Glow plug light always comes on

[matysik]

Truck info: 2004.5 lly with 170k miles on it.

I noticed recently with the warm weather that no matter what my glow plug light still comes on and stays on for its cycle no matter what ambient temperature it is outside. I tested all 8 plugs with a simple circuit tester and all checked out good from the battery to the plug terminal.

Is this a good enough test on plugs or does the glow plug indicator always being on mean there is a glow plug code?

 

[duratothemax]

There will be a code stored in the glow plug control module.

You cant just test them with a circuit tester. You need to first disconnect each glow plug, and check the resistance of each glow plug between the terminal and ground.

The glow plug module is "smart"...its not just a relay like most people think. It runs diagnostics on each glow plug circuit individually and if there is a fault within one or more glow plugs, it will isolate/disable that one (or more) circuits, and run the others normally. As an indicator that there is a problem with the glow plug system, it will keep the glow plug light on for an extended period of time.

Basically...you need to check the ECM for codes. It will either show a code for one or more bad glow plugs, or a P0670. P0670 generally (but not always) means the glow plug control module itself has failed internally and you need to replace it. Its not unheard of on LLY's for the GPCM to fail. Its not too expensive, $99 new from partszoneonline.com or rockauto.com

But anyway...check if there are any codes and go from there.

Ben