Glow Plugs: The Unsung Heroes Your Diesel Ignores Until January

In mild weather with only one failed plug, you might get away with it — the car will probably start after a longer crank and run roughly until warm. In cold weather or with multiple failed plugs, you risk not starting at all. More importantly, the longer you leave it the more carbon deposits build up, and the more likely the plug is to seize in the head and turn a routine replacement into a far more expensive extraction job. So: driveable-ish, but inadvisable.

That's your diesel's general engine management warning light moonlighting as the glow plug symbol — confusing, but standard. When it stays on or flashes while driving it typically means a fault code has been logged, which could be a failed glow plug, a faulty control module, an emissions or fuel system issue, or something else entirely. A diagnostic scan is the only way to know what it actually is — don't assume it's definitely plugs without checking.

Not necessarily — if only one or two have failed and the rest test healthy, replacing just the faulty ones is perfectly reasonable. That said, if the plugs are original and the car has covered 80,000+ miles, fitting all four at once makes sense: the remaining ones are living on borrowed time, and you're already paying for the labour once. We'll give you an honest recommendation based on what our tests show.

On most modern diesels with accessible plug locations — a VW Golf, Ford Focus or similar — it's a one to two hour job. Vehicles with plugs buried deep in the engine bay, or where corrosion makes extraction slow and careful, will take longer. If we find a seized plug when we get in there, we'll let you know before proceeding, because that changes both the time and the cost.

Not directly — the MOT doesn't test glow plugs themselves. But a failed glow plug that causes a rough cold start can produce excess smoke emissions during the MOT emissions test, which can fail you. An engine management light on the dash will also fail the MOT automatically. Fix the plugs before the test and you remove both risks at once.

P0670 means the ECU has lost confidence in the glow plug control module — the separate unit that manages when and how hard each plug fires. It doesn't automatically mean the module is dead; wiring faults, a corroded multiplug and even a single shorted glow plug dragging down the circuit are all common real-world causes. Start by scanning all the related codes (P0671–P0678 flag individual cylinders) before condemning the module itself. A proper diagnostic will tell you whether it's a £15 connector or a £200 module — worth knowing before you guess.