Ford's 6.0L Power Stroke diesel engine (2004.5–2007 F-250, F-350, F-450, F-550 Super Duty) is a powerful engine, but it has a well-known weakness: the glow plug wiring harness. Ford's original wiring harness design uses connectors that crack and corrode over time, cutting off power to the glow plugs and causing a number of starting problems.
1. Difficulty Starting in Cold Weather
If your truck takes longer than normal to crank (especially below 40°F), the first thing to check before assuming the glow plugs themselves are faulty is usually a damaged wiring harness.
2. White Smoke on Starting
Unburned fuel from cylinders that don't receive enough glow plug heat appears as white smoke until the engine warms up.
3. Irregular Idling or Ignition Problems, Especially in Cold Weather
A cracked wiring harness can deliver inconsistent voltage to individual cylinders, causing an erratic idle that corrects itself when the engine warms up.
4. Engine Malfunction Light Illuminating with Glow Plug Circuit Codes
Codes like P0670-P0677 directly indicate glow plug circuit failures; in high-mileage 6.0L trucks, this is almost always related to the wiring harness.
5. Visible Cracks or Corrosion in the Wiring Harness
Open the hood and look at the wiring harness insulation near the valve covers; brittle, cracked, or green-corroded connectors confirm that it's time to replace them.
Solution:
Replacing the wiring harness and glow plugs together (not one or the other) will prevent a repeated repair in the future. An original Motorcraft OEM kit includes both driver and passenger side wiring harnesses and all 8 glow plugs, and the job is generally a 1-2 hour DIY project with basic hand tools.