What is the cylinder numbers and firing order? P0676 Number 6 glow plug

Got a multimeter? Disconnect the GPCM connector and check your resistance between the negative battery terminal and pin #1(Yellow/White trace wire) coming from the glow plug control module. If the resistance is less than 5 ohms, typical resistance is around 1 ohm, then your wiring and glow plug is good, no need to remove the valve cover and replacing good parts, and indicates your GPCM has likely failed.

If the resistance is above 5 ohms, then disconnect the connector on the outside of the valve cover and ohm the wire from the GPCM to connector and VC gasket to ground to determine where the problem lies.

Buddy

The GPCM is only found on California vehicles and Excursions for glow plug control. It would be found the same place as the glow plug relay on 49-state vehicles, which is by the passenger side valve cover, and has two connectors. Picture

Find the appropriate wire color and ohm that wire from the connector to ground like I described before. I am betting you likely have a failed GPCM. Typical failures are a bad GPCM, UVC Harness, or faulty glow plugs.

HTH
Buddy

All the readings of exactly 0 ohms concern me a bit, in the shop here, I do not like it when another tech says he has 0 ohms resistance. Some DMM's can read .0 or 0. when reading an open circuit, so is that what you're seeing, or is there some resistance but you're rounding to zero? Typical resistance is .4-.8 ohms(less than 1, but greater than 0 ohms.) If you do have some resistance in the circuit and your multimeter is rounding, then yes, you have a faulty GPCM.

As for the pricing, several people here have given good reviews to powerstroke shop, I see they sell it for $124 plus shipping here.

Buddy

Sounds like your meter could be rounding down then, but it will not hurt to double check again if you are in doubt even, check in a higher scale range also, I know some non auto-range DMM's will read 0 if the resistance is higher than what scale is selected.

Do not check the red wire this time, especially if the key is on, that one is 12v hot in start/run. Only the Yellow and brown wires with tracers are for the glow plugs.

Buddy