It has been reported that Autozone (and I suppose other) car
parts stores will read your codes for free. Seems like a great
service many places would do if it sells parts. I'd try one
of them to get the actual coses since that is the only way to
know for sure.

If it came on going up hill you might also check the fluid
level of the coolant (but that turns on the coolant light for
just a few seconds, not the CEL light). Still, low coolant
triggering a dash light is a pretty common problem.