MOT failure on CO2 and lambda sensor ever since cat was stolen

[Claireg]

Hello,

I have just joined in the hope someone may be able to help. My Jazz had the cat stolen 5 years ago, ever since its failed its MOT every time on CO2 and lambda sensor exceeding limits on '2nd idle'. Im not sure what the garage did so it passed after the failure, but every year it then fails again.

I discovered serious concerns with the garage I had been using for the MOT's so no longer using them but with the MOT soon coming up concerned the car will fail again and could be a very costly repair.

I was not able to replace with a genuine Honda cat as they were too expensive so got the best one I could. I  was able to source genuine Denso sensors although even that was difficult at the time as they were out of stock everywhere due to the level of thefts.

I never had any issue atall with the car until this happened. Even with the MOT failure, the car runs fine, the fuel consumption doesnt seem to be any different and no warning lights. Its low mileage for its age and always been looked after.

[Marco1979]

I read (cannot remember where) that the sensors near the cat are sensitive for the exact positioning. Maybe one is not exactly in the flow, so the engine cannot adjust its mixture optimally. Repositioning might do the trick, if this would be the cause.

[Lord Voltermore]

I also vaguely recall reading that  aftermarket cats, or possibly aftermarket sensors sometimes have problems due to the depth the sensor tip screws into the gas flow. Also vaguely recall   it might be possible to adjust the depth by adding or removing shim washers etc.  As its reoccurred on later MOT's the first one was probably fiddled.   

Many years ago my daughters Citroen failed its first  MOT on emissions at a Citroen main dealer.
Technicians were mystified as to the cause.   But  shortly after, with no apparent work being done,  they produced a 'pass' printout.  Maybe it passed on the second attempt but I suspect  having several other identical cars  available nearby  and no cctv monitoring in those days  may have helped    Sold before the next MOT.

[cavosavosk]

Could be the position of the O2 sensor in the pipe as others pointed out, probably even some minor leak in the system, messing up the correct fuel ratio. At this age, it will be pain in the butt to figure out the exact issue that causes it.

I've had it other way around at the inspection with aftermarket CAT. On idle i passed without an issue, but with raised revs it was going into the red zone. Luckily the technician there was a good guy, he revved it a bit harder then released the accelerator and all of the values came right down to green fields