Pro Remapping — Stoke-on-Trent & Staffordshire
Glow Plug Fault: Symptoms, Fault Codes and Replacement Cost UK
Hard to start in the cold? White smoke on first startup? Here’s what’s actually happening and what it costs to fix.
Glow plugs are one of those components that work perfectly for 80,000–100,000 miles — and then start failing quietly enough that most drivers don’t immediately connect the symptoms to the cause. According to the RAC, glow plugs typically last between 80,000 and 100,000 miles, but this varies considerably with driving conditions, fuel quality, and the engine’s combustion temperature cycles.
The frustration is that a glow plug fault mimics several other diesel faults. Hard cold starts, rough idling, and white exhaust smoke can all point to injectors, EGR valves, fuel pressure, or glow plugs — and diagnosing without a proper scan tool is guesswork. Here is what each symptom and fault code actually tells you.
What Do Glow Plugs Do?
Diesel engines rely on compression heat to ignite fuel — there’s no spark plug. At operating temperature that works fine, but when the engine is cold, the cylinder walls and pistons absorb heat that would otherwise drive ignition. Glow plugs are electric heating elements threaded into the combustion chamber that pre-heat the air before cranking, raising the temperature enough for reliable cold ignition.
Modern diesel engines heat glow plugs in 2–10 seconds before cranking begins. The coil warning light on the dashboard is the timer: when it goes out, the plugs are up to temperature and the engine is ready to crank. Some systems continue heating briefly after the engine starts to improve initial combustion stability.
Glow Plug Fault Symptoms
Primary Symptoms
- Hard starting when cold — engine cranks longer than usual
- White or grey-white exhaust smoke on cold startup
- Rough idle, shaking, or misfiring until warm
- Coil warning light illuminated on the dashboard
- Engine takes longer than normal to warm up smoothly
Secondary Symptoms
- Increased fuel consumption (incomplete cold combustion)
- Engine hesitation on cold starts before smoothing out
- No-start in very cold weather (below 0°C)
- Active fault codes on OBD scan
- Possible acceleration hesitation until operating temp
Glow Plug Fault Codes: P0670 and P0671–P0674 Explained
When a glow plug fault is detected by the engine management system, it stores a fault code. Knowing which code you have significantly narrows down the diagnosis:
| Code | Description | What It Means |
|---|---|---|
| P0670 | Glow Plug Control Module Circuit | The control module itself is faulty or has lost communication — affects all cylinders |
| P0671 | Cylinder 1 Glow Plug Circuit Open | Plug or wiring fault specific to cylinder 1 |
| P0672 | Cylinder 2 Glow Plug Circuit Open | Plug or wiring fault specific to cylinder 2 |
| P0673 | Cylinder 3 Glow Plug Circuit Open | Plug or wiring fault specific to cylinder 3 |
| P0674 | Cylinder 4 Glow Plug Circuit Open | Plug or wiring fault specific to cylinder 4 |
A P0670 code points to the glow plug control module — the unit that distributes power to all plugs — rather than the plugs themselves. Replacing individual glow plugs will not fix a P0670. A P0671–P0674 code points to a specific cylinder and usually means one plug has failed, though a wiring fault or corroded connector can produce the same code.
Will a Glow Plug Fault Fail an MOT?
This depends on what the warning light is doing and what the emissions test shows.
A flashing coil warning light indicates an active fault code stored in the ECU. According to Stoneacre, a flashing coil light will cause an MOT failure because it indicates a confirmed fault with the emission control system. A steady coil light that illuminates briefly at cold start and goes out after the engine warms is the normal pre-heat cycle, not a fault — this will not cause an MOT failure on its own.
Separately, white exhaust smoke caused by incomplete cold combustion from faulty glow plugs can trigger an emissions-related failure if excessive smoke is present during the test. The safest approach is to fix the glow plugs before presenting the vehicle for MOT.
