Getting codes for glow plugs

[05f-350superduty]

My truck is showing codes for the # 2, #4,#5 and #6 glow plugs. They were all replaced new about a year ago. I've replaced the relay but its still throwing the codes. Would this mean that bolth my harnesses are bad to ?

[sinner6.0L]

the easiest thing to do is a resistance check where the glow plug harness goes into the engine harness. There's a 4 pin connector on each bank with each pin relating to one glow plug. Resistance to ground should be approx 2-3 ohms. Any higher and you can suspect either the glowplug or harness has internal high resistance which will generate a DTC.

In my opinion its not worth it to change the harness or the plugs individually. Just get 4 plugs and a harness and be done with it. It would be wise to use motorcraft glow plugs too.