Under California regulations fuel tank breather vapours must pass through a carbon canister on motorcycles. This means that the fuel cap can not have the small breather hole normally found on motorcycle fuel caps and that the tank has to be built with a special breather tube. I have no idea what they do on cars. I read somewhere that some cars use a positively pressurised fuel tank and that, if the tank does not hold pressure, say if the fuel cap is not fitted, you can get a warning light.

I found an explanation at http://www.autoshop101.com/forms/h62.pdf

The other common cause of an engine warning light on cars with performance exhaust systems is the reading of the second sonda lambda sensor which indicates the catalyser is not functioning correctly.

Peter

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