Help - MOT failure on CO - oxygen sensor? Which one?

[MiniNinjaRob]

Ok my 1997 940 Turbo HPT has failed its emissions on CO only. All other emissions were perfectly fine.
The tester said, after asking a few questions, that it was probably the O2 sensor that was faulty.
Engine has been fully serviced, new ignition, everything has been changed recently so I am rule a LOT out apart from the O2 sensor.

So I need to replace it ASAP.

However this is where the confusion lies as I don't know much about these things on these cars. I have read there might be 2 sensors but I can only find one.

Here is the one I have found:



It's the gold bit in the middle of the picture. It's about a foot after the turbo.

I can't find any others, I read there might be one at the Cat but I can't find one round there.



Here are the connectors I've found under the bonnet which might be the connectors to the sensor but I'm not sure as I've read they are blue but these evidently are not blue!





Sooooo - have I got one or two oxygen sensors? And where is best to buy one from and what brand/type do I need?

Help!!

[Forrest]

In my experience replacing the O2 or lambda sensor is a good thing to do since if you are still on the original it will probably improve fuel economy and hence likely to reduce CO emissions too. The thing in your first picture looks a bit like the sensor, it is in the exhaust downstream of the turbo. However both the ones I have changed have been underneath the front pipe just behind the turbo. Yours looks in a slightly odd place. I suggest you look under the pipe near the turbo (using a small mirror may help) and see if a previous owner has messed up trying to remove the original and installed a new one further down the pipe. If possible, use the original mounting point because the exhaust gasses will be hotter there and it will work better. Maybe consider replacing the front pipe if it has been butchered and you're planning on keeping the car. Best replacement is NTK (NGK) OZA446-E58 OZA446E58 (0483) which will have the correct connector. The original will have been Bosch if you want to stay OEM.

A 1997 car will most likely only have one sensor. In the unlikely event that there are two only replace the one nearest the front. The back-most one won't affect the emissions.

[MiniNinjaRob]

Thanks for the help - quick question though. That sensor code I've looked up and it's for a Saab, the NTK code for the Volvo is different. Or have I mixed things up?

[Forrest]

Check on http://www.ngkpartfinder.co.uk if you're not sure, but I've used that part on two cars, 1994 and 1997.

[Ashc]

Another possible culprit is the coolant temperature sensor under the inlet manifold. They can fail, making the ecu think that the engine is cold. It keeps the mixture rich most of the time, which obviously causes terrible emissions.

[Chooch84]

If that doesn't help you the Bosch part number should be 0 258 003 376. Bosch part numbers for other bits and bobs can be found at this link.

[James_EG_940]

You will almost certainly only have one oxygen sensor. That orange connector is the connector for it - the other one on passenger side of engine bay is crank position sensor; just follow the wires.

I would suspect o2 sensor or temp sensor - how does your exhaust smell?

[omc 47]

Hi NinjaRob,
Yes that is the Lambda sensor in your pickys, I have 2 940's and they are both on the down pipe from the turbo.
I've had issues with the CO readings before now and found the 3 bolts (holding the turbo on the down pipe) needed clamping up a little tighter.
This connection is a dry perfect fit and if not completely sealed fools the ECU into making the mixture richer because the Lambda sensor is sensing too much oxygen in the first part of the pipe after the turbo................

[TonyS9]

Its also possible someone has fitted a wideband sensor for tuning purposes. Or the original has been blanked off and it is loose.

I would look for other problems first before condemning the sensor, definately air leaks on the exhaust at the turbo and manifold is a potential cause.

[MiniNinjaRob]

Originally Posted by James_EG_940

You will almost certainly only have one oxygen sensor. That orange connector is the connector for it - the other one on passenger side of engine bay is crank position sensor; just follow the wires.

I would suspect o2 sensor or temp sensor - how does your exhaust smell?

Exhaust tends to smell like engine is running rich. MOT tester said he could tell it was the oxygen sensor from the smell and how the gas analyser reacted when he came off the gas during the test.

Car has done 180000 miles now and non of the sensors have been changed so far