Steve,

Don't you mean if the car has an exhaust, it may flag up a P0240 CEL?

I dont see how it can flag up a cel if its been disabled?

Modifying the routine ususally means its never going to come back up again as the check value has changed. Assuming its a CAT related code, other codes such as TGV reference a bank / voltage check.. but again those can be eliminated with some knowledge.

But you are right, definately get a workshop or dealership to investigate the cause of the error. In some causes a CEL will disappear as part of its "drive cycle" if the error is nothing serious.