Volkswagen Glow Plug Warning Light — What It Means & How to Fix It

What it means

The coiled-wire light on a TDI means the diesel preheating system has a fault.
Glow plugs warm the combustion chamber so a cold engine will start — when one fails, you get hard starts and rough running until the engine warms up.
Pull the codes; they're numbered by cylinder so you'll know which plug is dead.

Affected Models

VW Golf TDI
VW Passat TDI
VW Tiguan TDI
VW Polo TDI
VW Caddy TDI
VW Touareg TDI
VW T-Roc TDI
VW Transporter TDI

Common Causes

Failed glow plug — they last 60,000–100,000 miles
Glow plug relay or control module dead
Open circuit somewhere in the wiring
Corroded or loose connector at the plug
Plug seized in the head (common over 100k miles)

How to Fix It

Pull the codes with any OBD-II scanner.

P0670–P0679 are individual glow plug circuit codes for cylinders 1–8.
A standard reader gets them — no VW-specific tool needed.
The code tells you which cylinder.
Check the plug with a multimeter before buying anything.

Disconnect the connector and measure resistance from the plug terminal to ground.
Healthy: 0.5–2 ohms.
Open circuit (infinite resistance): the plug is dead.
Heat the head before you try to remove a plug on a high-mileage engine.

Steel plug, aluminium head, decades of heat cycling — they bond.
Force a seized plug and it snaps off inside the head, which is an extraction job and an expensive one.
Heat gun on the area first, penetrating oil overnight if you can wait.
Test the relay if all the plugs read fine.

Right resistance on the plug but the fault code keeps coming back? It's the relay or the wiring upstream.
The relay sends power to all the plugs at once — when it dies, none of them fire.
Replace all four if more than one has failed.

They wear at similar rates.
If two are gone, the other two are close.
Doing them as a set saves a return visit.

The coiled-wire warning light only appears on diesel VWs — petrol engines don’t have glow plugs, so if you’re seeing this light you’re definitely driving a TDI.
The system warms the combustion chamber for a few seconds before a cold start.
When part of it fails, the engine starts harder, idles rough until it warms, and the check engine light often comes on alongside.

Why diesels need glow plugs at all

Petrol engines have spark plugs to light the fuel-air mix.
Diesels don’t — they compress the air so hard it self-ignites at over 700°C, hot enough to fire the injected fuel without help.
That works once the engine is warm.
On a cold engine, the cold cylinder walls and piston soak up the compression heat before it can reach ignition temperature, and the engine either won’t start or starts and stumbles.
Glow plugs solve that by adding their own heat to the chamber for those critical first few seconds.

The seized-plug problem

Single most important thing to know if you’re working on a high-mileage TDI: glow plugs seize.
The steel plug body bonds to the aluminium head over years of heat cycling, and the threads can effectively weld in place.
Try to remove one with normal torque and the plug snaps off, leaving the ceramic tip and the steel base stuck in the head.
Getting that out requires extraction tools, drilling, and sometimes head removal — a £20 plug becomes a £500+ job.

Any decent VW diesel mechanic will heat the head around the plug with a heat gun, soak it with penetrating oil, work the plug back and forth gently, and stop immediately if it doesn’t move.
If yours is over 100k miles and the mechanic doesn’t mention seizure risk before they start, find a different one.

One plug, or all four?

Tempting to replace only the one that threw the code, but the four plugs degrade together.
If one has failed, the others are within a few thousand miles of failing too — and on a high-mileage engine, every additional plug-removal is another seizure risk you take on.

Doing all four together costs more in parts but saves the labour and risk of going back in three months later for the next one.
On any TDI past 80,000 miles, all four is the safe call.

Frequently Asked Questions

Can I drive a VW diesel with the glow plug warning on?

Yes once the engine is warm.
Glow plugs are only used at cold start — once the engine is running, combustion is self-sustaining and the plugs sit idle.
Expect rough cold starts until you fix it, but otherwise the car is fine to drive.

Why do VW glow plugs seize in the head?

Steel plug threaded into aluminium head, with combustion temperatures going through it on every cycle.
Over years that interface corrodes and chemically bonds.
Penetrating oil and heat reduce the risk substantially — never just lean on a breaker bar.

How often should VW glow plugs be replaced?

60,000–100,000 miles in normal use.
VW doesn't list a service interval — they're run-to-failure components.
Some specialists recommend proactive replacement at 100k specifically to avoid seizure risk, since the longer you leave them the harder they come out.