EngineLightCodes
Powertrain code

P0304 Cylinder 4 Misfire Detected

Severity
Stop soon
Typical repair cost
$30–$880
Do it yourself
Possible, easy
Shop time
0.3–4 hours

The engine computer has detected that cylinder 4 is misfiring, so the engine shudders or hesitates, often worst at idle. On many four-cylinder engines cylinder 4 is the last one, next to the transmission, where oil from a leaking valve cover can pool in the plug well. A failing coil or a fouled spark plug is the usual cause.

How the engine computer detects it

The engine module detects misfire from crankshaft speed: a cylinder that does not fire gives no acceleration impulse. At a catalyst-damaging rate (CARB: at least 5 % per 200 revolutions) the warning light flashes; at an emissions-relevant rate (at least 1 % per 1,000 revolutions – GM about 2–5 %, Ford 0.9–1.5 %) it comes on after two trips. Ford and GM may shut off a misfiring cylinder's injector to protect the catalyst. P0304 means more than 90 % of the detected misfires fall on cylinder 4. Usual causes are the ignition coil or spark plug of that cylinder, its injector, or low compression; a quick test is to swap coil or plug with a neighbouring cylinder and see whether the misfire moves. Bulletins for four-cylinder engines name internal coil faults (Mazda) and plugs cracked during installation (FCA).

Written from public technical sources and not yet checked by a mechanic. How we write these pages

Symptoms

  • Check engine light on, flashing at a catalyst-damaging rate
  • Rough idle, jerking, vibration
  • Loss of power, hesitation
  • Smell of unburnt fuel, higher consumption

Common causes

  1. Ignition coil of cylinder 4 faulty (coil, connector, ground) Most likely
  2. Spark plug of cylinder 4 worn, sooted, cracked or wrongly gapped Most likely
  3. Injector of cylinder 4 clogged or leaking, or its control Possible
  4. Low compression on cylinder 4 (valve, rings, head gasket) or local false air Less likely
  5. Water or oil in the plug well Less likely

How to diagnose P0304

  1. Locate cylinder 4. On most inline four-cylinder engines it's at the transmission end; on V engines check the numbering diagram for your engine.
  2. With the ignition off, remove the cylinder 4 coil and look into the plug well. Oil there means the valve cover or spark plug tube seal is leaking and should be replaced along with any damaged parts.
  3. Swap the cylinder 4 coil with a neighbor, clear the code and drive. If the misfire moves with the coil, replace it.
  4. Check the cylinder 4 spark plug for oil fouling, wear or cracks. Replace the full set if they're due.
  5. If the misfire stays on cylinder 4, have the injector tested and a compression test done to check the valves, rings and head gasket.

Vehicles that report P0304 most

VehicleYearsReportsMedian miles
Ford Escape2013–20195396k
Ford Edge2015–20183262k
Honda CR-V2000–201922110k
Honda Odyssey2003–20162084k
Honda Accord2011–20222044k
Ford Fusion2013–201918101k
Honda Pilot2011–20191487k
Chevrolet Silverado 15002008–20239124k
Kia Sorento2013–20229—
Honda Fit2016–20199—

Counts of owner complaints to the NHTSA that mention P0304 (380 in total), not failure rates: popular models get more complaints simply because more of them are on the road. Models with fewer than 3 complaints are left out.

Reading the code

P
Powertrain
0
Generic (SAE) code
3
Ignition system or misfire
04
Specific fault

Questions drivers ask

Can I drive with code P0304?

If the light flashes, stop driving as soon as it's safe: the catalytic converter can be damaged quickly. If it's steady and the engine only stumbles a little, you can drive gently to a shop, but don't wait long.

Will my car pass an emissions inspection with P0304?

Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.

Will P0304 clear by itself?

The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.

Can I fix P0304 myself?

Often yes, with an OBD-II scanner and basic tools. Difficulty: easy. Start with the cheapest checks before replacing expensive parts.

Often reported together

Codes that owners mention in the same NHTSA complaints as P0304. They can point to a shared cause, so it's worth checking them together.

This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.