(this is something I still haven't solved.)
My CEL came on 2 years (50k) ago. I fixed the suggested problem, truck drives perfect, cel warns of other problems OK, and the CEL code still comes on everytime I start the truck. (I clear is with my code reader and it stays off until I restrart the truck). I was told the ECU was malfunctioning after a tech swapped it with his identical one. Since a lot of the used parts like this are non-returnable, I'm debating whether to try a replacement ECU.
any thoughts appreciated.

 

What code is it throwing?

 

You said the tech swapped it with his identical one. Did you not get the CEL when he did that? That would seem pretty convincing evidence to me.

Like mastacox asked, what is the code?

 

Yes, ECU`s can go bad, they can either go bad and quit working all together or part of it can fail were it will throw a code thats not really there, but your truck will still run.

 

With my old 3.0 my CEL was on pretty much constantly. Funny part is that it would only go off when it rained.

Truck drove great, got "good" mileage (17mpg w/ 33x12.5's on stock 4.56's), and very gave me any quams. I actually got worried when it turned off.