We would expect a car that has been serviced according to schedule to pass an emissions test, but there are number of issues that could cause it to drive normally but be outside of the emissions limits. The emissions report should indicate on which element or elements of the test it failed on, and this could help point towards the cause.

A faulty oxygen sensors, catalytic converter, a dirty air filter or worn out spark plugs could all cause emissions to be higher than normal. If your car is averaging only 5,000 miles a year it could be that it is mostly making short journeys which could lead to a build of carbon deposits which could also contribute to excessive exhaust emissions. A longer drive using higher engine revs can help this, and ensuring the engine is thoroughly warmed before an MoT will help the catalytic converter perform optimally. It is unlikely that changing the cambelt has caused the emissions to spike unless the repair was carried out incorrectly.