Exhaust emission warning light

[Isambard]

Hi all
For better or worse I have just replaced my Scorpion exhaust on my 172 with a Klarius pattern exhaust (couldn't stand the noise or look of the scorpion).
The car is driving fine, but on the way home the emissions warning light came on. I am hoping to get a code reader on it on Wednesday, but before that I'm trying to work out the most likely culprit, and whether it is ok to continue driving:
- Both Lambda sensors were disconnected (and reconnected) during the exhaust swap. I was planning to change the cat but couldn't get the bolts undone!
- The fuel was very low when exhaust was changed, so I then filled up at a different filling station to usual (though 95RON as normal)
- Is it possible that the new exhaust is causing a build up of gasses around the cat, as it is less free flowing than the old one?

Do any of these sound like the most likely cause? I have double checked the connection for the post-cat lambda, and I think if it was the pre-cat one, that would definitely cause running issues?
Thanks for any thoughts!

 

[Isambard]

Quick update: I got the fault code checked today and it is as follows:
P0420: Catalyst Efficiency Below Threshold (Bank 1)
Any help?

 

[Diagro]

Sounds like the Post cat sensor is reading the wrong value.

May be the fact you undid it and reattached it which has casued this.
Take it off, give it a clean and then reapply it. if its still throwing the light then replace it.

 

[Panda]

Cat on its way out i think

 

[Isambard]

Thanks guys, I do have a spare lambda sensor, so I might try swapping it over as my first attempt.
I'll report back!