Mk3 mondeo 1.8 duratec decat

I had to replace the cat on the father in laws mk3 mondeo, from memory it was only £80 from Ses exhausts so might be worth doing at the same time as the lambda. (make sure you have access to new bolts as they all sheared when I did it). We had to run his for a few months before hand with the catalyst knocked out and I can't remember it flagging up fault codes. Hope this helps

No, if you replaced the cat but left an iffy lambda in then there's a good chance of corrupting the new cat, probably what's happened to break your existing cat. Have you checked the lambda harness to make sure it's not fouling something in the engine bay???

You don't need a new sensor by the sounds of it?

And is your sensor before or after the cat?

Hmm... I don't know these engines so can't really pass comment, but if your O2 sensor is looking for a value, and it aint getting it, it will alter fuelling until it does. Would it not make your engine either run super lean, or limp mode, engine light straight back on?

In your shoes I would leave it as it is, running and working, and then buy a new cat and fit it when it arrives. Sounds like too much to go wrong if you go smashing the cat out when you don't seem to need to?

If you remove the cat the bank 1 sensor 2 O2 sensor isn't going to feed the correct voltage back to the ECU and it will most likely put the EML back on for a catalyst ageing fault or a bank 1 lambda loop fault either for s1 or 2.