P0301-P0304: misfire on a specific cylinder and how to localise it

P0301, P0302, P0303 and P0304 name the cylinder. That is a gift. Unlike P0300 random misfire, which could be anything feeding all cylinders, a numbered code narrows the search to one set of parts and one combustion chamber.

Find cylinder 1 before you touch anything

Cylinder numbering is manufacturer-specific and getting it wrong wastes an afternoon.

• Inline engines — usually numbered from the timing belt or chain end, but not always from the front of the car on transverse installations.
• V engines — bank 1 contains cylinder 1. On many transverse V6 layouts bank 1 is the rear bank, against the bulkhead.
• Boxer engines — Subaru numbers odd cylinders on one side, even on the other; see Subaru EJ engine diagnostics.

Confirm against a manual for your specific engine. Every mechanic has replaced a perfectly good coil on the wrong side at least once.

The swap test decides everything

This is the whole diagnosis, and it costs nothing.

1. Note the misfiring cylinder.
2. Swap its ignition coil with a neighbouring cylinder’s. Clear codes, drive, re-read.
   • Misfire followed the coil → the coil is faulty.
   • Misfire stayed → coil is fine.
3. Repeat with the spark plug.
4. Repeat with the injector, if accessible.

When the misfire refuses to move after all three swaps, the fault is in the cylinder itself: compression, valves, or the head gasket.

Reading the plug is free information

Pull the plug from the misfiring cylinder and compare it against its neighbours. Detail in spark plug reading diagnosis, but the short version:

• Wet with fuel — no ignition. Coil or plug.
• Wet with oil — valve stem seals or rings.
• Steam-cleaned, unusually clean — coolant entering. Suspect the head gasket.
• Damaged electrode — detonation, or a foreign object.
• Sooty black — running rich on that cylinder. Leaking injector.

A clean plug in a row of dirty ones is not good news. It means something is washing it.

When the parts are innocent

Compression

A burnt exhaust valve, a broken ring or a jumped timing tooth all produce a stubborn single-cylinder misfire. A compression test settles it; a leak-down test tells you where the air escapes — hissing at the intake means an intake valve, at the exhaust means an exhaust valve, into the cooling system means the gasket.

Injector

A partially blocked injector on one cylinder gives a misfire that appears under load and clears at idle. Injector circuit codes P0201-P0206 cover the electrical side; a mechanically clogged injector sets no circuit code at all.

Vacuum leak at one runner

A cracked intake gasket at a single port leans out exactly one cylinder. Look for it when the misfire is worst at idle and improves under load — the opposite pattern to a fuel delivery problem.

Carbon on the valves

Direct-injection engines with heavy intake deposits misfire cylinder-selectively as the deposits disturb airflow unevenly. Background: GDI carbon buildup.

Reading misfire counters

Most ECUs expose per-cylinder misfire counters through enhanced data or Mode 06. Watching them live is far more informative than the code:

• Counts rising only at idle → vacuum leak or fouled plug.
• Counts rising only under load → weak coil breaking down under cylinder pressure, or a fuel delivery limit.
• Counts rising on cold start only → see P0316 misfire on startup.

Why you should not keep driving

Unburnt fuel from a misfiring cylinder goes straight into the catalyst and burns there. A sustained misfire can destroy a catalyst in a couple of hundred miles. If the check engine light is flashing rather than steady, that is precisely what the ECU is warning about — see flashing vs solid check engine light.

Cost expectations

• Spark plugs: $8–25 each, and always replace as a set.
• Ignition coil: $30–150.
• Injector: $60–300.
• Head gasket: $600–2500 depending on engine.

After the repair

Clear the codes and watch the misfire counter for the repaired cylinder stay at zero through a full drive cycle including load. A count that creeps up slowly means the fault is still present, just below the threshold that sets a code.

Try it on the free tier of the app — pair an ELM327 adapter and watch per-cylinder misfire counts while a helper varies the load. Seeing which cylinder climbs, and when, turns the swap test from guesswork into a two-step process.

Read next

• P0263 to P0266: cylinder contribution and balance faults
• P0305 to P0308: misfire on cylinders 5 to 8 and V-engine numbering
• P0351 to P0358: ignition coil primary and secondary circuit by cylinder