Glow Plug Replacement Cost UK (2026)
According to ClickMechanic and Autodoc UK, the average cost to replace all four glow plugs in a diesel car in the UK sits between £90 and £435, with most common vehicles averaging around £170–£212 including parts and labour.
| Vehicle Type | Typical UK Cost (all plugs) | Labour Time |
|---|---|---|
| Common diesels (Vauxhall, Toyota, Ford) | £90–£180 | 1–2 hours |
| Mid-range (VW, Audi, Peugeot, Citroen) | £150–£250 | 1.5–2.5 hours |
| Premium (BMW, Land Rover, Mercedes) | £250–£435 | 2–4 hours |
| Seized plug extraction (any) | Add £80–£200 | Additional half-day |
Can the Glow Plug Control Module Be the Cause?
Yes — and this is the diagnosis step most DIYers miss. If you have a P0670 code and fitting new glow plugs doesn’t clear the fault, the glow plug control module (sometimes called the relay or timer unit) has failed. The module is a separate component, typically mounted near the valve cover or bulkhead, and can fail independently of the plugs themselves.
A glow plug control module replacement costs £120–£350 parts and labour depending on the vehicle. Always read the fault code before buying parts — a P0670 versus P0671 makes the diagnosis entirely different.
How a Proper Glow Plug Diagnosis Works
At Pro Remapping we diagnose glow plug faults using live data from the vehicle’s OBD system alongside a resistance test on each plug. A healthy glow plug shows a resistance of 0.5–2 ohms when cold; an open circuit or very high resistance reading confirms the plug is dead. This takes about 20–30 minutes and removes all guesswork before any parts are ordered.
If you’re experiencing cold start issues and have related fault codes alongside your glow plug symptoms, it’s worth also ruling out diesel injector faults — injector issues can produce nearly identical cold start symptoms and are sometimes wrongly attributed to glow plugs without a proper diagnosis. Similarly, a P0300-series misfire code alongside a glow plug code usually points to the plug as the primary cause, but confirmation via live cylinder balance data is quicker and cheaper than exploratory parts fitting.
Glow Plug Diagnosis & Replacement — Stoke-on-Trent
Hard starting or white smoke? We’ll scan, test, and fix it properly. Call us for a same-day diagnostic or drop-off booking.
Workshop location:
201 King Street, Fenton, Stoke-on-Trent, ST4 3ER
Glow Plug Fault — Frequently Asked Questions
What are the symptoms of a glow plug fault?
The most common symptoms are difficulty starting when cold, white or grey exhaust smoke on cold startup, rough or shaky idle until the engine warms, increased fuel consumption, and a coil warning light on the dashboard. Even a single faulty plug causes noticeable symptoms on a 4-cylinder diesel.
Will a glow plug fault fail an MOT?
A flashing coil warning light indicates an active stored fault code and will cause an MOT failure. A steady coil light that goes out after warmup is the normal pre-heat cycle and won’t cause a failure. Excessive white smoke on cold start, however, can trigger an emissions-related failure — so it’s best to replace faulty plugs before the test.
How much does glow plug replacement cost in the UK?
Replacing all four glow plugs on a typical diesel costs between £90 and £435 in the UK, with an average around £170–£212. Common models like Vauxhall or Ford are at the lower end; BMW, Land Rover, and Mercedes tend to cost more due to part prices and labour time. Seized plugs requiring extraction add £80–£200 to the job.
Can I drive with a faulty glow plug?
In warm weather, a single faulty plug may only cause a rougher than normal cold start. In cold or wet conditions the symptoms are worse — harder starting, persistent misfire, and higher fuel consumption. It’s safe to drive short distances, but leave it too long and the remaining plugs bear more load and fail sooner. Get it diagnosed promptly.
Written by the Pro Remapping Diagnostics Team · Published 15 September 2026 · 201 King Street, Fenton, Stoke-on-Trent, ST4 3ER · Pro Remapping is an independent ECU remapping and diagnostics specialist serving Stoke-on-Trent, Staffordshire, and the wider West Midlands.