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P0673: Cylinder 3 Glow Plug Circuit

Moderate

Quick answer

P0673 means the glow plug circuit for cylinder 3 (and only cylinder 3) has an electrical fault, usually a burned-out glow plug on that cylinder. Your diesel will likely still start in mild weather but protest on cold mornings. Test the plug’s resistance before buying anything, and remove it with patience: a snapped plug is the expensive version of this story.

What it means

Diesels ignite fuel with compression heat, and when the engine is cold each cylinder leans on its own glow plug: an electric heating rod, to get the first combustion events going. Modern glow control modules monitor the current through every plug individually, which is how the computer can be this specific: P0673 says the circuit for cylinder 3’s plug reads open, shorted, or out of spec, while the other cylinders’ plugs check out.

That per-cylinder precision is a gift. Unlike the whole-circuit codes (P0380/P0382) or the module code (P0670), this one hands you a single suspect: cylinder 3’s plug, its terminal connection, or the short run of wiring between it and the module. The overwhelming majority of the time it’s the plug itself: they’re heating elements living inside a combustion chamber, and like any heating element, they eventually burn out. Where cylinder 3 sits varies by engine; your service manual’s cylinder numbering diagram settles it before you wrench on the wrong hole.

One plug down is the mildest glow fault there is: five (or seven) healthy neighbors still preheat their chambers, so the engine usually starts. But the dead cylinder fires late and rich while cold, which is the rough first minute and the white smoke. And the same monitoring that caught this plug will keep catching its siblings: on a high-mileage engine where one original plug has died, the rest are the same age. The six-cylinder light-truck diesels we cover in our vehicle hubs: the EcoDiesel Ram and Grand Cherokee, the 3.0 Power Stroke F-150, the 3.0 Duramax Silverado, all monitor plugs this way.

P0673 symptoms: what you'll notice

  • A rough, uneven idle for the first minute on cold starts: cylinder 3 joining the party late
  • White smoke at cold startup, clearing as the engine warms
  • Slightly longer cranking in cold weather; in deep cold, noticeably harder starts
  • Check engine light, in warm weather, often the only sign
  • No symptoms at all once the engine is warm: glow plugs retire the moment the engine can heat itself

Common causes

Ordered from most to least likely.

  1. 1.

    Burned-out glow plug on cylinder 3

    The cause behind most of these codes. Glow plugs are consumables, heating elements with a service life, often 80,000–100,000+ miles.

  2. 2.

    Corroded or loose terminal at the plug

    The small nut-and-terminal (or spade) connection on top of the plug lives in engine heat and weather. A bad connection reads like a bad plug, and costs nothing.

  3. 3.

    Damaged wiring between module and plug

    A short harness run, but it crosses a hot, vibrating engine. Chafe and heat damage set the same code.

  4. 4.

    Glow module output failure on this channel

    Rare but real: one driver inside the module dies. Suspect it when a known-good new plug doesn’t clear the code.

How to fix it: diagnosis, step by step

Cheapest and most likely checks first.

  1. 1 Find cylinder 3 and inspect its connection

    Confirm cylinder 3’s location from your engine’s numbering diagram: front-to-rear and bank conventions differ between engines, and guessing wastes the whole afternoon. Inspect the plug’s terminal and short harness run: a corroded terminal or backed-off nut is the free fix hiding under this code.

  2. 2 Resistance-test the plug

    Disconnect the plug’s feed and measure from its terminal to a clean engine ground. Healthy glow plugs read very low, typically a couple of ohms or less (verify your spec). An open reading condemns the plug; matching its neighbors’ readings acquits it and sends you to the wiring and module.

  3. 3 Verify the module’s output reaches the plug

    During a cold-start glow cycle, check for voltage at the plug’s terminal. Voltage present at a plug that tests open confirms the plug. No voltage at a plug that tests fine moves the fault to the wiring or the module’s output channel, and just saved you a plug job that wouldn’t have fixed anything.

  4. 4 Replace the plug with patience, not muscle

    This is the step that separates a $30 repair from a machine-shop visit. Work on a warm (not hot) engine, soak the plug threads with penetrating oil, and back it out slowly with hand tools, if it binds, stop, re-soak, work it back and forth. Seized plugs that snap off in the head can require extraction tools or worse. Thread the new plug by hand and torque it to spec; they’re easy to overtighten.

  5. 5 Consider the other plugs while you’re there

    If this engine is past 80,000–100,000 miles on original plugs, the rest are the same age as the one that just died. Replacing the set while the job is open is a judgment call your wallet and your winter get to negotiate, but it’s the reason mechanics rarely replace just one.

Parts & tools you may need

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Frequently asked questions

What does code P0673 mean?
P0673 means the glow plug circuit for cylinder 3 (and only cylinder 3) has an electrical fault, usually a burned-out glow plug on that cylinder. It’s moderately serious: you can usually keep driving gently, but diagnose it soon.
Can I drive with P0673?
Yes, with one glow plug down, the other cylinders still preheat and the engine starts in all but the harshest cold, running normally once warm. The costs of waiting are a smoky, rough cold start, slightly more strain on the remaining plugs, and a worsening bet against winter mornings. Schedule it; don’t shelve it.
Should I replace one glow plug or all of them?
The honest answer is a question: how old are the others? If the set is young and one plug failed early, replace the one. If they’re original on a high-mileage engine, they share the dead plug’s birthday, and since the labor of getting to them is the real cost, doing the set at once is usually the cheaper path over the next two winters.
What happens if the glow plug snaps off during removal?
That’s the disaster scenario this page keeps warning about: the broken tip stays in the cylinder head, and getting it out ranges from a specialized extraction kit to pulling the head at a machine shop, hundreds to thousands of dollars. Prevention is everything: warm engine, penetrating oil, hand tools, zero forcing. If a plug fights back hard, stopping and letting a diesel shop with extraction experience take over is the cheap decision.
Why is it always cylinder 3? The code came back after I cleared it.
Because clearing a code doesn’t heal a heating element. The module re-tests every plug each glow cycle, so an open plug re-sets its code on the next cold start, every time. If the code returns after a new plug, the suspect changes: recheck the terminal connection, then the short harness run, then the module’s output for that cylinder.
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