Car running badly? Losing power, misfiring, stalling or juddering
How a car misbehaves narrows the cause quickly. A sudden loss of power with a warning light usually means the engine has put itself into limp mode to protect itself. A misfire or rough idle points at ignition or fuel. A judder only when pulling away is often the clutch or dual-mass flywheel rather than the engine.
Pick what the car is doing below for the likely causes, ranked, and what to do next.
Free interactive diagnosis
Is it petrol or diesel?
A hybrid counts as petrol. The likely causes are completely different between the two.
Is it safe to keep driving?
For a typical car with each symptom. These are general guidance, not a diagnosis — if you're unsure, don't drive it.
- Car losing power or in limp modeDrive carefully
- Car misfiring, idling rough or shakingDrive carefully
- Car stalling at junctions or when slowing downDrive carefully
- Car juddering when pulling awayDrive carefully
- Car hesitating or jerking when acceleratingDrive carefully
Car losing power or in limp mode
Drive carefully — book it in
You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.
Likely causes
- Turbocharger or a leaking boost pipeMost likely
A whistle or hiss under acceleration, loss of power, and sometimes blue or black smoke.
If it's this: get it checked soon.
- Blocked diesel particulate filter (DPF)Likely
The DPF light is on, sometimes with limp mode. If the car mostly does short trips, a steady 20-minute run at motorway speed can clear it — if the light stays, it needs a forced regeneration.
If it's this: get it checked soon.
- EGR valve stickingPossible
A rough idle, black smoke and a P0400-series code. Common on diesels that do mostly short, slow journeys.
If it's this: get it checked soon.
- Air flow sensor or a split boost hosePossible
A flat spot or limp mode, codes P0100–P0104 or a boost fault. A split intake hose often hisses under load.
If it's this: get it checked soon.
What to do next
Limp mode protects the engine. Have the fault codes read before anything is replaced — the code narrows this list down more than any question can.
Car misfiring, idling rough or shaking
Drive carefully — book it in
You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.
Likely causes
- Ignition coil or spark plugsMost likely
A misfire code (P0300–P0304), shaking at idle, and a loss of power. Swapping a coil to another cylinder and seeing the fault move confirms it.
If it's this: get it checked soon.
- Fuel injectorLikely
A misfire or knock on one cylinder, and sometimes smoke. On a diesel a garage can do a leak-off test to find the bad one.
If it's this: get it checked soon.
- EGR valve stickingPossible
A rough idle, black smoke and a P0400-series code. Common on diesels that do mostly short, slow journeys.
If it's this: get it checked soon.
- Worn engine or gearbox mountsPossible
A clunk when pulling away or changing gear, and the engine rocking visibly when someone blips the throttle.
If it's this: not urgent.
What to do next
If the engine light is flashing, stop driving it. Otherwise have the codes read — a misfire code names the cylinder.
Car stalling at junctions or when slowing down
Drive carefully — book it in
You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.
Likely causes
- Dirty throttle body or idle controlMost likely
It fires, then dies unless you hold the revs up. Cleaning the throttle body is often all it needs.
If it's this: not urgent.
- EGR valve stickingLikely
A rough idle, black smoke and a P0400-series code. Common on diesels that do mostly short, slow journeys.
If it's this: get it checked soon.
- Air flow sensor or a split boost hoseLikely
A flat spot or limp mode, codes P0100–P0104 or a boost fault. A split intake hose often hisses under load.
If it's this: get it checked soon.
- Fuel pump, fuel filter or air in the fuelLikely
On a petrol car, listen for a two-second hum from the back seat when you switch the ignition on — silence points at the pump. Rule out an empty tank and a faulty fuel gauge first.
If it's this: get it checked soon.
What to do next
Stalling can leave you without power steering or brake assistance mid-junction. Get it looked at within days.
Car juddering when pulling away
Drive carefully — book it in
You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.
Likely causes
- Dual-mass flywheel or clutchMost likely
Judder pulling away, a rattle at idle that stops when you press the clutch, or revs rising without the car speeding up.
If it's this: get it checked soon.
- Worn engine or gearbox mountsLikely
A clunk when pulling away or changing gear, and the engine rocking visibly when someone blips the throttle.
If it's this: not urgent.
- Ignition coil or spark plugsPossible
A misfire code (P0300–P0304), shaking at idle, and a loss of power. Swapping a coil to another cylinder and seeing the fault move confirms it.
If it's this: get it checked soon.
- EGR valve stickingPossible
A rough idle, black smoke and a P0400-series code. Common on diesels that do mostly short, slow journeys.
If it's this: get it checked soon.
What to do next
If it's worse in first and reverse and the clutch bite point has changed, it's the clutch side. A garage can test the flywheel before quoting.
Car hesitating or jerking when accelerating
Drive carefully — book it in
You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.
Likely causes
- Air flow sensor or a split boost hoseMost likely
A flat spot or limp mode, codes P0100–P0104 or a boost fault. A split intake hose often hisses under load.
If it's this: get it checked soon.
- Turbocharger or a leaking boost pipeLikely
A whistle or hiss under acceleration, loss of power, and sometimes blue or black smoke.
If it's this: get it checked soon.
- Ignition coil or spark plugsLikely
A misfire code (P0300–P0304), shaking at idle, and a loss of power. Swapping a coil to another cylinder and seeing the fault move confirms it.
If it's this: get it checked soon.
- Fuel pump, fuel filter or air in the fuelLikely
On a petrol car, listen for a two-second hum from the back seat when you switch the ignition on — silence points at the pump. Rule out an empty tank and a faulty fuel gauge first.
If it's this: get it checked soon.
What to do next
Listen for a hiss under acceleration — a split boost hose is cheap. Otherwise have the codes read.
A shortlist, not a diagnosis
The ranking is the one a mechanic would give over the counter for a typical car — nobody can road-test a car over the internet, and the real cause may not be listed. Add the registration in the checker above to have it checked against that car's MOT advisories, mileage and engine. Have the car checked by a qualified mechanic before relying on it, and if you're unsure whether it's safe, don't drive it.
Common questions
What is limp mode?
A protection mode. When the engine computer detects a fault that could cause damage, it limits power and revs so you can get home or to a garage. Drive gently and have the fault codes read — switching off and on may clear it for a while, but the fault is still there.
Can I drive a car that's misfiring?
Only gently and not far. A misfire sends unburnt fuel into the exhaust, which can overheat and destroy the catalytic converter. If the engine management light is flashing, stop driving.
What are the symptoms of a failing dual-mass flywheel?
A judder when pulling away, a rattle at idle that stops when you press the clutch, and a knock when you switch off. It's common on diesels and higher-mileage manuals, and it's usually replaced together with the clutch.
Related symptoms
Engine management light
Engine management light on: causes and is it safe to drive?
Exhaust smoke
Smoke from the exhaust: what white, blue and black smoke mean
Noises
Strange noise from your car? Find what's making it
All symptoms: Engine management light · Other warning lights · Won't start · Exhaust smoke · Leaks · Brakes · Steering & vibration · Noises · Symptom checker