Took my 1999 F350 to the shop and they pulled 2 codes off as the reason the check engine light is on.

1) P1393 - Glow Plug Circuit Low Input (Bank 2)
2) P1391 - Glow Plug Circuit Low Imput (Bank 1)

I assume this is more then just changing the glow plugs, but any info and help would be greatly appreciated. I am in California if that makes a difference (I know sometimes it does)

Thanks in advance!

 

Looking in to your problem, there may be too many causes for this problem. An open in the glow plug circuits, glow plug relay, glow plugs or monitoring circuit could be the possible causes. If you can able to find some more symptoms then pls tell me so that i can help you.

 

There could be several causes. Your gp shunt or relay, the glowplugs themselves, or your PCM. Get a hold of a multimeter and a 9v battery to test the relay and shunt. This next thing may be a little dangerous with glow plugs, but if you have a gp connected and out of the cylinder, you can ground it on the block and check the temp. Some multimeters will allow you to check temp by putting both leads on an item in question. The PCM is something the dealership is going to have to look into.
If im forgetting someone I apologize but that is all that I can think of right now.

 

I'd check the wiring going into you're valve covers. It seems to be a common problem.