fault code guide
P0300: Random / Multiple Cylinder Misfire Detected
Fault code P0300 means the engine computer has detected incomplete combustion across more than one cylinder. Your engine will feel rough or down on power, and continuing to drive can damage the catalytic converter.
How it shows up: The amber engine management light illuminates steadily or flashes, accompanied by engine shaking, sluggish acceleration, or a rough idle. The specific P0300 code is retrieved by connecting an OBD scanner to the diagnostic port.
Updated 9 November 2026
Can I keep driving?
Service soon
A steady engine light allows you to drive gently to a nearby garage, but if the light flashes, you must pull over immediately to prevent costly catalytic converter damage.
What to do right now
- 1Note whether the engine management light is steady or flashing.
- 2If the light is flashing, pull over safely, switch off the engine, and call for breakdown assistance.
- 3If the light is steady, ease off the accelerator, avoid motorways or heavy towing, and drive straight to a garage.
Likely causes
Most likely first.
- Likely
Worn spark plugs or ignition coils
Over time, spark plug electrodes erode and coil insulation weakens, preventing strong sparks across multiple cylinders. This leads to intermittent ignition failure and unburnt fuel entering the exhaust.
- Likely
Intake vacuum leak
Cracked rubber vacuum pipes or leaking intake manifold gaskets let unmetered air into the engine. This thins out the air-fuel mixture across all cylinders, causing lean misfires, particularly at idle.
- Possible
Low fuel pressure or dirty fuel injectors
A failing fuel pump, blocked fuel filter, or gummed-up injectors starve the cylinders of adequate petrol. Without enough fuel, the mixture fails to ignite reliably under load.
- Possible
Faulty mass air flow sensor
A dirty or failing sensor misreports the amount of incoming air to the engine computer. This results in an incorrect fuel calculation that causes erratic misfiring across several cylinders.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Check the warning light behaviour
Observe the instrument cluster while running the engine. A flashing light signals an active, severe misfire that can ruin the exhaust catalyst, requiring an immediate stop, whereas a solid light means you can drive gently to a garage.
- 2
Listen for vacuum hiss under the bonnet
With the car parked and engine idling, open the bonnet and listen carefully around the plastic intake manifold and rubber hoses. A distinct hissing or sucking noise indicates an air leak that draws unmetered air into the engine.
- 3
Inspect wiring to the ignition coils
Switch off the engine and let it cool, then examine the wiring harness leading to each ignition coil pack. Look for loose push-fit connectors, rodent chewing, or broken insulation that could disrupt electrical signals.
- 4
Check the air intake ducting
Examine the large rubber or plastic intake hose running from the air filter box to the engine throttle body. Ensure the hose clamps are tight and there are no splits or tears that allow bypass air into the system.
Rough cost to fix
Rough estimateTypical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. Costs vary depending on whether the misfire stems from simple consumable items like spark plugs or requires diagnostic labour to trace fuel pressure and intake air leaks.
Spark plug replacement
- Do it yourself
- €30–90£25–80 in the UK
- At a garage
- €90–220£80–190 in the UK
Ignition coil pack replacement
- Do it yourself
- €80–260£70–230 in the UK
- At a garage
- €160–420£140–370 in the UK
Vacuum hose or intake manifold gasket repair
- Do it yourself
- €20–80£15–70 in the UK
- At a garage
- €110–310£95–270 in the UK
Fuel injector replacement or professional cleaning
- Do it yourself
- Garage job
- At a garage
- €220–650£190–560 in the UK
When to go to a garage
- The engine management light starts flashing or the car shakes violently.
- You notice a strong smell of unburnt petrol or see smoke coming from the exhaust or catalytic converter area.
- The engine loses power suddenly, cuts out when coming to a halt, or enters limp-home mode.
- Fitting fresh spark plugs fails to clear the misfire and the fault code returns.
Questions drivers ask
Can I drive my car with a P0300 fault code?
You can drive a short distance at moderate speed if the engine light remains steady and the car feels reasonably smooth. If the light begins flashing or the engine shudders heavily, pull over safely because unburnt petrol will quickly overheat and destroy your catalytic converter.
What is the difference between P0300 and codes like P0301 or P0302?
Codes ending in a specific number pinpoint an isolated cylinder, such as P0301 for cylinder one or P0302 for cylinder two. Code P0300 means multiple cylinders are misfiring simultaneously or the misfire is jumping randomly, which often points to a shared issue like an intake air leak or fuel starvation rather than a single failed component.
Will my car pass an NCT or MOT with code P0300?
On the UK MOT, a lit engine management light on a petrol car first used from July 2003 is an automatic fail. For the Irish NCT, an engine warning light or logged emissions code produces an advisory note, but the car will fail the test if the misfire causes tailpipe exhaust emissions to exceed legal limits.
Will clearing the P0300 code solve the problem?
Erasing the code with an OBD scanner only wipes the computer memory and turns off the dashboard light temporarily. As soon as the engine management system completes a drive cycle and detects combustion irregularities again, the light and code will return.
Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.
The warning light you may see
Related fault codes
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General guidance for most cars, not a diagnosis of yours. Details differ by make, model and year - check your handbook, and if in doubt, don't drive.