VW or Audi will not start: working out which problem you have

Before you can fix it, you need to know which of three completely different faults you have: the engine does not turn over at all, it turns over but never fires, or it fires and then dies within a second or two. Those three point at almost entirely separate parts of the car, and once you know which one you are looking at you have already ruled out most of the possibilities. Go and sit in the car, turn the key (or press start) and listen carefully once — the sound it makes is the single most useful piece of information you have. This guide walks you through what to listen for, what to try next, and, just as importantly, when to stop trying and call for recovery instead. If you can already smell fuel strongly, skip to the last section now and stop.

Step one: split the symptom into one of three cases

People describe all three of these as "it won't start", but they are different problems. Turn the ignition on, wait for the dashboard lights to settle, then try to start it, and pay attention to what actually happens.

No crank means the engine does not turn over. You may get silence, a single clunk, or a rapid machine-gun clicking, but the engine itself stays still. This is nearly always electrical: battery, connections, starter, or something in the car telling the starter it is not allowed to work.

Cranks but will not fire means the engine turns over at its normal speed, over and over, and never catches. The battery and starter are doing their job. The engine is missing one of the things it needs to run: fuel, spark or compression, or the ECU's permission to inject fuel at all.

Starts then dies means it catches for a moment, sometimes only half a second, then cuts out. That is a very specific pattern and it points hard at the immobiliser or at fuel supply that cannot be sustained.

One more free clue before you go further: look at the dashboard warning lamps with the ignition on. Do they all come on brightly and steadily? Is there a key or padlock symbol — on some models a car outline, sometimes with a key inside it — that stays lit or flashes when it would normally go out after a second or two? Warning symbols differ between models, years and even trim levels, so check your handbook rather than assuming what one means. One thing not to be alarmed by: on many VWs and Audis a small light that blinks steadily while the car is locked and parked is simply the alarm telling you it is armed, and that is normal.

Then write down what you saw and heard, on your phone if nothing else, before you start trying things. The first attempt is often the clearest one — batteries get weaker with every crank, and a fault that was obvious at 8am can be muddied by lunchtime. Change one thing at a time after that, and note what changed with it.

• Silence or clicking, engine still = case one, no crank.
• Engine spinning at a normal speed, never catching = case two.
• A brief catch then stall = case three.
• Note any warning symbol that behaves differently from usual, especially a key or padlock.
• Record the first attempt. It is your cleanest piece of evidence and you only get it once.

Before you touch anything: two minutes of safety

None of what follows is difficult, but a no-start is one of the few times an ordinary owner ends up with their hands near a starting engine and a battery capable of hundreds of amps. A short pause here costs nothing.

If somebody is going to crank the engine while you look or listen under the bonnet, agree that first, and keep hands, sleeves, hair and cables well clear of belts, pulleys and the cooling fans. Electric cooling fans on VAG cars can spin up on their own, with no warning, even with the engine off and the key out. Never reach into the engine bay on the assumption that a still engine will stay still.

Working at the battery: ignition off, keys out, take off rings and a watch, and do not let a spanner touch the positive terminal and any metal bodywork at the same time — that is how people get burns and ruin a battery. If the battery is hot, swollen, hissing or leaking, walk away from it and call for help rather than testing anything.

If you are jump starting, use the jump points the manufacturer provides rather than clamping straight onto the battery. Many VAG cars put the battery somewhere awkward and provide a positive post under a plastic cover in the engine bay plus a separate earth stud, and the handbook shows both and the order to connect them. Never jump a battery that is visibly damaged, leaking, or frozen solid. If your car is a mild hybrid with a 48-volt system, follow the handbook to the letter and never touch orange cabling or connectors.

Finally, do not sit cranking a car repeatedly in a closed garage. A car that fires briefly still produces exhaust, and a garage fills up faster than people expect. Open the door, or push it out first.

Case one: it will not crank at all

Start with the battery, because a weak battery is by far the most common cause and it can fool you. A battery can light the dashboard perfectly brightly and still collapse the instant the starter draws current. The give-away is what happens during the attempt, not before it.

The old test is to turn the headlights on and try to start it: if the lights dim heavily, or you hear rapid clicking that gets weaker, the battery or its connections cannot deliver the current. That test still works on older cars, but treat a healthy-looking result with suspicion on a newer one — modern VAG load management deliberately dims or drops consumers during cranking to protect the start, and LED lighting is voltage-regulated so it will not visibly sag anyway. Take dimming as evidence; do not take steady lights as proof of a good battery.

The clearer signals are these. Rapid weak clicking is almost always supply — battery, terminals or earth. One single solid clunk with a bright dashboard means current is arriving but the engine is not turning: starter, solenoid, or a mechanically stuck engine. Complete silence with a bright, healthy dashboard points at the control side, something not telling the starter to go at all.

