Motor Vehicle Expert
OBD fault code guide

P0300 Code Meaning UK

P0300 means “Random or Multiple Cylinder Misfire Detected”. In plain English, the engine is not burning fuel properly and the misfire is not limited to one clear cylinder. It can be caused by spark plugs, ignition coils, injectors, vacuum leaks, fuel pressure problems, timing faults, compression issues or sensor faults.

📅 Updated June 2026 🇬🇧 UK diagnostic guide 🛠 Misfire and rough running fault
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✓ P0300 meaning ✓ Symptoms and causes ✓ UK repair costs ✓ MOT-aware advice
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Quick answer

What does P0300 mean?

P0300 means the ECU has detected a random or multiple cylinder misfire. A misfire happens when one or more cylinders do not burn the air and fuel mixture correctly. Because P0300 is not tied to one specific cylinder, the problem may affect several cylinders or the ECU may not be able to identify one clear cylinder.

Most likely area

Ignition, fuel delivery, air leaks, compression, timing or engine management sensors.

Urgency

High if the engine light is flashing, the car shakes badly, smells of fuel or loses power.

MOT risk

High if the misfire affects emissions, causes smoke or leaves the engine warning light on.

Code meaning

P0300 — Random or Multiple Cylinder Misfire Detected

The full description is usually P0300 Random or Multiple Cylinder Misfire Detected. Unlike P0301, P0302, P0303 or P0304, which point to a specific cylinder, P0300 means the misfire is random, moving between cylinders or affecting more than one cylinder.

This matters because a random misfire can be caused by a shared problem such as low fuel pressure, an intake air leak, contaminated fuel, timing issues, weak battery voltage, crankshaft sensor problems or a general ignition fault.

Mechanic view

Why P0300 should not be ignored

A misfire can send unburnt fuel into the exhaust. That fuel can overheat and damage the catalytic converter, turning a simple ignition fault into a much more expensive repair. If the engine warning light flashes, that usually means the misfire is severe enough to risk catalyst damage.

The correct repair depends on diagnosis. Replacing all coils, plugs or injectors without testing can get expensive and still miss the real cause.

Symptoms

Common symptoms of a P0300 code

P0300 is usually easier to feel than some other fault codes because a misfire often changes how the engine runs.

Engine shaking

The car may vibrate, judder or shake at idle because the engine is not firing smoothly.

Flashing engine light

A flashing engine management light usually means the misfire is serious and could damage the catalytic converter.

Rough idle

The engine may idle unevenly, hunt, dip, stumble or feel like it wants to stall.

Poor acceleration

The car may hesitate, jerk, feel flat or struggle under load when accelerating.

Fuel smell

A strong fuel smell can suggest unburnt fuel is entering the exhaust due to incomplete combustion.

Limp mode

Some cars reduce power to protect the engine and catalytic converter when a serious misfire is detected.

Common causes

What causes a P0300 fault code?

P0300 can be caused by ignition faults, fuel problems, air leaks, mechanical engine issues or sensor faults.

Common

Worn spark plugs

Old, fouled or incorrect spark plugs can cause weak combustion and random misfires.

Common

Faulty ignition coils

Weak coils can break down under load and cause shaking, hesitation and flashing engine lights.

Fuel system

Injector problems

Blocked, leaking or weak injectors can cause one or more cylinders to run incorrectly.

Air leak

Vacuum or intake leak

A split hose, loose intake pipe or leaking gasket can make the mixture too lean and trigger misfires.

Fuel pressure

Weak fuel pump or restriction

Low fuel pressure can cause multiple cylinders to misfire, especially under load.

Mechanical

Low compression

Valve, piston ring, head gasket or timing problems can cause a misfire that ignition parts will not fix.

Timing

Timing chain or belt issue

Incorrect valve timing can cause rough running, poor starting, misfire codes and reduced power.

Sensor

Crank or cam sensor fault

Incorrect engine position signals can confuse ignition and injection timing.

Basic issue

Low voltage or poor earth

Weak battery voltage, alternator faults or poor earth connections can cause unstable engine control.

Diagnosis

How a garage should diagnose P0300

✅ Scan all stored, pending and permanent codes.

✅ Check for cylinder-specific codes such as P0301, P0302, P0303 or P0304.

✅ Record freeze frame data before clearing the code.

✅ Inspect spark plugs for wear, oil fouling, coolant marks or incorrect gaps.

✅ Check ignition coils and swap-test only where appropriate.

✅ Check for vacuum leaks, split intake pipes and loose hose clips.

✅ Check fuel pressure and injector operation if ignition parts test correctly.

