Need help diagnosing glow plugs

[99' Powerstroke]

My service engine soon light has been on for a long time and I borrowed a generic scanner to check. Came up with codes P1391(Continuous glow plug circuit low input, bank 1), P1393(Continuous glow plug circuit low input, bank 2), and P0603(Continuous powertrain control module keep alive memory error). Not exactly sure what the last one means.

This would make sense since my truck does take a couple seconds longer to start when in gets down into the 30's. I checked the GPR and it shows 9-10 volts so it seems good, and I checked the GP's at the valve cover plug and all eight show .8-1.0 ohms. Since the GP's test good I assume the under cover harness is also good. All I can think of to check now is the continuity in the wiring harness from the GPR to the valve cover.

Is there anything I am missing? What else can I check?

[jesilvas]

9 volts is low for the gpr

I made my Samsung SGH-I997 send this.

[jesilvas]

Wait, do you have an excursion or california truck with a glow plug module, not relay? Cause if it's a gpr, idk how you're getting codes for them.

I made my Samsung SGH-I997 send this.

[99' Powerstroke]

I live in California and I bought my truck in 2003 from a used car lot in California, but my truck has a GPR. I guess it could have been bought new out of state, but it has California emissions stickers under the hood. It also never had a catalytic converter, but I'm not sure if all California trucks came with one.

If 9 or 10 volts is low for the GPR then that would be disappointing since it is a fairly new relay that I just threw at the problem about 7000 miles ago hoping it would fix it. I guess I shouldn't be surprised since it is a chinese relay I got from Pep Boys, but it apparently wasn't working even right out of the box.

[floorboy]

Put on a stancor and be done if the old gpr is bad.

[99' Powerstroke]

I think I will try a Stancor. I was doing some research on these and I found a couple forums where people were saying their new Stancors were going bad. They were claiming that when Stancor started making these in Mexico the quality got worse. Oh well, I think they are still cheaper than an OEM replacement.