by Glow plugs are a simple idea: heating elements that warm the combustion chamber so a cold diesel will start. They are also one of the few jobs where the diagnosis is straightforward and the removal is the genuinely difficult part, because a glow plug that has been in a cylinder head for years can seize solid.
What they do and when
Diesel ignites by compression rather than by spark. On a cold engine there is not enough heat to ignite the fuel reliably, so glow plugs preheat the combustion chamber before and during cranking.
Modern systems also run the plugs for a period after starting, called post-heating, which reduces white smoke, improves running quality while cold and helps emissions. That means a failed glow plug can cause rough running and smoke for the first few minutes even on a van that starts acceptably.
Safety notes
Allow the engine to cool before working near the exhaust, turbo or manifold, as these retain heat long after the engine is switched off. Never work under a van supported only by a jack; use rated axle stands on firm level ground with the wheels chocked.
Keep hands, tools and loose clothing clear of belts, pulleys and cooling fans whenever the engine may run. A cooling fan can start without warning.
Symptoms
- Hard starting when cold, improving or disappearing when warm. The classic pattern.
- Extended cranking before the engine catches on a cold morning.
- Rough running and misfiring for the first minute or two after a cold start.
- White or grey smoke on cold start, from unburned fuel in cylinders that are not firing properly.
- Glow plug warning light flashing or staying on, which usually indicates a fault rather than normal preheating.
- Engine management light with glow plug circuit codes.
- Worse in cold weather, fine in summer. A seasonal complaint is a strong indicator.
One important nuance: a modern common rail diesel in good condition may start acceptably in mild weather with several failed glow plugs, so plugs can be dead long before the owner notices. The first real symptom often arrives with the first hard frost.
It is not always the plugs
Worth establishing before ordering parts.
- The glow plug relay or control module. Fails commonly and takes out all the plugs at once. If every plug appears dead, suspect the relay before believing all four failed simultaneously.
- Wiring and connectors, which sit in a hot part of the engine bay and corrode.
- Low battery voltage, which affects both preheating and cranking. See battery and charging fault testing.
- Other cold start causes: low compression, injector faults, fuel supply problems or waxed fuel in winter. See diesel cold start problems.
Diagnostic sequence
1. Read the codes
Check: Glow plug circuit codes, which frequently identify the specific cylinder or point at the control module.
Next action: A code naming one cylinder narrows things immediately. A code for the module or a general circuit fault points at the relay.
2. Test each plug for resistance
Check: Disconnect the supply and measure resistance between the plug terminal and earth with a multimeter.
Result and next action: A healthy glow plug reads a low resistance, typically a small number of ohms. An open circuit reading means the element has failed. Compare all plugs against each other, since the comparison is more informative than any absolute figure, and manufacturer specifications vary between plug types.
3. Test current draw
Check: With a clamp meter around the supply lead, measure current to each plug during preheating.
Result and next action: This is more revealing than resistance alone, because a plug can read acceptable resistance but draw insufficient current. A plug drawing noticeably less than its neighbours is failing even if it does not read open circuit.
4. Check for supply voltage
Check: Voltage at each plug terminal during the preheat period.
Result and next action: No voltage at any plug points at the relay, its control circuit or the module. Voltage present at some but not others points at wiring.
5. Check the relay itself
Check: Listen for it operating on ignition, and check its output.
Next action: The relay is frequently the actual fault and is usually cheaper and far easier to replace than the plugs.
The removal problem
This is where the job goes wrong, and it deserves to be understood before you start.
A glow plug sits in the cylinder head, exposed to combustion, for years. Carbon builds up around the shaft and the threads corrode. When you apply torque, there is a real risk the plug shears, leaving the body in the head.
Extracting a sheared glow plug is a specialist job. It may require drilling out the remains with the engine in situ, using specific extraction tooling, and in bad cases removing the cylinder head. A job that started as four inexpensive parts can become a very large bill.
Practical measures that reduce the risk:
- Work on a warm engine, not a hot or cold one. Gentle warmth helps break the bond; a hot head risks damaging threads.
- Use a proper penetrating fluid and give it genuine time, ideally applied over several heat cycles rather than five minutes.
- Do not simply lean on the bar. Work the plug back and forth in small increments rather than applying continuous increasing force.
- Stop if it feels wrong. A plug that will not move with reasonable effort is telling you something.
- Clean the bore with a suitable reamer before fitting the new plug, so carbon does not seize the new one immediately.
- Use anti-seize sparingly and correctly, and torque to specification. Overtightening is another route to a sheared plug next time.
Given all this, many owners reasonably conclude glow plugs are a job for a garage, particularly on an engine with poor access or a known reputation for seizure.
Should you replace them as a set?
Generally yes. The labour is largely shared, they have all done the same work, and having the head open again to do the next one is worse than doing them together. Where one has failed on a high-mileage engine, the others are usually not far behind.
Indicative UK costs
Broad context only. Glow plugs themselves are typically inexpensive, often ten to thirty pounds each. A relay or control module is usually a modest part. Labour is the variable: straightforward access might be an hour or two, while poor access or a seized plug can multiply that considerably. Extracting a sheared plug is a specialist charge and can be substantial. Get quotes, and ask how the garage handles seizure.
Common mistakes
- Assuming all four failed at once. Check the relay first.
- Testing resistance only. Current draw catches plugs that resistance testing passes.
- Forcing a seized plug. The single most expensive mistake available here.
- Working on a stone cold or fully hot engine. Warm is the compromise.
- Not cleaning the bore. Carbon seizes the new plug from day one.
- Ignoring battery condition. Preheating draws significant current and a weak battery undermines it.
When to stop and get help
Stop and get help the moment a plug feels like it might shear, if one has already sheared, or if access requires removing other components you are not equipped to disturb. Also seek help if replacing the plugs does not resolve hard cold starting, since the cause may be compression, injectors or fuel supply.
Prevention
Replace glow plugs at reasonable intervals rather than waiting for failure, because a plug that has been in place for a decade is far more likely to seize. Keep the battery healthy. Address rough cold running promptly, since a failed plug allows unburned fuel into the cylinder, which is not good for bores or the DPF.
Frequently asked questions
Can I drive with a failed glow plug?
The van will usually still run, but cold starting suffers, cold running is rough and unburned fuel loads the DPF. It is worth fixing rather than living with.
Why does it start fine in summer but not winter?
Because preheating matters far more when cold. Plugs can be dead for months before the first frost reveals it.
Should I replace all of them?
Usually yes. The labour is shared and the remaining plugs have had the same life.
The glow plug light is flashing. What does that mean?
Generally a fault rather than normal preheating, often in the glow plug circuit but sometimes elsewhere in engine management. Have the codes read.
This guide is general information for UK van owners, not vehicle-specific repair instructions. Procedures, tolerances and component layouts vary by engine code and model year. Confirm any figures against your VIN and manufacturer workshop information, and see our Disclaimer before carrying out any work.
