P0301: Cylinder 1 Misfire Detected

What does P0301 mean?

The ECM measures small changes in crankshaft speed to detect when a cylinder does not burn its mixture properly. P0301 means it has linked the misfires to cylinder 1. Codes P0302 to P0312 mean the same for other cylinders.

Where cylinder 1 is located depends on the engine. Check a service manual or a reliable source for your engine's firing order and cylinder numbering before swapping parts.

A flashing check engine light means the misfire is severe enough to damage the catalytic converter.

Common symptoms

• Rough idle or shaking
• Loss of power or hesitation
• Flashing check engine light during heavy misfires
• Fuel smell from the exhaust
• Lower fuel economy

Common causes

The causes below are the ones commonly seen with P0301. The actual cause can vary by vehicle, engine and mileage, so treat this list as a starting point, not a diagnosis.

• A worn spark plug in cylinder 1
• A faulty ignition coil or plug wire on cylinder 1
• A clogged or failing fuel injector on cylinder 1
• A vacuum leak near cylinder 1's intake runner
• Low compression from a valve or head gasket problem
• Damaged wiring or connector to the coil or injector

What to check with a scanner

• A common DIY test is to swap the cylinder 1 coil with a neighboring cylinder, clear the code and drive. If the misfire moves to that cylinder, the coil is the likely cause. Do this only if you are comfortable working on the engine.
• Check Mode 06 misfire counters, where supported, to confirm which cylinder is affected and whether others are close to the limit.
• Check freeze frame data for rpm, load and temperature when the code set.
• Look at fuel trims in Live data to rule out a wider lean condition.

How to read and clear P0301 with OBD Master Pro

1. Plug in your adapter. Connect a generic ELM327 (or ELM329) OBD2 adapter to the diagnostic port, usually under the dashboard on the driver's side. Bluetooth Classic, Bluetooth LE, Wi-Fi and USB OTG adapters are supported.
2. Switch the ignition on. Turn the key to ON or start the engine, depending on your car.
3. Connect in OBD Master Pro. Open the app, go to Connection and pick your adapter. Classic Bluetooth adapters connect from the Bluetooth tab, Bluetooth LE adapters from the BLE tab with no pairing needed. Use the generic OBD2 / EOBD profile or your car's brand profile.
4. Read the codes. Open the fault codes screen to see stored, pending and permanent codes. Confirm P0301 is listed and note any other codes, since they often explain the cause.
5. Check the details. Look at freeze frame data, Live data and, where your car supports it, Mode 06 test results (see the checks above).
6. Fix, then clear. After the repair, clear the codes to turn off the check engine light. Clearing also resets the emissions readiness monitors, so drive normally and check the readiness screen before any inspection. Permanent codes cannot be erased with a scan tool; they clear on their own once the car's self test passes.

Tip: you can also look up P0301 and other codes in the app's DTC search without connecting to a car. If your adapter will not connect, use Connection > Share connection log and email it to support. Many cheap ELM327 clones only handle basic engine OBD2, which is all you need for P0301.

When to see a mechanic

Get professional help if any of these apply:

• The check engine light is flashing
• The misfire stays on cylinder 1 after swapping the coil and plug
• You suspect low compression or a mechanical problem
• There is coolant loss or white smoke

If you are unsure, a qualified technician can confirm the cause with tests such as smoke testing, pressure testing or scope readings. Clearing a code does not repair the fault.

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