There was a check engine light on in my 2006 Ford F350 6.0 diesel truck. P0671, cylinder 1 glow plug circuit. In the end it was a zero dollar fix. We tested the glow plug and wiring harness. All things pointed to the glow plug module but it seems the fix was just reconnecting the connector as the light cleared itself. I am assuming it was just a poor connection that has been resolved.