OBD-II trouble code
P0672: Cylinder 2 Glow Plug Circuit
The glow plug controller found a fault on the cylinder 2 channel. The thing that catches people out is when this test runs: it only happens during the brief preheat window on a cold engine, so a warm vehicle in a workshop cannot reproduce it and a cleared code proves nothing until the next cold morning.
Quick facts
- System
- Powertrain
- Category
- Auxiliary Emissions Controls
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $60 – $600
- DIY difficulty
- Intermediate DIY
What does P0672 mean?
A diesel has no spark. It relies on the heat of compression to ignite fuel, and when the engine is cold that heat is not quite enough, so each cylinder gets a glow plug — an electric heating element that warms the combustion chamber for starting and for the first minute or two of running. The glow plug control module operates all of them and watches each channel separately, which is why the codes are numbered per cylinder. P0672 is the cylinder 2 channel.
The single most useful thing to understand about this code is **when the module is allowed to test.** Glow plugs are only energised during preheat, before cranking, and during afterglow, for a short period once the engine is running. That is the entire diagnostic window: a few tens of seconds, once, on a cold engine. Outside it the channel carries no current and there is nothing to measure. So the module cannot check the plug on a warm engine, cannot check it while you are driving, and cannot check it at all on a vehicle that has been sitting in a warm workshop for two hours.
That has two immediate practical consequences. First, this is not an intermittent fault just because it will not reappear on demand — it is a test that has not had the chance to run. Second, and more importantly, **clearing the code proves nothing until the vehicle has had a genuine cold soak.** A repair verified by clearing the code and driving around the block has not been verified. Leave the vehicle overnight, start it cold, and then read the codes. Plenty of glow plug repairs have been handed back as complete and returned the following morning for exactly this reason.
The second thing worth knowing is how the module decides there is a fault. Most systems measure current through each channel rather than simply asking whether the circuit is connected. A glow plug is a low-resistance element that draws a substantial current when cold, and as plugs age their resistance climbs. Long before a plug is fully open it can be drawing noticeably less current than its neighbours and still warm up a little — which is exactly why a bench test can mislead. A plug that glows when you power it can still be the one the module is complaining about.
The better test is comparison, not pass or fail. Measure the resistance of all the plugs on the engine and look at cylinder 2 against the others. A healthy set reads closely matched, usually well under two ohms and often under one. The odd one out is the answer, whether or not it would technically still light up.
One more note on the dashboard. The glow plug indicator lamp is not a fault light on most vehicles — it is a wait-to-start lamp, and it will behave normally with one plug dead. Do not take a normal-looking lamp as evidence the system is healthy.
Common causes
- Cylinder 2 glow plug with an open or high-resistance element from normal ageing
- Corroded or loose terminal at the cylinder 2 plug
- Broken, chafed or heat-damaged wire in the branch feeding cylinder 2
- Bus bar or connector strip terminal that has lost its grip on the plug stud
- Failed cylinder 2 output channel in the glow plug control module
- Poor engine-to-body ground affecting the return path for the glow circuit
- Water or road salt intrusion at an exposed plug terminal
- Damage to the plug terminal from over-torquing during previous work
Symptoms
- Extended cranking on a cold start
- Brief rough idle or a single-cylinder misfire for the first minute after a cold start
- White or grey smoke on cold start that clears as the engine warms
- Check engine light with P0672 stored
- Symptoms that disappear entirely in warm weather and return in winter
- The code refusing to reappear at a workshop because the engine is already warm
- Normal glow plug indicator behaviour despite the fault, since that lamp is a wait-to-start indicator
Diagnostic steps
- 1.Record all stored codes and note whether any other cylinder glow plug codes or the module-level code are present. One channel points at that branch; several point at something shared.
- 2.Plan the diagnosis around a cold engine. The glow plug test only runs during preheat and afterglow, so a warm vehicle cannot demonstrate the fault.
- 3.Measure the resistance of every glow plug on the engine and compare them against each other rather than against a pass or fail threshold. A healthy set is closely matched and the outlier is your answer.
- 4.Inspect the cylinder 2 plug terminal, its nut and the connector or bus bar strip for corrosion, heat damage and a loose fit on the stud.
- 5.Check continuity from the module output to the cylinder 2 plug terminal, and inspect the wire where it passes near the exhaust or a bracket.
- 6.Where the tooling allows, clamp a current probe on the cylinder 2 feed during a cold preheat cycle and compare the current draw with the other cylinders.
- 7.Verify the engine-to-body ground straps, since the glow circuit's return path depends on them and a poor ground affects every channel.
- 8.If the plug, terminal and wiring all test good, evaluate the module's cylinder 2 output channel.
- 9.After the repair, leave the vehicle to cold soak overnight and check for the code on the next cold start. Anything less has not verified the fix.
Repair cost
$60 – $600
The glow plug itself is inexpensive, typically $20 to $70 for a quality unit, so the bill is almost entirely labour and access. A straightforward replacement where the plug is reachable from above runs $120 to $300. Where intake ducting, a turbo pipe or the manifold has to come off first, expect $300 to $600. Repairing a corroded terminal or a damaged branch wire is $90 to $250. Diagnosis is $80 to $180. If the module's cylinder 2 output channel has failed, a glow plug control module is typically $250 to $700 fitted. Note that the low end of this range assumes the plug releases cleanly — see the cylinder 3 page for what happens when one does not.
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.