Ignition and misfire

P0303Cylinder 3 misfire detected

Cylinder 3 Misfire Detected

Stop driving or get it looked at now

Generic OBD-II code

Ignition/Combustion System (Misfire Monitor)

What this part does

The engine control module monitors crankshaft speed fluctuations to detect a cylinder that is not firing or burning fuel completely; the underlying cause is usually ignition, fuel delivery, or mechanical (compression/valve-train).

What the computer is measuring

The computer detects a misfire by watching crankshaft speed rather than by listening to the engine. Each firing cylinder accelerates the crankshaft a little, and the crankshaft position sensor resolves those individual contributions. A cylinder that fails to fire leaves a measurable gap, and because the computer knows the firing order it can name the cylinder responsible.

P0303 names cylinder three in the manufacturer’s numbering. That last part matters more than it sounds. On an inline four, cylinder three is the third one along and there is little room for confusion. On a transverse V6, numbering commonly runs one to three down the front bank and four to six down the rear one, or the reverse, depending on the make. If you guess, you may end up replacing a perfectly good coil on a cylinder that was never misfiring.

Once you have the right cylinder, the diagnosis is short. Only four things belong to that cylinder on its own, and three of them can be moved to a different cylinder to see whether the fault moves too. That swap test is the fastest honest answer available on this code, and it costs nothing but time.

What you will notice

  • Flashing check engine light while misfiring, which means stop and shut it off.
  • Rough idle and vibration through the car.
  • Hesitation and reduced power.
  • Unburnt fuel smell and a drop in economy.

Likely causes, most common first

Ordered by how often each one actually turns out to be the problem, not alphabetically and not by part price.

  1. A failed ignition coil on cylinder 3

    The most common cause on coil on plug engines, and the first thing to swap.

  2. A worn or fouled spark plug

    Cheap, common, and the deposits on the old one narrow down what caused the failure.

  3. A restricted or failed injector on cylinder 3

    Behaves exactly like an ignition fault from the seat, so it has to be ruled out by test rather than by feel.

  4. Low compression in cylinder 3

    The expensive outcome. Confirm it with a gauge before accepting it.

  5. A cracked intake runner or leaking port gasket at cylinder 3

    Unmetered air at one port leans that cylinder enough to misfire while the rest run normally.

What to check, in order

  1. Confirm which physical cylinder is number three on this engine.

    On a transverse V6 the numbering usually runs along one bank and then the other, not left to right across the front. Working on the wrong cylinder makes every later test meaningless.

  2. Swap the coil from cylinder 3 to another cylinder, clear the codes and drive it.

    If the misfire follows the coil, that is the fault. If it stays on cylinder 3, the coil is good.

  3. Swap the spark plug and inspect the one you removed.

    A fault that follows the plug is the plug. A wet or oily plug points at fuel delivery or oil control instead.

  4. Compare the injectors, then compression test cylinder 3 against its neighbours.

    Even injectors and even compression put the cause back on the intake side, so look for a port leak.

Where this one turns up most

  • Rear bank cylinders on transverse V6 engines, where coils run hotter and are rarely replaced preventively.

Codes that often show up alongside it

What a DriveDecode adapter can read

A DriveDecode adapter reads engine and emissions systems: the powertrain and emissions codes the OBD-II standard requires on every US vehicle it covers. It does not read ABS, airbag (SRS), TPMS or transmission-specific manufacturer codes. Those need make-specific decoding that generic OBD-II hardware cannot do.

Reading P0303 on your own car

DriveDecode pulls P0303 straight off the vehicle along with the freeze frame the computer stored when it set: the coolant temperature, engine load and speed at that exact moment. That context is what separates a fault that happens on a cold start from one that happens under load, and it is the first thing the list above asks you to narrow down.

See how DriveDecode works

Last reviewed 2026-09-05. Definition sourced from the published standard.