2003 Dodge 2500 Diesel, 2wd, Auto. This is the 3rd time that this has happened, pay dealer and private garage to diagnose a bad ECM, change ECM, still nothing, have electronics guy check original ECM and find nothing wrong with it. Reinstall original ECM after 2 months of truck sitting and truck runs for 2 years without a problem (No changes to anything at all)...Now same old same old.....truck will not start, turns over good, no communication with ECM. After key is turned, 'Park Brake' light illuminates and dings, 'Air Bag' light illuminates and dings, gauges do not move and red security light stays lit. I see another thread for a 2006 with similar problem, they note that it may be a viscous fan issue and to remove #6 fuse however #6 fuse on my truck is 'Air Bag', that did not solve my problem upon removal of said fuse. I don't think it is an ECM problem, there has to be something like a sensor or module failed or failing that is blocking the ECM from functioning normally or even identifying a problem. Dealer ran through their checklist and their solution is bad ECU, so why does ECM check out good but still not run?? And then when I put the original ECM back on so I can send to the scrap yard, have 2 years of absolute trouble free driving?? Truck ran great, shut it off and now this again.

 

On your 03 there is a plug near the bottom of the Rad that supplies the fan, You could try unplugging that to eliminate the Viscous Fan.
There are a few plugs that are prone to problems you could try taking apart, cleaning and plugging back in. Behind the Drivers side Mud Flap. Back of engine at the top where it bolts to the transmission. Large plug behind Drivers side battery ... ( I had to use drill guides to drill the head of that bolt off then replace the bolt ) Plus there are 2 wires taped together running down to the computer from the Batt terminal of the fuse box and a ground (Mine showed signs of having ran hot distorting the tape)
My problem ended up being the ECM and I In Boxed you information on that. With the off again on again nature of your problem I would be going after the plugs I mentioned and take a day to find and clean all the grounds in and around the engine compartment even if they look good.