I have a 2001 Ford Excursion, got the SES light about a month ago, I had a local diesel guy look at it and he replaced the GPCM, light was off for about 2 days them came back on. I've looked it over an don't see any loose wires. The local shop can't tell me what it is exactly, but they are more than willing to research it for $90/hour. While checking the wiring i looked at the GPCM, it looks pretty dirty for being new so I'm starting to wonder if he just reset the OBD? Either way, does anyone have any thoughts as to what would be causing the 4 error codes P0671, P0673, P0675, AND P0677?
Thanks.

 

Welcome to FTE

I would also be suspicion of a dirty looking NEW GPCM. The codes point to all four glow plugs in cylinders 1, 3, 5, 7 as bad, which I doubt. Maybe he found one of the connectors loose and just re-seated it and it has come loose again. Here is a diagram of the GPCM and wiring/connectors. May even be a bad GPCM if it was not really replaced or replaced with one removed from another vehicle. You should go talk to this guy.