You said the engine is running well. In my opinion, this rules out the
water pump having seized. (Not sure what led you to that potential root
cause...) No smoke out the tail pipe is another good thing. Changing oil
regularly is excellent preventive maintenance for your engine, but again,
not related in any way to the check engine light being on.
The only thing you know for certain is that the engine computer has
determined there is a problem. This is based on the information it received
(or did not receive) from the various feedback sensors used to determine
overall health of the system. You really need to know the error code that
was set to provide insight into which sensor provided the bad data and set
the fault condition. If you are in the US, Autozone reads the OBD_II error
codes for free and can tell you the cause. Post the error code and
description once you have it and you are likely to get some useful direction
on what to check from this group.
Also post vehicle mileage, maintenance history, and all other symptoms you
are seeing (starts hard, poor acceleration, rough idle after warms up, poor
mileage, overheating, alternator light on, etc.)
Bob