my car was on the rolling road last friday for power map with normal exhaust and an iva map with the mnr test exhaust with cat.i then recieved a phone call saying lambda was good,hydrocarbons good but co2 way out. and nothing they were doing could change the levels.
he said it was as if the cat just wasnt working at all but this cat has been used on loads of mnrs for iva and never had a problem before! any idas what could be causing the high co2 levels?

Could be a knackered cat. Especially the one your borrowing as they are swoped from car to car and being run very lean or very rich until set up

Over cooling ?

What exactly were the readings?

Was it definately CO2 which was out and not CO ?

Cheers
Davie

Sounds like either cat is knackered or not big enough for your engine config the fact it has been fine on similar cars is no guarantees it will be on yours. Note that for MOT now if your car is presented “de-catted“ it will fail

i had exactly the same thing with a moto bike engine in my kit. Lamba and hydro was spot on but co2 was about 2 when it should be under 0.2.

we pulled the fuse on the radiator fan and removed the cooling fan that they put at the front of the car when on rollers. we let it get hot but we monitored the temp and kept it reasonable. reving about 4000 rpm and suddenly the cat came alive and co2 went to 0.02.

we replicated this on IVA test, took about 3 attempts as kept failing but the tester was really good a let us rev it up to get hot and then drop the revs to the requirement once cooking for the emissions test. it passed third time round.

hope this is helpful.