blew the relay again because the new one is making the same noises.

Sorry for the length of this, thanks if youre taking the time to go through this!

So as of now i have coming in the mail:

- A cam position sensor (mine was on its way out and it could keep the engine from starting)
- 4 New premium BOSCH sensors (confirmed from 12v.org that they are a direct match)
- A new ECU
- an OBD interface (Vgate-Scan model, made for VW/Audi)

Can anyone confirm the O2 sensor circuit on these cars? Is there really no fuse between the sensor and the ECU? This seems absurd that a small sensor could fry the whole car!

I found this on a VW forum: VWVortex.com - O2 Sensor problem, car wont start now. P1176,P1198,P0140

Someone with a 2000 Jetta had the same issue, and it turned out a new ECU fixed it.

How should I approach this? Do I just switch in all the new parts and cross my fingers? Are there any other circuits I should be testing before frying a new ECU? Sorry for the lengthy post but any help is appreciated!!

 

Seems no one wanted to touch this issue, totally understandable. Electrical stuff can be a pain...

So I was able to fix the original issue successfully. I replaced a few different parts so its tough to say exactly what would have caused the car to stop starting in the first place, but im placing my bet on the ecu.

total, i changed out:

ECU
Cam position sensor (was cracked and oily)
both pre-cat (upstream) O2 sensors with Bosch #13913
All 6 main relays
ALL fuses


Started right up the first time. It indeed seems that there are no fuses between the O2 sensors and the ECU. Really have to be careful there. Luckily there was no permanent electrical damage. Hopefully this helps anyone who runs into this issue in the future!

Instead of 9 codes, I now only have 3 codes in regards to the rear O2 sensors (computer says that both are bad, electrical testing reveals only passenger side needs replacement, but ill be doing both anyways), and then a code for system too lean, most likely due to the faulty O2 sensor.

 

Glad you were able to fix the original issue! Always good to hear success stories. Many people don't update their threads with their successes. Sounds like you've put a few pretty pennies into this car, always unfortunate to hear. These are beautiful cars, but more often than not are quite expensive to fix.

Keep us up to date about the system too lean. That could be due to a vacuum leak somewhere too. If you replace the O2 sensors and that code continues to be present, that's where I'd look next.

 

Thanks for the reply! As far as audis go, i've found that the 96 is not a terribly costly car to maintain. Its the labor and parts from shops and dealers that are honestly a scam. Thats what makes sites like this so valuable!

The ECU is telling me now that one of the rear sensors is failing. I lifted up the hood and realized I forgot to plug one of them back in! Plugged it back in, cleared the codes, and they havent gone away yet. Ill be replacing both rear sensors on monday, so hopefully i can have more results then.

I did notice however, that there is a rubber tube coming out of the passengers side rear wheel well, at first I thought this may have been like a sunroof drain or trunk lid drain, but Ive been reading about the leak detection system on these cars, and Im starting to think it may be a vacuum line that has fallen off. I am also finding that the plenum chamber and crank case breather hoses are filled with a yellow mayonnaise-like gunk, that appears to be oil mixed with moisture.

If the leak detection system functions in the way that i think it does, i believe it may be possible that this hanging tube is sucking in moisture from the road and injecting it into my intake, which is causing the mixture of oil and water....

I will try to post pictures later but I may post a separate thread as it is a separate issue. Thanks again for the reply!