I have a 98.5 diesel giving me the codes P1693, something to do with the companion module, and P0381, glow plug / heater indicator circuit malfunction. The wait to start light is not coming on and I don't hear the lift pump when I first turn the key to the on position. I brought it to the shop and they told me it was an ignition relay. They charged me a bunch of money and sent me on my way and it didn't fix the problem. Any ideas on what causes these codes, how to fix them and how serious are they?