P0300
Random / multiple cylinder misfire detected
OBD code
RP-1002
Random or multiple cylinder misfire (P0300)
Narrow a P0300 random misfire to a single cylinder, a shared ignition component, a fuel delivery problem, an air leak or a mechanical fault — and prove the cause before replacing parts.
Intended outcome
A confirmed misfire cause demonstrated by an improvement in misfire counter data or a repeatable cylinder balance result.
Before you start
Safety first
Read before you start
Safety warnings
A flashing check engine light means the misfire is severe enough to damage the catalytic converter. Reduce load and diagnose before continuing to drive. Never touch ignition components while the engine is running.
Record freeze-frame data for the code before clearing anything: engine speed, load, coolant temperature and fuel trims at the moment the fault set. Check whether the misfire is worse cold, hot, at idle or under load.
Tools, parts and consumables
| Tools, parts and consumables | Category | Applies to |
|---|---|---|
| OBD scan tool with misfire counter and freeze-frame support |
Tool
Required
|
— |
| Spark plug socket and torque wrench Tighten plugs to the figure published for the exact engine — never to a generic value. |
Tool
Required
|
— |
| Fuel pressure gauge |
Tool
Required
|
— |
| Compression or leak-down tester |
Tool
Required
|
— |
| Leak-detection spray |
Tool
Optional
|
— |
| Spark plugs, correct specification for the engine Confirm the plug part number against the vehicle documentation. |
Part
Optional
|
— |
Compatible vehicles
-
petrol
Not verified
This vehicle applicability has not been confirmed against the manufacturer documentation for your exact vehicle.Applies to gasoline engines with per-cylinder misfire detection. Compression and plug specifications must be taken from the vehicle documentation.
Possible causes and priorities
-
Worn or fouled spark plugs, or an incorrect plug gap
high -
Failing ignition coil or coil-on-plug module
high -
Vacuum or intake air leak affecting one bank
medium -
Low fuel pressure or a restricted injector
medium -
Mechanical fault: valve sealing, worn camshaft lobe or low compression
medium -
Engine management input fault (MAF, crank/cam correlation) affecting all cylinders
low
Diagnostic workflow
-
Diagnostic
Capture freeze-frame and misfire counters
Read stored and pending codes, then record freeze-frame data. Display per-cylinder misfire counters where the tool supports them and note which cylinders increment.
What did you find? A known set of affected cylinders and the load, speed and temperature at which the fault sets.
-
Caution
One cylinder is clearly affected
Treat it as a single-cylinder fault: ignition and injector first.
-
Caution
Several or all cylinders are affected
Look for a shared cause: air leak, fuel pressure, ignition supply or engine management input.
-
Caution
One cylinder is clearly affected
-
Diagnostic
Shared-cause checks
Read long- and short-term fuel trims at idle and at 2500 rpm. Spray a suitable leak-detection agent around intake joints with the engine idling. Measure fuel pressure and check the supply to every coil.
What did you find? Fuel trims near neutral, no change in idle when a joint is sprayed, and fuel pressure within the general range for the system.
-
Fault
Positive fuel trims and a reaction to leak detection
Unmetered air is the cause. Repair the leak, then recheck trims.End of this branch
-
Fault
Fuel pressure below the general range
Fuel delivery fault. Confirm at the pump and at the rail before replacing the pump.End of this branch
-
Normal
Shared checks are normal
Continue to the single-cylinder checks.
-
Fault
Positive fuel trims and a reaction to leak detection
-
Diagnostic
Isolate the cylinder with a swap test
Move a known-good coil (and, where accessible, the injector) to the affected cylinder and observe whether the misfire counter follows the part or stays with the cylinder.
What did you find? The misfire follows the component, or stays with the cylinder.
-
Fault
The misfire follows the component
That component is faulty. Replace only that part.End of this branch
-
Caution
The misfire stays with the cylinder
The cylinder itself is the problem: injector, compression or valvetrain. Continue.
-
Fault
The misfire follows the component
-
Diagnostic
Inspect the plug and run a cylinder test
Remove the plug from the affected cylinder and compare it with a known-good cylinder. Run a relative compression test, and a cylinder leak-down test if the result is uneven.
What did you find? A plug in the same condition as the others, and compression comparable with the neighbouring cylinders.
Measurement: bar
-
Caution
The plug condition differs
Oil, coolant or fuel fouling each point at a different root cause. Investigate the fouling rather than just replacing the plug.End of this branch
-
Fault
Compression is low on that cylinder
Mechanical fault: valve sealing, ring sealing or a camshaft fault.End of this branch
-
Inconclusive
Plug and compression are normal
The injector is now the leading suspect. Test injector flow or replace with a known-good unit to confirm.End of this branch
-
Caution
The plug condition differs
Technical specifications
| Value | Applies to | Source | Technical verification status |
|---|---|---|---|
|
Spark plug gap
As published for the exact engine
|
All engines | Unverified | Not verified |
|
Not confirmed against the manufacturer documentation for your exact vehicle. Verify with the correct workshop manual before relying on this value. Confirm every measurement and torque value using the correct vehicle documentation before you work on the vehicle.
This library deliberately does not publish a generic plug gap. Direct-injection and turbocharged engines differ from older naturally aspirated engines, and an incorrect gap causes the fault you are chasing. |
|||
|
Compression variation between cylinders
Generally within about 10 % of the highest cylinder %
|
General guidance |
General educational guidance
Established general engine diagnostic practice
|
Not verified |
|
Not confirmed against the manufacturer documentation for your exact vehicle. Verify with the correct workshop manual before relying on this value. Confirm every measurement and torque value using the correct vehicle documentation before you work on the vehicle.
A rule of thumb for spotting an uneven cylinder, not a pass/fail specification. Always compare against the manufacturer figure for the engine. |
|||
Verification checklist
- Misfire counters stop incrementing on the affected cylinder
- Code does not return after a road test covering the original conditions
- Fuel trims return within the normal band at idle and under load
- Freeze-frame conditions reproduced without a repeat fault
Common mistakes
Replacing all coils and plugs at once "to be safe" hides the actual cause and leaves the fault unresolved. Isolate the cylinder first. Second: treating a P0300 as an ignition fault by default — a shared air leak or a fuel pressure fault produces exactly the same random misfire pattern.
Frequently asked questions
Is P0300 always more serious than P0301?
P0300 means the module cannot attribute the misfire to a specific cylinder, or that several cylinders are misfiring. It is a broader finding, not automatically a worse one, but it does point at shared causes first: air, fuel pressure, ignition supply or engine management inputs.
Why does the misfire disappear above idle?
A small vacuum leak or a marginal injector is most disruptive when the engine needs little fuel and is most sensitive to mixture. That pattern points at a leak or a partially restricted injector rather than a failed coil.
References and sources
- What does P0300 mean and how to fix it General educational guidance · AutoTools
- How to change spark plugs (DIY guide) General educational guidance · AutoTools
- Fault code P0300 — random / multiple cylinder misfire Trusted technical reference · AutoTools OBD reference
Follow this procedure step by step
Mark each step complete, branch on what you actually find, and keep your notes.
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