St 170 emissions failure.

[New to this]

Hi so recently just put my st through an mot and it failed due to emissions, before the test the car was "ragged" around a bit with the hope of it passing but it still failed, my question is I know it's either the sports cat or the O2 sensor but of of those 2 which would be the one you would repair if you only had the option to fix one, as I have sunk far to much into repairs for the car and am now only left with the option of replacing one of those things in the hope it passes otherwise I won't have a car till I can replace the other part.

Someone also told me to unplug the battery for a bit then put it for the mot, is there any truth to that?

Thanks for any advice.

[Stoney871]

O2 sensor is more important tbh.

Have you thought of trying a Terraclean service?

Sent from my SM-G930F

[artscot79]

IDE look at the o2 sensor but in truth you need the fault codes which there should be if the emissions are high or to look at the live data and see what's doing what

[New to this]

3 hours ago, Stoney871 said:

O2 sensor is more important tbh.

Have you thought of trying a Terraclean service?

Sent from my SM-G930F

I didn't even know what a terraclean was until I googled it just now, if it really does lower emissions and improve mpg then I will definitely be doing that.Have you had one before and did you find that it improved your mpg?

@artscot79

Thanks, I was unaware about it throwing up fault codes so will get that checked, also with the fault codes would that be able to tell me what exactly needs changing or is it a link in the chain kind of thing?

 

[iantt]

most important is what are the emmisions readings on the mot emmisions test, you can tell quite a bit from those, can you photo and post up to see what they are?

 

[New to this]

10 minutes ago, iantt said:

most important is what are the emmisions readings on the mot emmisions test, you can tell quite a bit from those, can you photo and post up to see what they are?

 

I can get the paper and post it up, car was taken to garage by friend so I'll get the paper off him soon and post it.

[iantt]

if it failed on co and or hc and past on lambda then cat likely to be at fault, if it failed on lambda reading also then lambda (upstream hego sensor) likely at fault