My truck is a 1993 F150. For the past few days (not longer than a week) while driving, the check engine light turns on and stays on until the truck is turned off. I don't drive over city speeds (45 mph) and I can get it to come on after accelerating hard for a few moments. There are no strange or new noises coming from the engine, no loss of power, or other strange happenings.

Yesterday I tried to get the codes out of the EEC using a paperclip, the same thing I've done dozens of times. Problem was, last night, the self-test would not start. The check engine light would not flash at all, not even right when I was turning the key to "on". While trying to run the test, the engine RPMs would dip, climb, and then dip again, but only once. In past tests the engine would do this several times before spitting out codes.

I figured the bulb had chosen that moment to blow out, and chalked it up to bad luck. The thought that something might be wrong with the test itself didn't cross my mind.

Until tonight. The silly light came back while driving home tonight, and it's doing the normal quick flash when I flip the key forward. Unfortunately, the paperclip method isn't frying the fish. I get no flashing light with the paperclip in.

The only thing that I can think could be wrong is the fact that about a month ago the truck was in a mechanics shop, getting an oxygen sensor replaced. The mechanic mentioned he had to run an EEC test to find out what was wrong, and I hadn't run the test myself since I got the truck back. Might he have disabled the self-test? (Yeah, right.) Or might he have broken something whilst screwing around down there?

If both of those are outside the realm of possibility, what could be the matter?

 

Usually that means the ECU is bad. Check the wire harness, and make sure your paperclip is making good contact in the connectors.