The first morning at minus five degrees arrives, and suddenly it's the same old struggle in car parks everywhere. One diesel takes several attempts to start, another fires up but shakes like mad, and a third leaves a cloud of white smoke behind it. If you drive a diesel, you've probably seen it yourself. Quite often, TDI glow plug problems are behind all this trouble. A tiny component can make a big difference. But here's the thing! Not every TDI that struggles to start in cold weather has faulty glow plugs. Sometimes the cause is somewhere else entirely, and it's worth finding it before it turns into a much bigger headache for you.
What do glow plugs actually do?
You know a diesel doesn't have spark plugs like a petrol engine. Instead, the fuel ignites because the air gets extremely hot when the engine compresses it. The trouble is, in winter, all that cold metal absorbs some of the heat, making the engine harder to start. That's where the glow plugs come in. They heat the combustion chamber for a few seconds before you start the engine and keep working afterwards. That helps your TDI start more easily and run more smoothly while it warms up.
TDI glow plug problems: how can you spot a faulty one?
- On cold mornings, the engine cranks for ages and struggles to start, but once it's warm, it starts without any trouble.
- The engine starts but immediately begins shaking, runs as though it's only firing on three cylinders, and blows white smoke from the exhaust.
- You're driving along when the glow plug warning light suddenly starts flashing on the dashboard.
- A diagnostic scan shows a glow plug circuit fault for a particular cylinder, such as P0671 to P0674.
There's something important I want you to understand. On VAG cars, a flashing glow plug light doesn't automatically mean a glow plug has failed. It can also come on because of other engine management problems. That's why I don't like replacing parts based on guesswork. We read the fault codes first and only then decide what needs doing. Otherwise, you're paying for parts that might not have been faulty in the first place.
Why replace all the glow plugs instead of just the faulty one?
I've said this before in my Passat B7 service schedule: when one glow plug fails, I recommend replacing the whole set. Think about it. They've all been working for the same number of years, at the same temperatures and under the same conditions. If one has already given up, the others probably aren't far behind. We replace just the faulty one, and a month later we're lifting the bonnet again to deal with the next one. Why put yourself through the same job twice?
I recommend replacing your glow plugs as a preventive measure at around 60,000 km, even if they're still working. There's a serious reason for this. As a glow plug gets older, it can start to break apart, and its tip could end up inside the cylinder. At that point, we're no longer talking about replacing a few glow plugs. You could be looking at an engine repair. I'm telling you this because replacing them in good time could save you a lot of trouble.
Why isn't replacing glow plugs as simple as it sounds?
You look at a glow plug and think, what's the big deal? Unscrew the old one, screw in the new one, job done. Not quite! These plugs spend years sitting in the cylinder head, constantly heating up, and over time their threads can seize solid. If someone grabs a spanner and starts forcing one out of a cold engine, it's surprisingly easy to snap it. And a broken glow plug stuck in the cylinder head is a proper nightmare. That's why, at Das Auto Service, we remove them carefully with the engine warm, using the correct torque. We tighten the new ones with a torque wrench, exactly as specified. Brute force won't help you here. It'll only make the repair much more expensive.
The glow plugs are fine, but your TDI still struggles to start. What else do we check?
- The battery. Cold weather reduces its capacity, while the starter motor needs more current to turn the engine over. If your battery is weak, starting the car on a winter morning can be a real struggle. I've explained more in my article about the battery and Start/Stop.
- The fuel filter. If it's old or has collected water, the fuel thickens in cold weather and struggles to pass through it.
- The injectors. If they're returning too much fuel, the pump can't build up enough pressure for the engine to start.
- The engine temperature sensor. If it's giving the ECU the wrong information, the engine management system might not even realise the engine is cold.
A tip from Kris Mladenov: Before every winter, have your glow plugs checked with a tester. It only takes a few minutes and could save you a lot of grief. If they need replacing, make sure the job is done by someone who knows how to remove them with the engine warm. I've seen what happens when someone gets impatient and starts forcing them with a spanner. Trust me, this is one of those jobs where the cheapest, quickest repair can end up costing you the most.
I hope your TDI starts first time every morning this winter, without endless cranking, shaking or clouds of white smoke. And if I've helped you understand something useful, send this article to a friend who drives a diesel. You might save them a fair bit of frustration too.
A helpful product from our shop
Audi battery charger. In winter, a healthy battery is half the battle when it comes to easy starting.
View in our shopRead more
- Vehicle diagnostics at Das Auto Service
- Routine servicing
- Passat B7 and CC 1.6 and 2.0 TDI service schedule
- Audi A4 B8 3.0 TDI service schedule
- Why does Start/Stop stop working in winter, and why does a new battery need coding?
- Diesel at -15°C: fuel, filters and glow plugs
- TDI common rail injectors: when to replace them and why we often replace all four
Frequently asked questions
How can I tell if my glow plugs have failed?
If your car struggles to start on a cold morning, shakes once it fires up and blows white smoke from the exhaust, the glow plugs could be to blame. But don't rush out and buy new ones. Those symptoms alone aren't enough to be sure. We test the glow plugs first.
Should I replace all the glow plugs at once?
Yes, that's what I recommend. If one glow plug has already failed, the others are probably nearing the end of their lives too. It's better to replace them together than have to do the same job all over again a little further down the road.
Why is the glow plug light flashing on my VAG car?
A flashing glow plug light means there's a problem with the engine management system. It doesn't necessarily mean a glow plug has failed. We need to run a diagnostic scan and read the fault codes to find out what's actually wrong.