Can you keep driving?
Can you keep driving?
Stop driving if any of these apply
- !The check-engine light is flashing.
- !The engine is running badly enough that traffic safety or engine damage becomes a concern.
What to check first
Step-by-step checks
- 1
Safety first
If the check-engine light is flashing or the engine is shaking badly, stop driving before you inspect anything else
- 2
Free - no tools
Check for other codes that point to a lean condition, coil fault, or cylinder-specific misfire
- 3
Basic tool needed
Look for obvious vacuum leaks, loose connectors, or anything recently left unplugged in the engine bay
- 4
Basic tool needed
If scan data is available, compare misfire counts and fuel trims before buying parts
- 5
Basic tool needed
If the engine is missing mainly at idle or under load, note that pattern before moving to the next step
If the code returns
- -If the code appears with a lean code, look harder for unmetered air or a fuel-delivery problem.
- -If the fault is strongest under load, ignition parts and fuel supply move higher on the list.
- -If the code returns immediately after clearing, focus on the active fault rather than the stored history.
Background
What this code means
P0300 is a generic OBD-II random misfire code.
It usually means the misfire pattern is spread across more than one cylinder, so the cause can be ignition, air, fuel, or sometimes a mechanical issue rather than a single bad plug.
Diagnosis
Common causes
Ignition weakness
Worn spark plugs, weak coils, or poor coil connections are among the most common first checks.
Vacuum leak
A split hose or intake leak can create a lean spot that shows up as a random misfire.
Fuel delivery issue
Low fuel pressure or a clogged injector can leave more than one cylinder short of fuel.
Mechanical fault
Low compression or valve sealing issues can cause a persistent misfire pattern.
Avoid these mistakes
What not to do
- xDo not replace major parts before checking the simple causes first.
- xDo not ignore a flashing check-engine light or obvious drivability symptoms.
Parts
Parts that may need replacing
See also
Related OBD codes
Source notes
Prototype Porsche overlay for generic misfire guidance.
- -Porsche engines can make coil, plug, and intake checks useful before broad parts replacement.
This guide is written with Porsche vehicles in mind, but bulletin history, engine variants, and component locations can still change the best repair path.
This is generic OBD-II guidance and should not override vehicle-specific service information. Some makes have model-specific misfire patterns, service bulletins, or coil-plug test procedures that should be checked before ordering parts.
Last reviewed: 2026-04-11