CodeXOBD

P0301 — Cylinder 1 Misfire Detected

P0301 means one specific cylinder is misfiring. That is good news diagnostically — the fault is isolated to one coil, plug, injector or cylinder — but a flashing check engine light still means stop driving.

High

A single misfire with a steady lamp will usually reach a workshop under light throttle. A flashing lamp means unburnt fuel is destroying the catalytic converter and the vehicle should not be driven. This code will cause an emissions test failure.

$10–$600Parts, typical range
0.3–3 hLabour
estimateNot a quote · 2026 basis

What P0301 means

The module counts the crankshaft deceleration that follows each failed combustion event and attributes it to a cylinder. When the count for cylinder 1 alone crosses the threshold, it sets P0301 rather than the random-misfire code. Because everything upstream — fuel pressure, air, timing — is shared with the cylinders that are firing correctly, a single-cylinder misfire points at the parts unique to that cylinder, and the diagnosis is largely a process of swapping them to see whether the fault follows.

Technical summary
SystemPowertrain — ignition and misfire

Symptoms you may notice

What causes P0301

Ignition coil on cylinder 1 has failed

Very likelylow costeasy

Coils fail individually and are the most common single-cylinder misfire cause on engines with coil-on-plug ignition. The classic confirmation costs nothing: swap the coil with a neighbouring cylinder, clear the code, and see whether the misfire moves with it.

Common misdiagnosis

Swap before buying. If the misfire stays on cylinder 1 after moving the coil, the coil was never the fault.

Spark plug on cylinder 1 fouled or damaged

Likelylow costmoderate

A cracked insulator, a closed gap or oil fouling affects one cylinder only. Read the plug when it comes out — oil fouling points at valve seals or rings, and replacing the plug alone will not hold.

Injector on cylinder 1 clogged or failed

Possiblemedium costmoderate

A partially blocked injector starves one cylinder while the others run correctly. It can often be heard: a healthy injector ticks steadily, a dead one is silent under a stethoscope.

Cylinder 1 has lost compression

Possiblevery-high costprofessional

A burnt valve, a failed head gasket or worn rings prevent that cylinder from building enough pressure to burn cleanly. This is the diagnosis that remains once spark and fuel are confirmed good, and the most expensive of the four.

How to diagnose it

1

Check whether the check engine light is flashing

Look at the lamp with the engine running. A flashing lamp is a different level of urgency from a steady one.

Safety

Continuing to drive with a flashing misfire lamp destroys the catalytic converter, typically at several times the cost of the misfire repair.

If it passes: The lamp is steady. Continue to step 2.If it fails: The lamp is flashing. Stop driving and have the vehicle recovered before the converter is damaged further.
2

Swap the coil with an adjacent cylinder

Move the coil from cylinder 1 to a cylinder that is firing correctly, and that cylinder’s coil to position 1. Clear the code and run the engine until the misfire returns.

Tools: Socket set, OBD-II scannerExpected: The misfire counter moves to the cylinder the suspect coil was fitted toIf it passes: The misfire stays on cylinder 1. The coil is good — continue to step 3.If it fails: The misfire follows the coil to the other cylinder. Replace that coil.
3

Remove and read the spark plug

Take out the plug from cylinder 1 and compare it against one from a healthy cylinder. Check the gap, the insulator for cracks, and the colour.

Safety

Let the engine cool before removing plugs from an aluminium head, or the threads can strip.

Tools: Spark plug socket, Feeler gaugeIf it passes: The plug is clean and within specification. Continue to step 4.If it fails: Replace the plug. If it was oil-fouled, investigate why oil is reaching the chamber before assuming the job is done.
4

Listen to the injector and test its resistance

With the engine idling, listen to the cylinder 1 injector through a stethoscope. Then measure its resistance and compare against the others.

Tools: Mechanic’s stethoscope, MultimeterExpected: A steady tick, and resistance within a few percent of the other injectorsIf it passes: The injector ticks and matches the others electrically. Continue to step 5.If it fails: Replace or clean the injector.
5

Compression test cylinder 1

With the fuel system disabled, crank the engine and record compression on cylinder 1 and on two healthy cylinders for comparison.

Safety

Disable fuel and ignition before cranking, and keep clear of belts and the fan.

Tools: Compression testerIf it passes: Compression matches the others. Re-examine the wiring to the coil and injector.If it fails: Cylinder 1 is significantly lower. The fault is mechanical — a leak-down test will locate it.

Related codes

P0300P0302

Common questions

Does P0301 always mean the coil?

No, but the coil is the most common cause on modern engines and the cheapest to rule out. Swapping it with a neighbouring cylinder tells you in ten minutes and costs nothing.

Can I drive with one cylinder misfiring?

Briefly, with a steady lamp and light throttle, to reach a workshop. With a flashing lamp, no — raw fuel is destroying the catalytic converter.

Which cylinder is number 1?

It varies by engine layout and manufacturer, and guessing wrong wastes a whole diagnosis. Check the firing order diagram for your specific engine before removing anything.

Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)

This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.