My mechanic has checked this and says he can't find anything wrong. The on board computer codes don't make sense.

Any ideas?

Could be anything, but since it is coming on after 5 minutes of driving, I would check the automatic Choke, leading off the fuel manifold, see if there is a loose vacuum hose anywhere. "Small black rubber hose" 3 to 5 minutes is the standard time for an automatic Choke to cut off.