If you get that single clunk and the engine will not budge, try twice and stop. An engine that is seized, or hydrolocked with water or fuel in a cylinder — which is a real possibility if the car has recently been driven through standing water — can be damaged badly by repeated attempts, and you will cook the starter into the bargain. That one is a recovery job, not a driveway job.

Connections deserve a proper look before you condemn anything. Corroded or loose battery terminal clamps, and a corroded engine earth strap, produce exactly the same symptoms as a flat battery and cost nothing to check. With the ignition off, try to twist each clamp by hand; if it moves at all, it is too loose.

The control side is where VAG cars have their own quirks. An automatic or DSG will not crank unless the selector is properly in P or N and, on most, the brake pedal is held down — try N as well as P, because a selector position sensor can fail in one position and work in the other. A manual usually needs the clutch pedal pushed fully to the floor, and the switch behind that pedal does fail. If the car has keyless start and the key battery is flat, the car may not wake up at all; many VAG models have a marked spot in the cabin, or the start button itself, where you hold the fob so it can be read, and the handbook will tell you where. Remember the brake pedal must still be pressed.

• Lights dim or clicking weakly under load: battery, terminals or earth.
• Bright lights and one loud clunk: starter or its solenoid, or an engine that is not free to turn — stop after two attempts.
• Bright lights and total silence: clutch switch, gear selector, relay, ignition switch, or a car that has not woken up.
• A jump start that cures it instantly tells you the fault is on the supply side, not the starter — but a battery that has gone flat once has either aged out or something is draining it.

Case two: it cranks but never fires

Because it cranks, you know the battery is at least roughly healthy — though on a diesel a slow, laboured crank can genuinely be too slow to start, so note whether the cranking speed sounds normal for your car.

Work through it in this order, because it goes from cheapest to check to most expensive to be wrong about: is fuel arriving, does the engine have permission to run, is the ECU getting a position signal, and is there still compression.

First, fuel. With the ignition on but before cranking, many VAG cars run the fuel pump for a second or two; on a quiet driveway you can often hear a whirr from under the rear seat or near the filler. Hearing it is a good sign. Hearing nothing is weaker evidence than people think, because some cars prime only when a door is opened or only while cranking — but combined with other symptoms it points at the pump, its relay or its fuse. Also ask the unglamorous questions: is there genuinely fuel in it (gauges and senders do fail), and did anything change at the last fill-up? Putting the wrong fuel in is common, and if you suspect it, do not crank the engine and do not even switch the ignition on — key-on can prime the pump and push it further through the system. Have the car recovered and drained.

Second, permission. If the immobiliser has not recognised the key, many VAG cars crank happily and never fire, because the ECU simply does not inject. Watch for a key or padlock symbol that flashes or refuses to go out. Try the spare key. If the spare works, the problem is the first key, not the car.

Third, the position signal. A failed crankshaft position sensor is the classic VAG cranks-but-no-start: without it the ECU has no idea where the engine is, so it will not fire at all. It can also fail intermittently with heat, which produces a car that cuts out when hot and restarts once it has cooled — if that pattern sounds familiar, say so to whoever works on it.

You will see advice to check for spark by pulling a plug lead or coil. Do not. Modern ignition systems carry enough energy to hurt you, and if there is fuel vapour around a bare spark is exactly what you do not want. That check belongs to somebody with the right tools, and the scan data described further down answers the same question without taking anything apart.

Fourth, compression. Listen to the crank itself. If the engine suddenly spins much faster, smoother and lighter than it ever used to, with none of the usual pulsing, that can mean it has lost compression across the board — a snapped timing belt or a jumped chain. Which of those your engine even has depends on the exact engine and year, and VAG has used both across its range, so do not assume from the badge on the boot. Either way the response is the same: stop cranking and get it recovered, because most modern VAG engines are interference designs where the pistons and valves share space, and continued cranking turns a bad day into a much more expensive one.

Case three: it fires and then dies

A car that catches and immediately cuts out is telling you that starting was fine and something then withdrew permission or supply. On VAG cars, the immobiliser is the headline suspect. The pattern is very recognisable: it fires for perhaps a second, dies, and a key or padlock symbol flashes. Depending on the age and generation of the system, an immobiliser fault can also show up as a car that cranks and never fires at all, so do not rule it out just because your symptom sits in the other box. Try the spare key first — it is free and it splits the problem in two. A worn key, a flat key battery, or a reader coil around the ignition barrel that has failed will all do this.

The other main family is fuel that starts out available and then is not. A pump that primes but cannot hold pressure, a badly blocked filter, or a failing pump relay can all give you a brief run on residual pressure and then nothing.

Note the conditions carefully, because they narrow it further. Only when hot points at a heat-sensitive sensor. Only when cold points at fuelling or, on a diesel, preheating. Only after refuelling points at fuel quality or the pump. Only after a service or a battery disconnect points at something disturbed during that work — air in a diesel fuel system after a filter change is a very ordinary cause of a car that runs for two seconds and stops.

