Worn or faulty spark plug on cylinder 1; Failed ignition coil or coil connector on cylinder 1; Faulty or leaking fuel injector on cylinder 1 (blocked, stuck or electrical fault); Vacuum or intake leak affecting cylinder 1 (split hose, intake manifold gasket); Low compression on cylinder 1 (valve, piston ring or head gasket problem); Damaged wiring or poor connection to the coil or injector on cylinder 1; Crankshaft or camshaft sensor fault, or a wider fuel supply fault causing intermittent misfires.

P0301

Cylinder 1 Misfire Detected.

Generic
Powertrain
Generic code
High severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
3: AreaIgnition system or misfire
01: FaultThe specific fault in that area

What it means

P0301 means the engine control unit has detected a misfire on cylinder 1. It monitors crankshaft speed, and when cylinder 1 fails to contribute its share of rotation, or the speed fluctuation is beyond a set limit, it logs the code. A severe misfire can flash the warning light and may also raise catalyst damage concerns.

Symptoms

Engine warning light on or flashing
Rough or unstable idle
Hesitation, stumbling or loss of power under acceleration
Engine shaking or juddering, worse at certain loads
Smell of unburnt fuel from the exhaust
Hard starting or occasional stalling

Possible causes

Worn or faulty spark plug on cylinder 1likely
Failed ignition coil or coil connector on cylinder 1likely
Faulty or leaking fuel injector on cylinder 1 (blocked, stuck or electrical fault)possible
Vacuum or intake leak affecting cylinder 1 (split hose, intake manifold gasket)possible
Low compression on cylinder 1 (valve, piston ring or head gasket problem)possible
Damaged wiring or poor connection to the coil or injector on cylinder 1possible
Crankshaft or camshaft sensor fault, or a wider fuel supply fault causing intermittent misfiresrare

How to diagnose

Read all stored and pending codes and the freeze frame data to see load, rpm and engine temperature when the misfire occurred, and check for other misfire or fuel trim codes.
Visually inspect the cylinder 1 coil, plug lead if fitted, connectors and wiring for damage, corrosion, oil or moisture.
Check the spark plug on cylinder 1 for wear, fouling, damage or the wrong gap, and compare it with the other plugs.
Swap the cylinder 1 coil with another cylinder, clear the code and see if the misfire follows the coil. Repeat with the plug if needed.
Check the fuel injector on cylinder 1 for a working electrical signal and correct resistance, and listen for operation. Check fuel pressure against the specification for the engine.
Inspect for vacuum leaks around hoses and the intake manifold, and check fuel trims for signs of unmetered air.
Carry out a compression and leak-down test on cylinder 1 if the ignition and fuel side are sound.
Check the crankshaft and camshaft sensor signals and the timing if no cylinder-specific fault is found. The procedure depends on the engine.

How to fix

Replace the worn spark plug, ideally as a full set to the correct specification.
Replace the faulty ignition coil or repair its connector.
Clean or replace the faulty fuel injector, or repair its wiring.
Repair the vacuum leak by replacing the split hose or the intake gasket.
Repair the mechanical fault found by the compression test, such as valve, piston ring or head gasket work.
Repair damaged wiring or terminals, then clear the code and road test to confirm the misfire has gone.

Can I keep driving?

High severity

Avoid driving if the warning light is flashing, as unburnt fuel can overheat and damage the catalytic converter. If the light is steady and the engine runs smoothly, drive gently and short distances only until it is repaired.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

This code for other makes

P0301AudiWorn out spark plugs, ignition wires, coil(s), distributor cap and rotor (when applicable), Incorrect ignition timing, Vacuum leak(s), Low or weak fuel pressure, Improperly functioning EGR system, Defective Mass Air Flow Sensor, Defective Crankshaft and or Camshaft Sensor, Defective Throttle Position Sensor, Mechanical engine problems (i.e.—low compression, leaking head gasket(s), or valve problems)