Problem with engine light on?

[comber1990]

hi having a problem with my engine light wen drive about then slow down say coming to traffic lights the engine light comes on an stays on till i turn ignition off its weird only does it wen slowing down and it doesnt matter if i boot it or not
this only happens since i put my green cotton air filter on any1 any idea
have been told before it cud be lambda sensor in exhaust?