Diesel Glow Plug Failure: Symptoms, Causes & Replacement Cost UK

What Are Glow Plugs and Why Do They Matter?

Unlike petrol engines, diesel engines do not use spark plugs to ignite the fuel. Instead, they rely on the heat generated by compressing air inside the cylinder to ignite the diesel fuel when it is injected. This process is called compression ignition.

The problem: on a cold morning, the cylinder walls, pistons, and air inside the engine are all cold. Compressing cold air does not generate as much heat as compressing warm air. The result is that diesel engines struggle to ignite fuel reliably in cold conditions.

Glow plugs solve this. They are electrically heated rods installed in each cylinder (one per cylinder) that pre-heat the air inside the combustion chamber before the engine starts. When you turn the ignition on and wait a moment before cranking (the brief glow plug warm-up period), glow plugs are doing their job.

On modern common rail diesel engines, glow plugs also assist combustion during the first few minutes of a cold drive, reducing white smoke and improving fuel burn until the engine reaches operating temperature.

Symptoms of Glow Plug Failure

1. Difficult Cold Starts

This is the most recognisable symptom. On cold mornings — particularly when temperatures are below 5°C — the engine cranks for longer than usual before firing. It may take two or three attempts to start.

One failed glow plug often produces only marginal cold start difficulty. Two or more failed plugs will make cold starting noticeably harder. In very cold conditions, the engine may not start at all if several glow plugs have failed.

2. White Smoke on Cold Start

A small amount of white smoke from a diesel exhaust on a cold morning is normal — it is mostly water vapour and condensation burning off. But sustained white smoke for several minutes after startup, or white smoke that has a slightly acrid or fuel-like smell, often indicates incomplete combustion from a cold engine — which can be caused by failing glow plugs not heating the cylinders adequately.

White smoke that clears completely once the engine is warm typically points to glow plugs or a cold-start injector issue rather than something more serious like a head gasket.

3. Rough Running When Cold

If one or more cylinders are not pre-heating properly, combustion in those cylinders is less efficient when the engine is cold. This can cause a rough idle, occasional misfires, or a noticeable shudder when the engine first starts. The roughness typically clears within a few minutes as the engine warms up.

This pattern — rough when cold, smooth when warm — is a classic glow plug symptom pattern.

4. Engine Management Light (Glow Plug Warning Light)

Most diesel cars have a dedicated glow plug warning light — usually a coil or spiral symbol, sometimes amber. This illuminates during the pre-heat cycle (normal) and should go out once the plugs are ready for starting.

If this light stays on after starting, flashes during the drive, or comes on alongside the general engine management light, a glow plug circuit fault has been detected. This could mean a failed plug, a failed glow plug relay, or a wiring fault.

The only way to confirm which plug has failed and why is a proper diagnostic scan with live data.

5. Increased Fuel Consumption

Glow plugs support efficient combustion during cold operation. When they fail, fuel is not burned as completely during the warm-up period. Over time, particularly in stop-start city driving with lots of cold starts, this contributes to slightly higher fuel consumption.

This symptom is subtle on its own. But combined with other signs, it adds to the picture.

6. Misfires or Fault Codes

Modern diesel engines monitor combustion quality in each cylinder. A failing glow plug can trigger misfire codes or cylinder contribution faults — not because the plug itself causes a misfire directly, but because incomplete cold combustion registers as a cylinder not firing properly.

If you have a scan showing codes P0670–P0679 (or manufacturer equivalents), these point directly to the glow plug circuit.

What Causes Glow Plugs to Fail?

Age and Mileage

Glow plugs are a wear item. They are subject to extreme heating and cooling cycles every time the engine starts from cold. Most manufacturers recommend replacement every 60,000–100,000 miles or every four to five years, whichever comes first. Many drivers do not change them until they cause a problem.

Carbon Build-Up

Carbon deposits from combustion can accumulate around the glow plug tip over time, reducing its ability to heat efficiently. This is more common on engines that do mostly short journeys, where combustion temperatures never fully burn off deposits.

Seized Glow Plugs

This is one of the more problematic failure modes. If glow plugs are left in an engine for a very long time without servicing, corrosion between the plug and the cylinder head can cause them to seize. A seized glow plug that is forced will snap — leaving the tip inside the cylinder head, which requires specialist extraction.

This is why replacing glow plugs preventatively on a mileage schedule is preferable to leaving them until they fail and seize.

Electrical Faults

The glow plug relay controls power supply to all plugs simultaneously. A failed relay can cause all glow plugs to stop working, even if the plugs themselves are fine. Wiring faults — corroded connectors, damaged insulation — can also cause intermittent glow plug circuit problems.

Glow Plug Replacement: What It Costs

All four (or six) plugs should always be replaced at the same time. Replacing a single failed plug leaves the others — which are equally aged — on borrowed time. Replacing the set once gives you a known baseline and avoids returning for the same job a few months later.

Can You Drive with Failed Glow Plugs?

In warm weather, yes — with reduced efficiency and some rough running on cold starts. A diesel engine can still start and run without glow plugs once ambient temperatures are above around 10–15°C, because the compression ratio alone generates enough heat to ignite the fuel.

In cold weather (below 5°C), failed glow plugs can make the car genuinely difficult or impossible to start from cold. Driving with known failed plugs in winter is not advisable.

More importantly, ignoring the glow plug warning light does not just mean tolerating a difficult start. The engine management system may restrict power or fuel economy, and unresolved fault codes can mask other developing problems. It is always better to get confirmed faults addressed rather than tolerate them.

Diesel Vehicles Particularly Prone to Glow Plug Issues

While glow plug failure can affect any diesel, some vehicles see it more commonly than others:

• BMW N47 and N57 diesel engines: Glow plug accessibility can be challenging and seized plugs are a known issue on high-mileage examples
• Ford 1.5 and 2.0 TDCi engines: Generally straightforward, but plugs left for too long can corrode in
• Vauxhall 1.6 CDTi and 2.0 CDTi: Glow plug replacement is routine on these and usually straightforward if done at recommended intervals
• Volkswagen Group TDI engines (2.0 TDI): Glow plugs are accessible and replacement is standard, but the relay (N108) is a known failure point
• Land Rover 2.2 SD4 and 3.0 TDV6: More complex to access, particularly on the V6 — labour time is higher

How We Diagnose and Replace Glow Plugs at Star Garage Mansfield

When a diesel comes in with cold start problems or a glow plug warning light, our first step is a full diagnostic scan to confirm which cylinder’s glow plug circuit has flagged a fault and whether the relay is also involved.

We then test the plugs individually for correct resistance and current draw, which tells us whether the plug itself has failed or whether the fault is in the wiring or relay.

Before removing glow plugs, particularly on higher-mileage vehicles, we apply penetrating treatment and allow adequate soak time to reduce the risk of seizure. If a plug does seize and break on removal, we have the tooling to extract the remaining piece without damaging the cylinder head.

We use quality replacement plugs — NGK and Bosch for most applications — and always replace the full set alongside the relay where appropriate. After fitting, we clear the fault codes and run a check to confirm the system is reading correctly before returning the car.

For diesel vehicles with multiple issues — cold start problems alongside EGR faults, DPF warnings, or fuel system faults — we diagnose the full picture before recommending repairs, so you understand what is causing what before any money is spent.

Frequently Asked Questions