Well, yesterday a friend of mine was to borrow my 'doo, and he called me telling me that when he plugged in the key it said "check exhaust"

After a few seconds it dissapeared, but a Temperature warning came on instead.
Just wanting to get the 'doo back he pulled in reverse and then the temp warning disappeared and "check engine" came on (with a code, which he didnt remember =( ah well) but it switched back to temp again when he put in forward?

Any ideas what this can be?