hi am new today, i’ve recently bought a 14 plate zetec fiesta with 78k on clock, few days ago yellow engine light popped up. took it to garage an they said the code is Catalyst System Efficiency Below Threshold they said they 95% sure it needs a new one, but also said could be O2 sensors, been quoted £792 can any one help me with this is this right, don’t know much about cars thanks. kyle 

They're right, it could be the cat or a faulty lambda sensor.

That quote seems expensive though.

You can buy a genuine used cat for around £200.  Or a new aftermarket cat for around £300.  Fitting should take less than an hour at a garage.  

 

Here is one used example, including the two sensors.  In your position, I would buy something like this and have it fitted at an independent garage for around £75.  

https://www.ebay.co.uk/itm/276429517051?

thanks for replying, they are 95% sure it needs new cat, what’s you’re opinion do you think that would fix it. the man i bought it of said it came up on the dash before he sold it an had to get new O2 sensor? then  light went off then came back on few days ago. 

How long ago was the new sensor fitted?  Was it pre-cat or post-cat?  Was it a decent brand?

I'm nowhere near 95% sure that it's the cat.  That's why I'd do as above, fit a replacement used cat with both sensors.  That covers all bases in the quickest and cheapest way.

i don’t really know, about what sensor it was sorry, not really clued up on all this stuff sorry. he didn’t really say much just wanted it gone tbh, and now i can see why. so do you think with getting what you suggested would sort it out an light will go off. thanks sorry for all questions i really do appreciate the help 

it was fitted two weeks ago 

14 hours ago, kylekella45 said:

i don’t really know, about what sensor it was sorry, not really clued up on all this stuff sorry. he didn’t really say much just wanted it gone tbh, and now i can see why. so do you think with getting what you suggested would sort it out an light will go off. thanks sorry for all questions i really do appreciate the help 

No need to apologise, everyone on the forum has a different skill level.  

If there wasn't any history of the car, I'd be about 60% sure it was the cat, 20% sure it was the post cat sensor, 10% sure it was the pre-cat sensor, and 10% something else entirely (wiring or ECU).

But as one sensor has already been changed, those percentages would also change, if we knew which sensor it was, and whether it was good quality or not.

As it was only changed 2 weeks ago, it could just be a cheap sensor that the ECU has taken some time to realise isn't quite within tolerance.  The genuine post cat sensor for these is over £300 new, so I doubt it was one of those if he was intending to get rid of the car quickly.

So that leaves us in a difficult situation now.  You could try replacing the post cat sensor again on it's own.  Or you could try replacing the cat.  Or you could try replacing the cat and the sensors from the link above.  We just don't have enough information to say whether it's more likely to be the cat or a sensor unfortunately.

If it was mine, I think I'd still buy the cat & sensors linked above.  But might try just swapping the post cat sensor (purple plug) from that onto yours first.  If the fault code returns after doing that, then chances are it's the cat at fault instead, but you already have that spare to swap on.  Would also recommend keeping the sensors from your old cat for now, just in case they're still good.  (PS - If it's still the original cat, they're worth a few quid at a scrap yard as well, so you can get a bit of money back from that).

thanks very much for you’re advice appreciate it