Flashing Glow Plug Warning Light: What It Means on a Diesel | Fastlane

Usually because the fault is intermittent and has not been present long enough to store a hard code, only a pending one a basic reader will not show. Generic OBD scanners also see emissions codes only, and the pre-heat system often logs in the engine module, which needs manufacturer-level equipment to read. Other possibilities are codes cleared recently, a sensor value drifting without crossing the threshold that sets a code, or a weak battery upsetting pre-heat before anything else. Send the registration to 01744 808586 and see car diagnostics.

That pairing means the car has entered limp mode to protect itself. The usual causes are a turbo or boost fault such as a sticking variable-vane mechanism or a split boost pipe, an EGR valve stuck open, a blocked DPF that cannot regenerate, or a fuel pressure or injector problem that shows up under load. It is rarely the glow plugs themselves — on most cars a failed plug alone will not derate the engine. Do not keep driving in limp mode. Ring 01744 808586.

An intermittent light means something is marginal rather than failed. Temperature-related appearances — cold mornings but not warm ones — point at pre-heat. A relay or module that works cool and drops out hot behaves the same way, as does a corroded connector or a chafed wire making and breaking with vibration. One plug on the edge of failure will draw the wrong current only sometimes, and a tired battery complains first because pre-heat draws heavily. Intermittent faults are cheaper to find while they are still intermittent.

The glow plug light itself is not an MOT item on most cars, so it will not fail the test on its own. What will fail it is the engine management light if that is on too, and excessive smoke at the emissions test — which is a real risk if the underlying fault is the DPF, EGR or injectors. In practice, a car flashing its pre-heat light is often close to failing on smoke. Get it looked at before the MOT, not after.

It can, and it is one of the more common causes when the light flashes alongside a loss of power. A DPF that cannot complete a regeneration will put the car into limp mode, and on many models the pre-heat lamp is the one used to tell you. Short journeys are the usual reason the regeneration never finishes. The fix is diagnosis first — a forced regeneration, a clean, or occasionally replacement. See DPF cleaning in St Helens.

It depends what is actually causing it, which is exactly why we test before quoting. If it is genuinely glow plugs, the cost varies with how accessible they are and whether any snap on removal — that is the risk on a high-mileage diesel. If it is the relay or module it is usually less. If it is a turbo, EGR or DPF fault it is more. You get a figure once we know, not before. Ring 01744 808586 with the registration.

Fastlane Autocare, 232-254 Boundary Road, St Helens, WA10 2PZ — diesel diagnostics with manufacturer-level equipment rather than a generic code reader, and a DVSA test bay on the same site. We read the engine module properly, test the pre-heat circuit and the battery, and tell you what is actually wrong before anything is replaced. Open Monday to Friday 8:30am to 5pm and Saturday 9am to 12:30pm. Ring 01744 808586 or see car diagnostics.

That curly symbol, which many drivers describe as a coil or a spring, is the diesel pre-heat or glow plug indicator. It looks like a loop of wire with two curls, and it is amber on almost every car. It has nothing to do with suspension springs, which is the usual assumption. On a cold start it should light for a second or two, then go out as the plugs finish heating. Our dashboard symbol guide and glow plug page show it.

Drive it gently and get it looked at within a day or two rather than carrying on for weeks. A flashing pre-heat lamp usually sits alongside a fault the engine has already noticed, and many cars will drop into reduced power to protect themselves if it worsens. Where the light flashes together with a loss of power, a smell of fuel or smoke, stop using the car and have it checked. Book it in on 01744 808586 or read about limp mode.

Glow plugs are small electric heaters, one per cylinder, that warm the combustion chamber so a cold diesel will fire. Diesel ignites on compression alone once the engine is warm, but from cold there is not enough heat, so the plugs glow for a few seconds before and often after starting. Modern systems keep them running for a short while after the engine catches to cut smoke and noise. Our page on the glow plug warning light explains the system in more depth.

The classic sign is a diesel that takes several attempts to start on a cold morning yet fires instantly once warm. Rough running for the first minute, a puff of white smoke from the exhaust after start up, a rattle that smooths out as the engine warms, and poorer economy all point the same way. Warm weather can hide it completely, which is why so many are found in October. See our guide to cold starting problems and exhaust smoke colours.

Replacing the full set is the sensible approach, because the plugs have all done the same mileage and the labour to reach them is the same whether you change one or four. Fitting a single new plug alongside three tired ones usually means going back in a few months. The parts themselves are modest; the access is what costs. On some engines the plugs sit under the inlet manifold. Ring 01744 808586, or book it alongside a full service.

Seized plugs do break off in the cylinder head, and it is the single reason this job is not a five minute task. Carbon builds up around the tip over the years and welds it in place. Doing it properly means penetrating fluid, heat cycles and patience rather than brute force, and working on a warm engine helps. Where one does snap, it can usually be extracted without removing the head. Our engine repair team in St Helens handles these regularly.

That combination almost always means the engine has entered a protection mode after logging a serious fault. Turbo boost problems, a blocked diesel particulate filter, a faulty EGR valve, a failing mass air flow sensor or an injector out of tolerance are the usual causes. The car limits power deliberately to prevent damage while you get somewhere safe. Reading the fault code is the only way to know which. See our pages on losing power and the DPF.

The pre-heat lamp doubles as the engine management warning on many diesels, and an illuminated engine management light is an MOT failure in its own right. Even where the tester treats it as a separate pre-heat indicator, the fault behind it often shows up as visible smoke on the emissions test, which fails too. Get the cause diagnosed and cleared before your test date. Our St Helens MOT bay and our common failures list explain what testers look for.

The control relay fails at least as often as the plugs themselves, and it is a far cheaper part. A relay fault typically kills the pre-heat on every cylinder at once, so the engine struggles badly from cold rather than running slightly rough. Individual plug failure gives a milder misfire that clears as the engine warms. Measuring the resistance of each plug and checking the supply voltage separates the two in minutes. Our diagnostics service does exactly that before anything is replaced.

Low battery voltage is a genuine cause of odd diesel warning lights, because glow plugs draw a heavy current and the control module monitors the supply closely. A battery that struggles on a cold morning can trigger pre-heat fault codes with nothing wrong with the plugs at all. Testing the battery and charging system first is always worth doing before spending on parts. It is a five minute check. See our page on battery life and charging faults.

Most sets last somewhere between sixty and a hundred thousand miles, though city cars doing constant short cold starts wear them out sooner because the plugs cycle every journey. Ceramic plugs on newer engines can go further but are more fragile to remove. There is no fixed service interval, so they are usually replaced once symptoms appear. Having them checked at service time before winter is the sensible move. Book a service in St Helens or read our winter checks.

We plug in first and read every stored and pending code across the engine module, because the flashing lamp is a messenger rather than the fault. From there we test each glow plug for resistance, check the relay output and battery voltage, and look at live data for boost, air flow and exhaust pressure while the engine runs. That tells us whether the problem is the pre-heat circuit or something else entirely. See our diagnostic cost page, or call 01744 808586.