Glow plugs preheat each cylinder for cold starts. When one or more fail, you get hard cold starts, white smoke after start, rough idle until warm, and codes P0670-P0678. Cheap to fix on most engines - except where access requires manifold removal.
Hard cold starts with bad glow plugs hammer the starter and battery. Replace at the first sign of failure. Most kits are under $300.
The most obvious symptom - cold starts that crank for 10+ seconds. Modern diesels often won't start at all below 40°F with multiple failed plugs.
View P0670 Diagnosis →The glow plug control module (GPCM) sets this code when one or more plugs draw incorrect current. Often paired with P0671-P0678 indicating which cylinder.
View P0670 Diagnosis →A dead cylinder during start dumps unburned fuel into the exhaust. White smoke that clears once warm = glow plug failure on that cylinder.
Get a free diagnosis →Misfire on the dead cylinder until combustion temps stabilize. Smooths out within 1-3 minutes of run time.
Get a free diagnosis →The relay or module that supplies the plugs can fail. All plugs stay cold. Common on 7.3L Powerstroke and older Cummins.
Get a free diagnosis →A worn glow plug tip can break off inside the cylinder, requiring cylinder head removal. Rare but expensive.
Get a free diagnosis →Run a free AI diagnosis tailored to your exact diesel. Get the most likely cause in under 30 seconds.
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Ford TSB 06-7-1 covers glow plug replacement on 6.0L Powerstroke. Ford TSB 14-0157 covers glow plug module updates on 6.7L Powerstroke. GM TSB 11-06-04-002 covers Duramax glow plug controller diagnosis. Cummins service letters address glow plug grid heater on 6.7L 2007.5+ trucks - the critical 2019-2022 grid heater bolt recall (NHTSA 22V-836) is separate from glow plug issues.
If you see a check engine light, these codes most often relate to the issues above. Click any code for full diagnosis steps.
80,000-150,000 miles. Replace as a complete set since they age together.
Above 50°F yes - the engine will fire without help. Below freezing you may not start at all.
$150-$700 for a full set installed. 6.0L Powerstroke is more because access requires lifting the cab on some trucks.
Yes. If one is bad the others are aging. Cost difference is minor since most labor is access.
Glow plugs are individual per cylinder. Grid heaters (Cummins) heat the intake charge for the whole engine. Different parts, different failure modes.
Yes - measure resistance with a multimeter. A good plug reads 0.5-1.5 ohms. Open circuit = dead. Different specs by engine.