PROBLEM!!! the car wont pass its emissions test its been through about 4 times and nothing and everytime the guy tells me to change or do something and it should pass so iv put the standerd air filter back on with a new air fiter changed the oil and oil filter, spark plugs and even the lamba sensor and iv put a bottle of that stuff in the fuel tank that cleans the engine out to to see if that helped but nothing i dont know what the problem could be and I'm starting to get .

is there any thing else i can check or do to put the emissions right ?

Which part of the emissions is it failing on?

I'm not sure the guy at the MOT station has kept the MOT certificate for some reason