Is replacing the spark plugs enough?

Spark plugs are a common cause, but with P0300 they are not the only one. If they are old, the mechanic replaces them; otherwise they check the coils, the fuel supply and the intake system first, so the new plugs are not wasted.

Will my car pass its MOT with code P0300?

In the UK, an engine management light that shows a fault is a major defect, so the car fails its MOT. This applies to most petrol cars first used from 1 July 2003. Elsewhere in Europe it depends on your country’s inspection rules; the EU roadworthiness directive lists an OBD read-out showing a significant malfunction as a reason for failure on petrol cars. This fault also affects exhaust emissions, so the car may fail the emissions test as well. It is safer to have it fixed before the test.

P0300 Code: Random/Multiple Cylinder Misfire

Q: Can I drive with code P0300?

Not if the check engine light is flashing. Stop when it is safe, because unburnt fuel can destroy the catalytic converter. If the light stays on steadily and the engine runs almost normally, drive gently to a garage.

When the code is set

Each combustion gives the crankshaft a brief push. The ECM uses the crankshaft position sensor to monitor how evenly the crankshaft turns. When a cylinder misfires, the crankshaft slows for a moment. When there are too many of these slowdowns and they cannot be pinned to one cylinder, the ECM stores P0300. With severe misfires the check engine light flashes.

Can I keep driving?

Have it checked soon

If the check engine light flashes, stop when it is safe and do not drive on: unburnt fuel can destroy the catalytic converter. If the light stays on steadily and the engine runs almost normally, drive gently without hard acceleration and have the car checked as soon as possible. If the engine shakes badly or loses power, it is better to have it taken to a garage.

Symptoms

The check engine light is on or flashing.
The engine shakes or runs unevenly, especially at idle.
Jerking and poor response when accelerating.
Hard starting.
Higher fuel consumption.

Common causes

Worn or damaged spark plugs.
Faulty ignition coils or spark plug leads.
An air leak into the intake system (a cracked hose or a gasket).
Low fuel pressure or dirty injectors.
Wrong sensor readings that make the mixture too lean (mass air flow sensor, oxygen sensor).
Mechanical faults, such as low compression, a damaged valve or a head gasket.
On some cars, simply running very low on fuel.

How to find the cause

A mechanic usually works like this:

Read all codes, since P0300 often comes with codes for individual cylinders (P0301 to P0304) or for the mixture.
Check the ECM data to see which cylinders are counting misfires and under what conditions.
Inspect the spark plugs and coils, and check the intake system for air leaks.
Measure the fuel pressure and check the mass air flow sensor and the oxygen sensor.
If ignition and fuel are fine, run a compression test.

Other ways the code is written

Scan tools and workshop printouts sometimes show the code with extra characters:

P030000

Related codes

Frequently asked questions

Evoca fault code diagnostics is an informational aid and does not replace a professional inspection of your vehicle. Only an inspection of the vehicle at a workshop can confirm the cause of a fault.

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P0300 code: random or multiple cylinder misfire