Pretty sure the post and pre cat sensors are different. Did you cross reference pn's? They do entirely different things pretty sure you gotta get the correct one

 

I did cross them! I thought they where the same sensor. Definitely worth a try.
I'm running a post 02 sensor as the pre cat sensor and vice versa.

I'll swap the sensor out to see if it affects it.

 

I took the ecu out and examined it under a magnifying glass and it looked flawless. No signs of water damage.
To be safe a did a brief cleaning and re installed.
after scanning the code came back.

I checked the measuring blocks on vag com and this is what i'm getting.
S8XoOl.jpg?1
The resistance for the before TWC sensor, is constant on 8.7 and does not budge, and it is always on Htg. S1 ON.

Where the other sensor seems to vary in resistance and in on and off.

 

Ok, so i think I've established that the sensor heats fine.
I've pulled it out with the key on its scorching hot, and i have tested the sensor pins 1 and 2 and they read 2.5 resistance like shown in this tutorial.
AudiWorld Tech Articles

So if that's the case somewhere between the sensor and the ecu the signal is being broken.
Or it is the ecu.

 

The heater circuit is a separate code from what you have.. I dont think it is that, especially because you should get a good reading when your exhaust temps are hot enough anyway..

When you check the resistance, you need to measure it across the entire harness, not from pin to pin at the oxygen sensors like you were doing (which is what you want to do to test the sensor, but we can assume its good since we have the same code with two different oxygen sensors youve used). Meaning, you connect one lead of your multimeter to one pin on the harness at the oxygen sensor, and connect the other lead to the same pin at the ECU.. The resistance should be about zero. This requires a wiring diagram for the harness at the ECU because there are so many pins, otherwise you could just measure resistance for every single one and make sure there is one with low resistance I guess.. You want a reading close to zero withOUT a k (k stands for the number displayed multiplied by 1,000), or M(M stands for the number displayed multiplied by 1,000,000) in the bottom right of your multimeter.

Next, you should measure the resistance from each pin to ground, so you will connect one lead of the multimeter to a pin on the harness at the oxygen sensor, and then connect the other lead to chassis metal or battery negative. (Here you want very high resistance, you will want the a resistance value with M or the same reading as you get when you dont connect the leads to anything, unless of course one of the pins is a ground).

 

I'm gonna go out slightly on a limb here and suggest that you use the appropriate pre-cat sensor, which I believe is this one:
Audi B5 A4 FWD 1.8T > Engine > Emissions > Oxygen Sensor > ES#1558 Oxygen Sensor - Priced Each - 13740

For a little while I was incorrectly using that particular sensor on my '98.5, but I'm supposed to use this sensor instead:
Audi B5 A4 FWD 1.8T > Engine > Emissions > Oxygen Sensor > ES#1563 Oxygen Sensor - 15160

And while I was running the first sensor the ECU wouldn't allow it's self-test of the pre-cat O2 sensor to pass because the timing was off. So I wonder if your usage of a post-cat O2 sensor in place of the pre-cat O2 sensor is the issue.

And I've actually still got the sensor in the first link laying around... I'd be happy to sell it to you for pretty cheap.

 

I'll buy it, im really open to any options at this point.
To be clear my car is a 1997 Audi A4 quattro 1.8t.
That sensor will work?
If so pm me your paypal and what you want for it and ill send the $.


Because mine definitely resembles the second sensor, not the first one.

 

Sent a PM.

 

I got my new 02 sensor in, and it still throws the code.

 

Next step I believe would be trying to figure out what pin it is at the ecu, for the 02 sensor.

If anyone has any advice or knowledge on it I would be very grateful.