P0304
- Powertrain
- Generic (SAE)
- Emissions
Get it looked at now
Unburned fuel entering the catalytic converter can melt the substrate, and a flashing MIL means that process is under way. A misfire also produces hesitation and power loss that can matter when merging or towing. Where the cause turns out to be mechanical, delay converts a valve repair into head or engine work.
- Safe to drive
- Flashing light means stop and get it towed. Steady light with rough running: drive gently and only as far as the nearest shop.
Cylinder 4 Misfire Detected
The computer detected repeated misfires on cylinder 4. It knows which cylinder is failing to burn its charge properly, but not the reason. The answer is spark, fuel or compression at that specific cylinder.
The ECM infers misfire from crankshaft speed variation, attributing a deceleration event to whichever cylinder should have been on its power stroke. A code naming only cylinder 4 excludes shared systems and narrows the field to that cylinder's coil, plug, injector, wiring, intake runner and mechanical condition. On many inline four engines cylinder 4 sits at one end of the block, where an intake or coolant distribution quirk can make it the first cylinder to reveal a marginal condition.
How the car detected itTwo counters run in parallel. Over roughly 200 crank revolutions, a misfire rate high enough to overheat the catalyst sets the code and flashes the MIL. Over roughly 1000 revolutions, a lower rate that still exceeds the emissions threshold sets the code with a steady MIL. Because detection relies on crank speed, a worn crank reluctor or a rough road can occasionally produce misleading counts, which is why the pattern should be confirmed on live data.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Flashing light means stop and get it towed. Steady light with rough running: drive gently and only as far as the nearest shop.
- Urgency
- Get it looked at now
- Will it pass emissions
- No
- If ignored
- Expect worsening roughness, poor economy and eventual catalyst failure, which shows up as a restricted exhaust and severe power loss. Fuel washing past the rings dilutes the oil and accelerates bearing and cylinder wear. A misfire caused by a valve sealing problem will deteriorate, and the cost climbs from a few hundred dollars to a few thousand.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Check engine light on or flashing
- Rough idle and uneven running
- Power loss, hesitation under acceleration
- Increased fuel consumption
- Possible smell of unburned fuel from exhaust
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Ignition coil for cylinder 4 failed | Very common | — |
| 2 | Spark plug for cylinder 4 worn or fouled | Very common | — |
| 3 | Cylinder 4 injector clogged | Common | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Whether the check engine light is flashing or steady
It determines whether this is a drive-it-carefully situation or a stop-now situation. Flashing means active catalyst damage.
Freeze frame conditions when the code set
Misfire only when cold points toward plugs, an injector that leaks down overnight or a warm-up enrichment issue. Misfire only under load points toward a weak coil breaking down under cylinder pressure.
Whether other cylinders also show misfire counts
P0304 alone means cylinder 4 hardware. P0304 alongside neighboring cylinders or P0300 shifts the diagnosis to fuel pressure, a vacuum leak, timing or crank sensor issues.
Whether coolant level and temperature are normal
An end cylinder is often the first to suffer from a head gasket seep or a localized overheat, and a low coolant level with an unexplained single-cylinder misfire is a meaningful clue.
The condition of the spark plug well and valve cover area over cylinder 4
Oil pooling in the plug tube shorts the coil boot and causes a misfire that returns after new parts unless the leak is fixed.
Confirm the misfire pattern on live data
Tools Scan tool with per-cylinder misfire counters
Watch per-cylinder counts at idle, at steady rpm and under light load. Establish whether cylinder 4 is the only one accumulating counts and under which conditions.
ExpectedCounts concentrated on cylinder 4 confirm the code and define the test conditions for later verification.
Swap the cylinder 4 coil to a known-good cylinder
Tools Hand tools, scan tool
Move the coil, clear codes, and reproduce the conditions. Whether the code follows the coil is the cheapest hard answer available on this code.
ExpectedCode following the coil confirms a coil failure. Code remaining on cylinder 4 eliminates the coil.
Inspect the cylinder 4 spark plug and its well
Tools Spark plug socket, gap gauge, inspection light
Read the plug against a healthy one and check the well for oil and the boot for carbon tracking. On an end cylinder, also note any sign of coolant intrusion, which a steam-cleaned looking plug suggests.
