More often than not, deciphering diagnostic trouble codes is like playing a high-tech version of the old circus sideshow "shell" game. In the trouble code shell game, we are led to believe that the pea (diagnostic solution) is hidden under one of the shells (DTCs) stored in the diagnostic memory. Unfortunately for the diagnostic technician, none of the shells (DTCs) on the diagnostic table may have a pea hidden underneath.

Are diagnostic technicians the victims of deception? Not really, because automotive engineers are constantly trying to out-smart their own creations by trying to devise an "on-board diagnostic" or OBD system that will detect any type of system failure.

Unfortunately, their results are often less than predictable since the relationship between the DTCs we see and the actual component failure at hand are, at best, tenuous.

Old and Tired
Most recently, I had a "tired" 1991 Dodge Dakota 5.2L V8 come into the shop with 125,000 miles on the odometer and two trouble codes, a DTC 41 (alternator field open or shorted) and a DTC 37 (TCC circuit open/shorted), stored in the diagnostic memory. Neither DTC caused the MIL to illuminate nor did either have anything to do with the customer's main concern, which was a random cranking, no-start complaint.

Just once during preliminary testing, I noticed on my lab scope that when I turned the key on, the TP voltage momentarily shot up to nearly 5 vo...