What P0301 means
P means powertrain (engine and transmission). The 0 marks a generic code defined by SAE, so the same number means the same thing across makes. The third character, 3, places it in the ignition system or misfire subsystem; the last two digits identify the specific fault.
The engine computer detects misfires by watching tiny changes in crankshaft speed: each cylinder that fires properly gives the crank a push, and one that does not shows up as a momentary slowdown. It counts misfires per cylinder and sets a code when the rate is high enough to raise tailpipe pollution or risk the catalytic converter.
Cylinder numbering is set by the manufacturer, so check the cylinder layout for your engine to find cylinder 1; it is not always the one nearest the timing belt or chain.
What “Misfire Detected” means here
The misfire monitor has counted enough misfires on cylinder 1 to exceed its threshold. Because one cylinder is named, the cause is usually local to it: that cylinder's spark plug, coil, injector, or compression.
What you may notice
Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany P0301:
- Shaking or rough running, often worst at idle or under load
- A flashing check engine light during severe misfiring
- Loss of power and hesitation
- Fuel smell from the exhaust
Causes to consider
- Faulty cylinder 1 ignition coil
- Fouled spark plug #1
- Clogged injector #1
Where to start diagnosing P0301
- Start with ignition: inspect the spark plug and coil for the cylinder concerned, and swap the coil with a neighbouring cylinder to see if the misfire moves.
- Then fuel: check the injector connector and resistance, and swap injectors if practical.
- Then mechanical: a compression or leak-down test identifies valve, ring or head gasket problems.
- Check for vacuum leaks near the affected cylinder's intake runner.
Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.
Live data to watch
These standard OBD2 values relate to P0301. Support varies by vehicle; a value your car does not report will simply be missing.
| Value | What to look for |
|---|---|
| Engine RPM (PID 0x0C) | Unstable idle speed. |
| Misfire counts | Per-cylinder counts are available through Mode 06 on many vehicles. |
| Short and long term fuel trim (PIDs 0x06–0x09) | A lean bank points to a vacuum leak or fuel delivery issue. |
Related codes
Codes from the same misfire monitor family. If several are stored together, the shared part — wiring, a common reference voltage, or the component itself — is the place to start.
Further reading
Symptoms that lead to P0301
If you have not scanned the car yet, or P0301 is one of several codes stored, these guides start from what the vehicle is actually doing.
Frequently Asked Questions
What does P0301 mean?
P0301 is the generic SAE J2012 code for "Cylinder 1 Misfire Detected". In plain terms, it means the engine computer's misfire monitor monitor has reported "Cylinder 1 Misfire Detected".
Can I keep driving with P0301?
If the check engine light is flashing, reduce speed and load and get the car looked at: unburnt fuel can overheat and destroy the catalytic converter.
What should I check first for P0301?
Start with ignition: inspect the spark plug and coil for the cylinder concerned, and swap the coil with a neighbouring cylinder to see if the misfire moves.
Definition source: SAE J2012 / ISO 15031-6 generic DTC definitions. This is the generic meaning that applies across makes; a manufacturer can define a code more narrowly or add its own sub-codes, so confirm against the service information for your vehicle before replacing parts. Written by the OBDAssistant engineering team — see our editorial policy.