OBD-II trouble code
P0673: Cylinder 3 Glow Plug Circuit
A fault on the cylinder 3 glow plug channel. The part worth reading before anyone picks up a socket is the removal risk: glow plugs seize into cylinder heads, and a plug that snaps off during extraction turns a modest job into one of the most expensive small repairs on a diesel.
Quick facts
- System
- Powertrain
- Category
- Auxiliary Emissions Controls
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $60 – $2,500
- DIY difficulty
- Intermediate DIY
What does P0673 mean?
P0673 tells you the glow plug control module measured something wrong on the cylinder 3 channel — most often an open or high-resistance element, sometimes the wiring or terminal feeding it. The electrical diagnosis is not the difficult part of this repair. The removal is.
A glow plug is a slender steel body threaded into the cylinder head with its tip exposed to combustion. Over tens of thousands of heat cycles, carbon builds around the shank and the threads corrode into the aluminium or iron around them, and the plug effectively becomes part of the head. When enough torque is applied to a plug that has bonded in place, the thin body twists apart, leaving the tip and threaded section inside the head with nothing left to grip. That is the failure that changes the economics of this job completely: a repair quoted in the low hundreds becomes specialist extraction work, and if extraction fails, cylinder head removal.
So the order of operations matters more here than the theory. **Prove the fault is at the plug before disturbing the plug.** A corroded terminal, a broken branch wire or a failed module channel produces exactly the same code, and every one of those can be repaired without ever unthreading anything. Measure the resistance of the plug at its own terminal, check continuity back to the module, and inspect the terminal and connector for corrosion and heat damage. Only when the element itself is proven open or badly drifted has the case been made for extraction. Gambling a seized-plug risk on an unconfirmed diagnosis is how a wiring fault ends up costing several thousand dollars.
When removal is justified, technique is what separates the two outcomes. Plugs come out far more willingly at operating temperature, when the head has expanded away from them, so warming the engine first is worth the wait. Penetrating oil applied and left to work, then several cycles of gentle loosening and retightening rather than one continuous pull, breaks the carbon bond progressively instead of loading the weakest section of the plug body until it fails. If a plug resists after reasonable effort, stopping and reassessing is a better decision than more leverage — the extra torque is not going to free it and is very likely to snap it.
There is a related question that this is the right moment to settle: whether to replace the whole set. The plugs on an engine are the same age and have taken the same number of heat cycles, so when one has drifted the others are usually not far behind. On engines where access is easy the set is a reasonable option and not much extra. On engines where the intake manifold, turbo pipework or an injector rail must come off first, labour dominates the bill and doing the set costs little more than doing one — but the flip side is real and should be said plainly: **every additional plug you unthread is another chance to snap one.** On a high-mileage engine with a known reputation for seized plugs, some experienced technicians deliberately replace only the failed plug for that reason. Neither answer is wrong; the decision should be made with the risk understood rather than by habit.
Common causes
- Cylinder 3 glow plug element open or badly drifted in resistance from age
- Corroded terminal or loose retaining nut at the cylinder 3 plug
- Branch wire to cylinder 3 broken or heat-damaged where it runs near the exhaust
- Bus bar or harness strip terminal no longer gripping the plug stud
- Failed cylinder 3 output channel in the glow plug control module
- Water and salt intrusion at an exposed terminal on a plug near the front of the engine
- Terminal damage from over-torquing during a previous glow plug replacement
- A previously broken plug repaired badly, leaving a poor thread or a leaking seat
Symptoms
- Hard starting or extended cranking on cold mornings
- Rough running and a single-cylinder misfire during the first minute from cold
- White smoke on cold start clearing as the engine warms up
- Check engine light with P0673 stored
- Noticeably worse cold-start behaviour in winter than in summer
- A diesel knock that sounds harsher than usual during warm-up
- Normal running once the engine reaches operating temperature
Diagnostic steps
- 1.Record all codes first. Cylinder 3 alone points to that branch; several cylinders or the module-level code point at something shared and mean no plug needs removing yet.
- 2.Prove the fault is at the plug before disturbing it. This is the whole point of the diagnosis on this code, because removal carries a risk that inspection does not.
- 3.Measure resistance at the cylinder 3 plug terminal itself and compare it against the other plugs on the engine rather than a pass or fail figure.
- 4.Inspect the terminal, nut and connector strip for corrosion, heat damage and a loose fit before assuming the element has failed.
- 5.Check continuity from the module output to the cylinder 3 terminal, and inspect the wire where it passes near hot components.
- 6.If the wiring and terminal are sound and the element is proven open or badly drifted, plan the removal deliberately rather than opportunistically.
- 7.Bring the engine to operating temperature before attempting removal, so the head has expanded away from the plug.
- 8.Apply penetrating oil, allow it to work, and loosen and retighten in small increments to break the carbon bond progressively instead of loading the plug body to failure.
- 9.Stop and reassess if the plug resists after reasonable effort. More leverage does not free a bonded plug, it snaps one.
- 10.Clean the threads and seat, torque the new plug to specification, then verify the repair on a genuine cold start rather than by clearing the code.
Repair cost
$60 – $2,500
The wide top end of this range is not padding — it is the seized plug scenario, and it is common enough to plan for. Diagnosis is $80 to $180. A terminal or branch wire repair, which avoids removal entirely, is $90 to $250. A straightforward plug replacement is $120 to $350 where access is direct, and $300 to $650 where the intake, turbo pipework or an injector rail must come off first. A glow plug control module is $250 to $700 fitted. If a plug snaps during removal, specialist extraction using dedicated tooling typically runs $400 to $1,200 per plug, and if extraction cannot be completed in the vehicle, cylinder head removal takes the job to $1,500 to $2,500 or beyond. Ask whether a quote assumes clean removal before treating it as final.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with glow plug / glow plug module replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.