My son has been driving a 1992 pickup this summer on a cross country work assignment. Before he left, the heads were rebuilt because of a burnt valve. At that time we were advised to replace the knock sensor, but the mechanic who did the work did not do this and my son had to leave the minute the truck came out of the shop.

Now he is on his way back, and he's driving about 2,000 miles to get home. the check engine light has been coming on a fairly regular basis ever since he's been gone. When the check engine light comes on, the truck will not go over 75 MPH. The light will eventually go out and then the truck will perform normally again.

Obviously he would be well advised to have the code pulled, but since he's on the road right now, I wondered if anyone here had an idea of what this could be.