✅ Carry out compression or leak-down testing if the misfire remains unexplained.

Avoid this mistake

Do not just replace random parts

P0300 is often misdiagnosed because people replace spark plugs and coils first, then keep replacing parts when the code comes back. Sometimes plugs and coils are the problem, but random misfires can also come from fuel pressure, air leaks, compression loss, timing issues or sensor faults.

A good diagnosis follows the evidence: codes, freeze frame data, live data, visual inspection, ignition checks, fuel checks and mechanical checks.

UK repair cost guide

How much does P0300 cost to fix in the UK?

Costs vary by vehicle, engine access, labour rate and the real cause of the misfire. These are rough UK guide prices only.

Diagnostic scan and checks

Usually around £40–£120 depending on how much testing is included.

Spark plug replacement

Often around £80–£200 depending on engine layout and plug type.

Ignition coil replacement

Common repair for misfires, often around £80–£300 depending on number of coils and access.

Injector diagnosis or replacement

Can become more expensive, especially on diesel or direct-injection petrol engines.

Vacuum leak repair

Small hose repairs can be affordable, but manifold gasket or intake work can cost more.

Compression or timing repair

Mechanical engine faults can be much more expensive and need careful confirmation first.

Safe driving advice

Can you drive with a P0300 code?

P0300 should be treated more seriously than many other engine codes because a misfire can damage the catalytic converter and leave the car unreliable. If the engine light is flashing, the car is shaking badly or there is a strong fuel smell, avoid driving and get it checked.

✅ Lower risk: mild stored code, no shaking, steady engine light and normal performance.

⚠️ Medium risk: rough idle, hesitation, poor fuel economy or repeated engine light.

🚫 High risk: flashing engine light, strong misfire, fuel smell, smoke, overheating or major loss of power.

Real experience

What I would check first

On a real car with P0300, I would first check whether there are also cylinder-specific codes. Then I would inspect the spark plugs, coils, intake pipes and live misfire data. If the misfire affects all cylinders, I would look closely at fuel pressure, air leaks, timing and sensor signals.

If only one cylinder keeps misfiring, the diagnosis should narrow towards that cylinder’s plug, coil, injector, compression or wiring.

MOT implications

Will P0300 fail an MOT?

P0300 can lead to an MOT failure if the vehicle runs badly, fails emissions, shows an engine management light related to emissions, produces visible smoke or has an obvious misfire during the test. A misfiring engine can also damage the catalytic converter, which can then cause further emissions faults such as P0420.

Emissions failure

Poor combustion can increase hydrocarbons and make the car fail an emissions test.

Warning light issue

An illuminated or flashing engine warning light is a serious concern before an MOT.

Poor running

A car that is visibly misfiring, smoking or running badly should be repaired before testing.

Engine light MOT guide Common MOT failures
Frequently asked questions

Frequently Asked Questions About P0300

Find answers to common questions about P0300 random misfires, driving safety, repair costs and MOT implications.

What does P0300 mean?

P0300 means the engine computer has detected a random or multiple cylinder misfire. The engine is not burning fuel properly on more than one cylinder, or the ECU cannot identify one exact cylinder.

Can I drive with a P0300 code?

You should be very careful. If the engine light is flashing, the engine is shaking, power is poor or the car smells strongly of fuel, avoid driving because unburnt fuel can damage the catalytic converter.

Will P0300 fail an MOT?

P0300 can lead to an MOT failure if the car runs badly, has high emissions, shows an engine warning light related to emissions, produces smoke or has an obvious misfire during the test.

What causes a P0300 code?

Common causes include worn spark plugs, faulty ignition coils, injector problems, vacuum leaks, low fuel pressure, poor compression, timing faults, sensor problems or contaminated fuel.

How much does P0300 cost to fix in the UK?

Simple ignition repairs may cost around £80 to £300, but injector faults, compression problems, timing issues or deeper engine problems can cost significantly more depending on the vehicle.

Should I clear a P0300 code?

Record the code, freeze frame data and any cylinder-specific codes before clearing it. Clearing P0300 without diagnosis can hide useful evidence and the misfire may return.

About this guide

Based on practical mechanic-style diagnostic experience

Motor Vehicle Expert publishes practical UK vehicle guidance based on real-world mechanical knowledge and hands-on diagnostic experience. This P0300 guide is written to help drivers understand random and multiple cylinder misfires before replacing expensive parts.

Fault codes should always be treated as a starting point for diagnosis. If your car has a flashing engine light, severe misfire, strong fuel smell, smoke, overheating or major power loss, stop driving and get professional help.