Flashing engine light and the car's shaking? Pull over
A flashing engine light with a stumbling, shaking engine means ease off now. Find somewhere safe to pull over and switch off, because driving on can overheat and damage the catalytic converter in the exhaust. If you're stranded on a busy road, put the hazards on, move everyone away from the traffic if it's safe, and ring roadside assistance if you have it. If anyone is hurt or in danger, call 000.
What does a misfire feel like?
A healthy engine fires each cylinder in a steady beat. When one cylinder misses, the beat stumbles, and you feel it as a shake at the lights, a shudder through the seat or a flat spot when you press the accelerator.
A rough idle is the same problem at low revs, where there's less momentum to hide it. You might also notice more fuel use, a raw fuel smell from the exhaust or the engine light. RAC says fixing a misfire should improve both fuel efficiency and how the car drives.
Diesels have no spark plugs, so a rough diesel points to injectors, fuel or compression instead. That's one for a mechanic who does diesel work regularly.
When it happens tells the mechanic where to look
| Usual suspects | What to mention | |
|---|---|---|
| Only on a cold start, then clears | Dirty or faulty spark plugs, high-tension leads on older cars, or the fuel injectors (RAC) | How long it takes to clear |
| All the time, one cylinder feels dead | A failed ignition coil, the part that makes the spark, or a worn plug | Whether the engine light is steady or flashing |
| Started soon after a fill | Contaminated fuel (NRMA) | The date, the servo and your receipt |
| At idle, with a hiss or whistle | A vacuum leak from an intake hose, letting in air the computer hasn't measured | Whether the hiss changes as you accelerate |
| Only uphill, overtaking or towing | A weak spark or a fuel supply problem that shows under load | What the car was doing at the time |
What to do next
Read the dash
Steady engine light and a small stumble: you've usually got time to reach a mechanic gently. Flashing light or anything red: stop.
Note when it happens
Cold or warm, idle or under load, wet weather or dry. A misfire that only turns up in the rain is a useful clue.
Think back to your last fill
If it began soon after refuelling, NRMA says to keep your receipt, write down the date, time and place, and tell the service station.
Leave the code alone
RACV says not to clear the code yourself before a mechanic sees the car. Clearing it can wipe the clue they need.
Go easy until it's fixed
Skip the overtaking, the steep hills and the trailer. Extra load makes a misfire worse.
How does a mechanic find the cause?
A scan tool usually shows which cylinder is missing. A good mechanic then proves the cause before replacing anything: moving a coil to another cylinder to see if the misfire follows it, pulling the spark plugs to read them, then testing fuel delivery and compression if the ignition checks out.
Look in your logbook too. RACQ says many modern schedules don't call for fresh plugs until 100,000 km or more, so an engine past that point with its first set is an obvious suspect. RACV says plugs always go in as a set, and RACQ says replacements should meet the maker's original specification, because standard plugs fire differently from the platinum or iridium ones some engines need.
Coils are a judgement call. RACV says replacing only the faulty ones may be possible, so ask what the mechanic would do on your engine and why.
Don't let it drag on. RACQ says catalytic converters are sensitive to misfires, and a converter is a far bigger job than a coil or a set of plugs.
What you can do, and what's the mechanic's job
You can
- Check the oil level on a cold engine.
- Note when the misfire happens and whether the light is steady or flashing.
- Keep your fuel receipt if it began after a fill.
- Take a photo of the dash if a light comes on.
Leave to the mechanic
- Reading the misfire codes and working out what they really mean.
- Testing coils, plugs, injectors and compression.
- Fitting ignition parts to the maker's specification.
- Checking whether the catalytic converter has been harmed.
Where we'd start
Small stumble, steady light, car otherwise fine: we'd keep it gentle and send the job today, because a misfire rarely fixes itself. Flashing light or a real shake: we'd stop and ask for a mobile mechanic to come to the car. Either way, tell them when it happens. That one detail usually saves the most time.