Hi Guys,

There are apparently two o2 sensors used on a blacktop Zetec.

I believe these are pre-cat and post-cat.

1. Are the two sensors physically the same?
2. What does each sensor do?
3. What is the likely result if the pre-cat sensor is missing/not working?
4. What is the likely result if the post-cat sensor is missing/not working?

Regards,
Andy

The first one is used for the emissions when running in closed loop. This is every thing but tick over and WOT after the engine has reached temperature.

The second is just to check on the cat.

If you run with out either it will go into limp home mode. You may get away with no cat by adding a resister to the post cat sensor. Google Mil eliminator mods.

Thanks mate, does that mean on your ECU install on your zetec you have both sensors running? I'm also not running a cat if that makes any difference.

I only had one connected (with the round green connector) but it's duff so have disconnected for now until the new one arrives, but the car is definitely not in limp mode!

If the sensor is disconnected the ECU will realise it is faulty and resort to standard mapped settings. It could well run better than if you have a sensor that is giving crap readings but not as well as when running properly, especially fuel economy.

The Only one ive done that was new enough for pre and post sensors was the St170. I think they come in around 2002~2004.

Yes the second sensor will know if the cats not there and it will through a wobbly unless you fool it or map it out.

You can see them on the mani

Ahh, my blacktop is around 1999 i think (from a mk2 mondeo), so i'm assuming i should only have one o2 sensor then.
I'll wait for the new one to arrive and hopefully she'll run a bit better!

So where are you getting the second sensor ?

I don't have one connected, but was chatting to a guy who "knows loads about zetecs" and told me every blacktop has two O2 sensors. I wasn't sure so atupidly listened to him...!

Phew, I was getting a bit worried then that I had missed a sensor on my Blacktop.

The only one I have is the Pre CAT Lambda sensor and nothing for Post CAT on a 99' Mondeo engine