Ok, so for the last 2 months I've been having a problem with the transmission skipping past 3rd gear. By pulling the ECT plug and shifting manually, I've assertained that 3rd gear DOES still exist physically and my problem is of an electrical nature. I pulled the shift solenoids, checked for resistance and sealing capacity (with compressed air) and found them all within spec. I changed the fluid (duh) and checked for solenoid operation from the cab by sending voltage through the ECT connector. All of them engage. Prior to pulling the solenoids I didn't get an ECT code, however once I put it back together my OD light blinks. Since everything was in acceptable condition, I figured my problem lie in the ECT, however having just replaced it the truck does the same thing.

Short of replacing the thermo sensor in the manifold, I've either checked or replaced all electrical components and checked their operation through the wiring harness. What else could there be??