ExpectedA dry, tan or gray plug with correct gap and a dry well. Oil, fuel or coolant deposits each point to a different failure.
Verify spark under load with a spark tester
Tools Adjustable spark tester
A marginal coil can fire freely in open air and fail against cylinder pressure. Use an adjustable spark tester and compare cylinder 4 to the others at the same setting.
ExpectedStrong blue spark, sustained, and equal to the other cylinders.
Test the cylinder 4 injector and its circuit
Tools Stethoscope or noid light, multimeter
Listen for an even click, compare resistance across all injectors, confirm the ECM drive signal, and where practical swap the injector with a neighbor and see whether the misfire follows.
ExpectedEven clicking and matched resistance across the set. Compare within the engine, since specifications vary by make.
Check for a vacuum leak local to the cylinder 4 runner
Tools Smoke machine
Smoke test the intake with attention to that runner's gasket, injector seal and any vacuum port teed off it. A leak feeding one cylinder makes it run lean enough to misfire while the others stay normal.
ExpectedNo smoke escaping at the cylinder 4 runner, gasket or injector seals.
Compression and leak-down test cylinder 4
Tools Compression gauge, leak-down tester, compressed air
If the misfire stays with the cylinder through every swap, test it mechanically. Compare against the other cylinders and use leak-down to hear where the air goes.
ExpectedWithin roughly 10 to 15% of the other cylinders. Air at the tailpipe indicates an exhaust valve, at the intake an intake valve, at the oil filler the rings, and bubbles in the coolant a head gasket.
Test the cooling system for combustion gases if compression is low
Tools Combustion leak tester, cooling system pressure tester
On an end cylinder with low compression, coolant loss or an unexplained overheat history, a block test on the coolant separates a head gasket from a valve problem before the head comes off.
ExpectedNo color change in the block tester fluid and a cooling system that holds pressure.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Replace the ignition coil for cylinder 4 | The misfire code following the coil to the cylinder it was swapped into | $130–$400 | DIY |
| Replace the spark plugs as a set with the correct heat range and gap | The misfire following the plug on a swap, or a worn, fouled, cracked or mis-gapped plug found on inspection | $90–$450 | DIY |
| Clean or replace the cylinder 4 fuel injector | No click, resistance out of line with the set, or the misfire following the injector when swapped | $180–$650 | Hard |
| Repair coil wiring, connector or ground for cylinder 4 | Missing supply voltage, poor ground, or no trigger signal at that coil connector | $100–$350 | DIY |
| Replace the valve cover gasket and spark plug tube seals | Oil found in the cylinder 4 spark plug well | $180–$650 | DIY |
| Repair an intake leak at the cylinder 4 runner or injector seal | Smoke escaping at that runner or seal under a smoke test | $150–$700 | DIY |
| Cylinder head repair for a burnt or leaking valve | Low compression on cylinder 4 with leak-down air heard at the intake or exhaust | $1,200–$4,000 | Shop |
| Head gasket replacement | Low compression with coolant loss and combustion gases detected in the coolant | $1,400–$3,500 | Shop |
US independent-shop ranges, parts plus labor. Most repairs land in the first two entries. Labor on plugs and coils is entirely about access, so an inline four sits at the bottom of the range while a rear-bank cylinder on a transverse V6 sits well above it. Add 100 to 200 dollars for diagnosis and a similar amount for compression and leak-down testing. If misfire has been left running long enough to destroy the catalytic converter, add 800 to 2500 dollars.
- Replacing coil, plug and injector together, which fixes the car but leaves the cause unknown if the misfire returns.
- Ignoring oil in the spark plug tube, which destroys the new coil in a few thousand miles.
- Not comparing cylinder 4 against a known-good cylinder. Almost every test on this code is a comparison test.
- Chasing fuel trims that are only lean because the misfiring cylinder is pumping oxygen past the sensor.
- Skipping a cooling system check when an end cylinder loses compression and the coolant level is unexplainedly low.
- Using a plug of the wrong heat range or an incorrect gap, which can cause a misfire that survives every other repair.
- Driving a flashing-MIL engine any further than absolutely necessary.
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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