Hi folks. Thanks for reading my post!

In short, there is 1.6v on the black wire (pin 4) of the oxygen sensor.

Vehicle:-
Audi A3, 1.6 (AKL engine code), Simos PCM, year 2000, pre EOBD

Faults:-
027B Oxygen sensor heating faulty – permanent
020D Oxygen sensor signal implausible – permanent

Live -
Reports 1.6v whether the sensor is connected or not. Plugged in, the voltage may drop a little to 1.2v when revved.
On my KTS 520 there is no fuel trim info. I should have checked OBD!

Checks:-
Open circuit on the sensor heater wires.
NBV at sensor heater circuit.
1.6v at pin 4 loom side whether the sensor is connected or not, using sensor ground and battery ground.
Battery earth volt drop to engine 0.01v.
Run engine with sensor disconnected and use DMM to check sensor output. At idle output = 0.9v. I disconnect the EVAP pipe to the manifold to introduce an air leak and the voltage drops to 0.4v. The sensor seems to be responding OK.
Cut the wire at the PCM (pin 25, black wire) and live data still reports 1.6v, confirmed with DMM.

Conclusion:-
The PCM is defective.
The heater side of the oxygen sensor is defective.

Questions:-
1, would anyone agree with my conclusion?
2, why is there 1.6v at the sensor wire?
3, have I missed any checks?

I didn't consider a bias voltage input at the time, however I feel this would of showed up when the sensor was plugged in (unless it was open circuit, which it was not since it produced a voltage unplugged)?

The car failed the MOT on emissions and other issues. The customer is likely to scrap the car if the problem is the PCM.

Many thanks,
Kieron

Hi Kieron,
what type of sensor is it? How many wires and what colours? If this was zirconia then it may produce a signal even when disconnected. This would mean rich mixture. It shouldnt reach ecu if diconnected, unless this is default setting. Is there any switching using scope. What does short term trim do?
HTH James

James, thanks for your repy,
The sensor is a four wire zirconia sensor. 2 white heater wires, one black signal wire and a grey signal ground. The sensor was piping hot when I got an output from it disconected, it was not switching as it was out of the loop!
With the sensor conected there was no switching. It just stayed at 1.6v. I saw no point in conecting the scope as I have a fast DMM which would detect the 1Hz cycle rate. Unless I've overlooked something.
I was unable to look at fuel trim with my KTS. I may have the opportunity again next Tuesday to look at the vehicle.
Fast idle emissions:-
CO = 1.37 / HC = 739 / Lambda = 0.948 / sorry no oxygen info.
Many thanks,
Kieron

Kieron

You need to sort the O2 sensor heating fault first,

If the ECU sees a fault with the heater circuit it will most likely ignore any signal and use defaults.

If I understand your post correctly you have checked heater circuit as OK, but confirmed sensor faulty. If so you need to replace the faulty sensor next.

cheers Aidan

Aidan,
You are of course correct. There is a problem with the oxygen sensor so the first step is to correct this.
The problem I have is the car is a traders car. He is reluctant to spend money when there is a possibility the PCM is faulty. If it comes to this he will scrap the car due to the other repairs needed.
Not the best situation for a committed diagnostic repair.
I was hoping to understand why there is 1.6v on the signal wire from the PCM. maybe it is a diagnostic bias voltage? In line with what you say, maybe its there because an open circuit is detected in the heater circuit.
I will take another look on Tuesday.
Thanks for your reply,
Kieron

Kieron.

"The problem I have is the car is a traders car."

I hope you are not trying to do them a cheap job !

Kev.

Hi
If I understand correctly, you have open circuit on the sensor heater, you could possibly eliminate this by introducing a dummy load on the circuit, isolated from the sensor. Something of similar resistance. Reset fuel trims if possible. Clear the codes and see where that leaves you.
Jim.

Kev,
Yes, I guess I am trying to do a cheap job. I have just started up on my own. I must remember not to take jobs personally. Its the customers problem not mine
I get too involved and interested in such jobs!
Thanks for your reply,
Kieron

Jim,
That sounds like a good plan. I will take another look at the car on Tuesday.
Thanks for your reply,
Kieron

Hi Kieron,
Tell your trader fella that you are a "Technician" not a "Magician" You've found a fault which is the 02 sensor, that needs to be replaced then you can move on !

Regards
Dave

FIXED
The 1.6v would appear to be a bias - diagnostic voltage. At the sensor, if I ground the voltage through a test light the voltage at the PCM drops to zero. My limited understanding of circuits would suggest if this was a (true) voltage the voltage would remain at this point and drop to zero after the test light bulb (load). Still need to do a little more reaserch to improve my understanding of how the voltage comes about and the enhanced fault codes as a result.
So, yes, I did replace the o2 sensor, which I should of done from the start. This cured the problem, o2 started switching and the emmissions came in line with what they should be.
Many thanks for everyones input.
Kieron