OBD-II trouble code
P0674: Cylinder 4 Glow Plug Circuit
A fault on the cylinder 4 glow plug channel. On a four-cylinder diesel this is the last plug in the set, which makes it the code where the most useful question gets answered: how many cylinders are reporting, because one is a plug and all four are almost never four plugs.
Quick facts
- System
- Powertrain
- Category
- Auxiliary Emissions Controls
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $60 – $900
- DIY difficulty
- Intermediate DIY
What does P0674 mean?
P0674 is the cylinder 4 channel of the glow plug system: the module energised that plug during preheat, measured what came back, and did not like the result. Taken on its own the diagnosis is the same one you would run on any glow plug branch — element, terminal, wire, module output.
What makes this code worth a separate page is that on the most common diesel layout in the world, the four-cylinder, cylinder 4 is the last plug in the set. That means P0674 is usually the code that completes the picture, and **the number of glow plug codes stored is more diagnostically valuable than any single one of them.**
Read the pattern this way. One cylinder code on its own is a branch fault: that plug, its terminal, its wire, or that one module output channel. Two adjacent cylinders is worth checking for a shared harness section or a connector strip that feeds them both, because plugs rarely fail in pairs on the same day. All four cylinders reporting at once is not four dead glow plugs — it is the thing they all depend on. That means the glow plug control module, the relay, the high-current fuse or fusible link that feeds it, the supply from the battery, or the common ground. Diagnosing four plugs individually in that situation wastes hours and, on an engine where the plugs are hard to reach, can waste a great deal of money too. Check the shared item first and the whole set of codes usually clears together.
The second thing this code demands is a lookup rather than an assumption, and it is the one that costs money when skipped. **Cylinder 4 is a number in the manufacturer's firing-order scheme, not a position you can point at.** On an inline four it is straightforward enough. On a V-configuration diesel — and there are plenty, in light trucks, large SUVs and European saloons — the cylinders are split between two banks, and which bank holds cylinder 4 depends entirely on how that manufacturer numbers them. Some number one bank straight through before starting the other; some alternate side to side. Get it wrong and you remove intake pipework, a heat shield or a turbo pipe to reach a plug that was never the problem, which on this repair is most of the labour.
Finally, calibrate the urgency by how many plugs are affected. A single failed plug on a four-cylinder gives a longer crank, a rough first minute and some white smoke, and in mild weather may be almost unnoticeable. Two or more, and the engine genuinely struggles to start when cold, because there is no longer enough combustion heat to bring the remaining cylinders up. If P0674 is stored alongside other cylinder codes, treat that as a vehicle heading for a winter no-start rather than an inconvenience — and go looking for the shared cause before ordering parts.
Common causes
- Cylinder 4 glow plug element open or high in resistance from age
- Failed glow plug control module, relay or high-current fuse where every cylinder is reporting
- Corroded or loose terminal at the cylinder 4 plug
- Broken or chafed branch wire to cylinder 4
- Connector strip or bus bar terminal that has lost tension on the plug stud
- Poor battery feed or engine-to-body ground serving the glow circuit
- Failed cylinder 4 output channel in the control module
- Corrosion at a plug terminal exposed to road spray or a leaking seal
- Wrong-specification plugs fitted previously, drawing outside the expected current range
Symptoms
- Extended cranking on a cold start, becoming severe if other cylinders are affected too
- Rough idle and misfire for the first minute from cold, smoothing out as the engine warms
- White smoke on cold start that clears with temperature
- Check engine light with P0674 stored, often alongside other cylinder glow plug codes
- A cold-start problem that is barely noticeable in summer and pronounced in winter
- No-start in cold weather when several plugs have failed together
- Normal running once at operating temperature
Diagnostic steps
- 1.Read and record every stored code before doing anything else, and count how many glow plug channels are reporting. That count decides the entire diagnostic path.
- 2.If all cylinders are reporting, stop testing individual plugs and go to what they share: the control module, its relay, the high-current fuse or fusible link, the supply and the common ground.
- 3.If two adjacent cylinders are reporting, inspect the harness section and connector strip that feed both before condemning two plugs.
- 4.Confirm which physical cylinder is number 4 on this engine before removing anything. On a V-configuration diesel it may sit on the opposite bank from cylinders 1 to 3, depending on the manufacturer's numbering.
- 5.Measure the resistance of every glow plug on the engine and compare them with each other. Matched readings with one outlier identify the faulty element.
- 6.Inspect the cylinder 4 terminal, nut and connector for corrosion, heat damage and a loose fit on the stud.
- 7.Check continuity from the module output to the cylinder 4 terminal and inspect the wire where it passes near hot or moving components.
- 8.Verify battery feed and engine-to-body grounds under load, since the glow circuit draws heavily and a marginal supply affects every channel.
- 9.Bring the engine to operating temperature before removing any plug, and work it loose in increments rather than forcing a seized plug.
- 10.Verify the repair on a genuine cold start after an overnight soak, since the glow circuit is only tested during preheat.
Repair cost
$60 – $900
Diagnosis is $80 to $180, and on this code it earns its keep because the code count determines whether you are buying one plug or one module. A single plug replaced with direct access is $120 to $350; where intake pipework, a heat shield or a turbo pipe must come off first, $300 to $650, and more on a V engine where cylinder 4 sits on the less accessible bank. Terminal or branch wire repair is $90 to $250. A glow plug relay or fuse is $30 to $150 and is the cheap answer when every cylinder is reporting. A glow plug control module is $250 to $700 fitted. A full set of four plugs, where the labour is being spent anyway, typically adds $60 to $200 in parts over doing one. Seized plugs can push any of these far higher — see the cylinder 3 page for what that involves.
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.