Flashing MIL: stop immediately. Steady MIL: short trips only. P0304 means the ECM has detected a combustion misfire specifically in cylinder 4 — a cylinder that is failing to ignite the air-fuel charge on enough power strokes to exceed the calibrated misfire threshold.
What P0304 means
Using the same crankshaft deceleration detection method as all cylinder-specific misfire codes, the ECM assigns each misfire event to a cylinder with the help of the camshaft position sensor. P0304 is stored when the misfire rate counted for cylinder 4 exceeds either the emissions deterioration limit or the catalyst damage limit. On 4-cylinder engines, cylinder 4 is the last in the firing order and is often physically the furthest from the engine management ancillaries, making it prone to slightly different thermal and fuel-trim conditions than cylinders 1–3. The same misfire severity rule applies as all P030x codes: a steady MIL means emission-level misfires; a flashing MIL means catalyst-damaging misfires requiring immediate attention.
Symptoms
- Rough idle with a vibration rhythm specific to cylinder 4's firing interval
- Check engine light on (flashing if catalyst-damaging misfire rate is active)
- Stumble or hesitation felt precisely at cylinder 4's power stroke frequency under light acceleration
- Reduced power and slower acceleration
- Fuel smell in the exhaust if raw fuel is passing through unburnt
- Possible oil fouling on the cylinder 4 plug if valve seal or ring wear is involved
Common causes
- Worn or fouled spark plug in cylinder 4 — the leading cause on high-mileage engines
- Weak or failed ignition coil on cylinder 4 (coil-on-plug systems) producing insufficient spark energy
- Clogged or electrically failing fuel injector on cylinder 4 causing a lean condition in that cylinder only
- Compression loss in cylinder 4 from a burnt or sticking exhaust valve, worn piston rings, or head gasket failure at cylinder 4
- Intake manifold vacuum leak at the cylinder 4 runner gasket introducing unmetered air to that cylinder
Severity & driving advice
Severity: High — Same risk as P0303 — a flashing MIL means the catalyst is receiving unburnt fuel at damaging levels. Address urgently.
Can I drive? Flashing MIL: stop immediately. Steady MIL: short trips only.
Diagnostic approach
- Confirm misfire is isolated to cylinder 4 via live scan data — View per-cylinder misfire counts on a scan tool while the engine idles. Cylinder 4's counter should be incrementing while others remain stable. On Toyota 6-cylinder engines, factory data confirms cylinder identification is performed by cross-referencing the CKP NE signal against the VVT cam sensor — verify the cam sensor circuit is fault-free first.
- Swap the cylinder 4 ignition coil with a known-good cylinder — Move the coil from cylinder 4 to cylinder 2 (or any cylinder that is running cleanly). Clear the codes and idle the engine. If the misfire follows the coil to its new location, the coil has failed and requires replacement. If P0304 persists with a fresh coil, proceed to check the spark plug and injector.
- Inspect and replace the cylinder 4 spark plug — Remove the spark plug from cylinder 4 and examine it. Carbon-fouled or oil-fouled plugs indicate combustion contamination. Check the gap and compare it against specification. Even if the plug appears normal, replacing it as part of a scheduled maintenance interval is worthwhile when diagnosing misfires — plugs degrade gradually and may test near-passable yet contribute to misfire under load.
- Perform a compression test and injector inspection — If the ignition components test good, perform a wet and dry compression test. Low compression in cylinder 4 relative to others (more than 15% lower, or below 150 PSI) points to internal wear. A wet test (adding a small amount of oil into the cylinder) that significantly improves the reading confirms ring wear. If compression is normal, focus on the fuel injector — test resistance and, where possible, balance-test injector delivery.
Make & model notes
Toyota: On Toyota's 1GR-FE V6 (4Runner, FJ Cruiser), cylinder 4 is the first cylinder in Bank 2. If P0304 appears alongside P0345 (Bank 2 cam sensor), resolve the cam sensor fault first before assuming the cylinder 4 ignition components are defective — cylinder identification depends on an accurate camshaft signal.
Hyundai / Kia: The 2.0L Nu and Theta II GDI 4-cylinder engines commonly develop P0304 from carbon buildup on intake valves (direct injection with no port wash). Walnut blasting the intake ports restores normal combustion in affected cylinders without replacing any sensors or injectors.
Dodge / Ram: 5.7L HEMI engines use cylinder deactivation (MDS). Cylinder 4 is one of the deactivated cylinders. A failed MDS lifter in cylinder 4 causes P0304 with a characteristic ticking noise. Listen for the tick before condemning the ignition or fuel system.
FAQ
Why does P0304 happen on the last cylinder more often than others?
Cylinder 4 (last in most 4-cylinder firing orders) sits at the end of the fuel rail, so any pressure variation or injector flow imbalance shows up more noticeably there. It is also the furthest from the coolant outlet on many transverse 4-cylinder engines, making it slightly hotter. However, statistically, no single cylinder fails more than others — P0304 is as common as P0301 or P0302 in fleet data.
Can a vacuum leak cause P0304 only in cylinder 4?
Yes, if the leak is at the intake manifold runner gasket or port directly feeding cylinder 4. This creates a locally lean mixture in cylinder 4 only while other cylinders run normally. Smoke-test the intake manifold with individual runners sealed to isolate runner-specific leaks.
My P0304 only shows up after the engine is hot. What causes that?
Heat-sensitive misfires typically point to an ignition coil that weakens as temperature rises, a spark plug with a marginal internal crack that expands with heat, or a fuel injector whose pintle seal swells and restricts flow when hot. Swap the coil first — it is the most heat-sensitive component and the easiest to swap test.
Do I need to fix P0304 immediately if the MIL is steady?
A steady MIL means the misfire is at emissions-level severity, not yet catalyst-damaging. However, the unburnt fuel being pushed into the exhaust system will begin coating the catalyst over time, increasing the risk of a P0420 catalyst efficiency code. Repair within a week or two to protect downstream components.