After paying to get the fuel filter canister changed, I drove my 1994 TD home and it seemed to run great. I drove it around for another 6 miles and then home. I parked it and then the next day I went out and it totally failed to start. I checked the ALDL and it now has started to give me DTC 13-Engine Shutoff solenoid circuit fault. If the solenoid wasn't retracting then it obviously wouldn't start so I borrowed another solenoid that was verified to be good. I took my original solenoid off and bench tested it and it tested perfectly with the plunger moving quickly. I guessed that maybe there was something else wrong with it so I took the borrowed solenoid and tested it on the bench and it moved exactly as the original one by not missing a beat with 12 volts applied to it. I installed the borrowed one anyway, just because. The truck made no effort at all to start and I verified that a fair amount of fuel was heading for the DS-4. This would lead you to believe that the solenoid (s) were acting up so I jumpered 12 jolts to the two lead wires on the installed borrowed solenoid.......nothing, even though I could hear the solenoid open up when I applied power. I then checked the voltage at the solenoid connector from the truck wiring harness and it showed a a scant 2.4 volts. This may be why I am getting DTC 13 but why didn't it start when I am 99.99 percent sure that the solenoid was open? Another question is where should I be checking for a substantial voltage drop on the solenoid lead in wires? Do you suspect that the fuel filter re-do messed with the wires right next to it? Thirdly (not related), if you are in front of the engine and look at the firewall, the tranny (auto) dipstick is visible slightly at the left of the firewall. Looking farther left about three inches there is a black and a white wire emerging from the main bundle. What are these for?