MOT Emissions failure, what could it be, help!?

My MK3 ford fiesta recently failed its MOT for high carbon monoxide emisions. Im not to sure what it was, but their was one thing i thought could be affecting the emisions. I recently swopped the air box for a cone air filter because the entire air box was broken and the car was running funny, but when a fiited the cone air filter the car has been running fine.

But when i changed it, one tube that came from the engine into the air box now goes nowhere (not sure how big of an issue, if its even an issue that is). I left the mass air flow sensor their, but the air temperature sensor is no longer their, i dont know if this could be enough to affect the emisions.

If its not this, what could if be?



ThanksMOT Emissions failure, what could it be, help!?
the one from the engine is proberly a oil breather hose, you can buy piper breathers from demon tweeks.



didnt know the mk3 fez had a air temp sensor.



personally i'd put the original airbox and filter back in and pour half a bottle of redex in the car. worked for my mot, my xr2i failed on emissions and the redex worked a treat.



p.s instead of a cone filter in a mk3 your better off buying a K%26amp;N panel filter, drilling holes in the bottom of the original airbox and using that, great airflow without the hot air of the engine bay.MOT Emissions failure, what could it be, help!?
It could just be dirty oil and an old oil filter.



My car's diesel. Just before the MOT I always use an engine flush and then change the oil and oil filter. It never fails on emisions now.... it use to, every time.
Maybe a faulty exhaust.
Possibly the Catalytic converter, but I would put the sensor back it is needed
could be a few things like faulty m.a.p sensor of faulty intake sensor, could be carbon buildup on the ports, or mite jst need a full service things like spark plugs, oil and filter, fuel filter and the cone filter mite need cleanin. Would also go 2 a scarp yard and get a air box and put a proper filter in. and as some of the other answers say mite be the cat converter
How long had the car been standing before it had its M.O.T. if the car was cold when it was being tested,then the emissions would have been higher as the choke would have automatic control started on it,best to run the engine for a while before the M.O.T. or better still go for a drive along a stretch of motorway before the test so the engine is warm and the exhaust has had chance to clear of any water,oil,debris etc