If it is a diesel and the weather is cold

Diesels have an extra system that petrols do not, and it fails quietly. Glow plugs and their control module heat the combustion chambers so the engine will light on a cold morning. When they fail, the usual picture is a car that starts perfectly all summer and in the afternoon, but cranks and cranks on a cold morning, sometimes eventually catching with a lot of white smoke and a rough first minute.

The coil-shaped lamp on the dash is worth understanding properly, because it is widely misread. Coming on at key-on and going out after a few seconds is normal preheating, and on a warm engine it may barely appear at all. A flashing coil lamp is different: on most VAG diesels that is the general engine-management warning — the diesel equivalent of a petrol car's engine light — and it means the ECU has logged a fault that may have nothing whatsoever to do with the glow plugs. Do not buy glow plugs off a flashing lamp. Read the stored codes first.

Never use easy-start, ether or any aerosol starting fluid on a diesel with glow plugs. It ignites far too early and too violently, and it can bend a rod or break a piston on an engine that was otherwise only suffering a tired glow plug. It is the most expensive five pounds an owner can spend on a cold morning.

Diesels are also less forgiving about fuel supply. Running the tank genuinely dry, or letting air into the system, usually means it will not simply start again after you add fuel — it may need priming or bleeding, and the correct procedure varies between engines. And a diesel needs a good brisk crank speed, so a tired battery hurts a diesel long before it would strand a petrol car.

Whatever the engine, crank in short bursts of a few seconds with a proper rest in between. Long continuous cranking overheats the starter and, on a diesel, washes unburnt fuel where you do not want it. If it has not started after several short attempts, stop and find out why rather than flattening the battery and removing your remaining options.

What the stored fault code adds, and why it saves guesswork

Everything above narrows the field using your ears. The car has already narrowed it further and written it down. Even a car that will not start usually logs what it noticed — no signal from the crankshaft sensor, fuel pressure never reached its target, glow circuit open, or no valid immobiliser authorisation. That converts an afternoon of swapping parts into a single answer.

There is a catch that catches a lot of people out. A basic OBD code reader only asks for the generic, emissions-related engine codes. A no-start fault often does not live there. Immobiliser and key faults are usually stored in the immobiliser function and the instrument cluster — and where that function actually sits varies by generation, sometimes the cluster, sometimes a separate module, sometimes spread across several — while the engine ECU may well hold its own manufacturer-specific authorisation code that a generic reader never requests. Either way, a basic reader can cheerfully report "no faults found" on a car that is refusing to start for exactly that reason.

What you need is a tool that reads every module in the car rather than the engine alone, and that can show live measuring values. Several tools can do that; VAGPULSE is the one we make, Windows software plus a cable, covering VW, Audi, Skoda and SEAT. The reason live values matter here is that you can watch whether the ECU actually sees engine speed while somebody cranks. If the ECU sees nothing, you have proved the position signal is not arriving — which is the crank sensor, its wiring, its connector or the trigger wheel, rather than a guess between four unrelated parts. Keep well clear of the engine bay while anyone is cranking.

Two practical notes. Charge the battery before you plug anything in, because a very flat battery throws false faults across half the car and you will chase ghosts. And if the battery has been replaced, on many modern VAG cars the new one is meant to be registered to the energy management system, and on a start-stop car it also has to be the correct type — otherwise charging behaviour is wrong from day one and the replacement ages early.

When to stop and call for recovery

Some of these situations are not diagnosis problems, they are safety problems, and pressing on makes them worse. In each of these, stop, get out, and phone for help.

Stop if you can smell petrol or diesel strongly. Do not keep cranking, and do not connect, disconnect or switch anything electrical, including the ignition. Stop if the battery is hot, hissing, swollen or leaking, and do not attempt to jump start it. Stop if you see smoke, or fluid pooling under the car. Stop if the engine suddenly spins fast and freely as described earlier. Stop if you have put the wrong fuel in. Stop if the engine clunks once and will not turn, especially after driving through deep water.

And if the car does start but the engine warning light is flashing rather than glowing steadily, do not drive it. A flashing light means damage is happening now, not that something needs looking at soon. Switch it off and have it recovered.

If you are somewhere dangerous, that outranks everything above. On a motorway or a fast road, do not attempt any of these checks. Get everyone out on the side away from traffic and behind a barrier, and call for recovery from there.

A word on bump starting: it is only ever possible on a manual, it will not help at all if the immobiliser is the cause, and it needs a safe stretch of road and a clear head. On an automatic or DSG it simply does not work. Remember also that while the engine is off you have no brake servo and, on many cars, no steering assistance — the brakes will feel alarmingly hard after the first push of the pedal and the wheel will be heavy. That catches people out at exactly the wrong moment. If you are unsure, recovery is the better call.

When you do phone for help, the three-way split at the top of this guide is exactly what they want to hear. "It cranks normally and never fires" or "it clicks once and does nothing" gets you the right van with the right kit, and often gets you a much better guess before they arrive.

Common questions