Hi, CO at idle approx. 8%, scoping O2 sensor at sensor showed flat line at 900mV and pulling off vac line to allow air in resulted in very minor signal change and sensor is very sluggish in its response, if any.
Using EOBD the O2 signal from ECU showed approx. 350mV constant.
Removed the O2 sensor to inspect and it was choked with soot, as expected really.
New sensor fitted - same result. I have checked signal wire back to ECU multiplug and have 0.1 ohms.
Next I scoped signal at sensor and signal at ECU multiplug and they were identical but the EOBD value was 350mV.
Am I looking at a faulty ECU here or have I missed something more obvious?

Thanks for your time, Martin.

Hi Martin.
Tech 2 may give you more help, but what other tests have you carried out ? what are the fuel trims ? what are the injector duration,s ? what is the fuel pressure ?
I think from the info supplied ECU is a long way down the list of possible,s.
Kev.

Hi Martin

Do you only have EOBD access ? if it was me, I would question how reliable EOBD info is on a 1998 vehicle, particularly if you are considering condemning the ECU on that basis.

Cheers Aidan

I have Vaux-Com but didn't think to try it, tomorrow I will. I also got the same O2 readings using Multiscan on Saab.
STFT is +23%, injectors 3 mS, MAP 39kPa, it is going into closed loop, have not checked fuel pressure but regulator is not leaking - checked with Mityvac.Engine runs smooth and I have used Smoke-Pro on intake and it is airtight.
I thought that if the ECU was seeing 350mV it would add fuel to richen hence +23% STFT and because the ECU does not see the O2 voltage increase it just keeps adding fuel?

Martin.

Hi Martin,
I take it this has a 4-wire sensor?
Have you checked the ground wire/ground reference to the sensor, also have you checked all the earth drops from battery to ECM/engine/chassis? A check of the the supply voltage as reported by the ECM against the true battery voltage may give it away.A 550mV error is easy to generate with a bad earth somewhere.
HTH Shaun

Hi Shaun, it has a 3 wire O2 sensor.
I did wonder if earthing could be an issue here but have not really any experience in voltage drop testing as you suggest, could you briefly suggest how best to do this.

Thanks, Martin.

Hope this helps

Volt drop testing

If you want to learn more I can highly recommend Bosch training course "VSE1 Essential Test Procedures".

Regards Glyn

Can you get any live data? would be foolish to condemn ECU at this stage

I will second that Glyn.
VSE1 is a good course(sitting both sides of the desk) and will help greatly with understanding volt drop testing.

Regards Mark

Hi, because the O2 sensor was a 3 wire and the signal earthed through the body of the sensor I checked the continuity between the sensor body and the heavily corroded exhaust - 3.5 kohms!
Removed the sensor filed the surface where the washer contacted, now 0 ohms. Also fitted an earth cable between exhaust bracket and gearbox.
Although the O2 sensor only had 3 wires it was using a 4 pin plug, the socket on the engine loom did have a wire connected to the unused position. This would have been the signal earth to the ECU, so I cut it and connected it to the cylinder head.
Hey presto the O2 sensor was now switching, the sensor signal reading on scanner coincides with scope, the STFT is now +-7% and CO level dropped from 9% to almost zero.
Thanks for the voltage drop testing pointers etc. have been reading up on this now for future use.

Regards, Martin.