OBD-II Code · Powertrain
P0304
Cylinder 4 Misfire Detected
Severity
high
Left alone, this one tends to take other parts with it.
Can you drive it
Depends on the light
Flashing: stop as soon as it is safe to — raw fuel is reaching the catalytic converter. Steady, and it only feels rough: a short drive to a shop is usually fine. Either way, stop if it shakes hard, loses power, overheats or makes a new noise.
Indicative cost (USD)
$30–$400
Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
Swap the coil to a quiet cylinder before buying parts
Cylinder 4 misfire.
How a shop works this
The ECU has already told you which cylinder. That makes the fastest honest test a swap: move one part to another cylinder and see whether the fault moves with it. It identifies the fault instead of guessing at it, and it costs nothing.
Photograph the coils before you touch anything, and note which is which.
Cylinder numbering is not the same across manufacturers, and V6 and V8 bank layouts differ by maker — look up the firing order and bank layout for your engine before you move parts, or you can spend an hour swapping coils on the wrong bank.
Move the cylinder 4 ignition coil to a cylinder that is not misfiring, put that cylinder's coil on 4, clear the code, and run it until the misfire returns.
On a modern coil-on-plug engine the coil is the most common single answer to a cylinder-specific misfire, which is why it is the first thing to move — not because it is the cheapest part. A misfire that is there even at idle is the classic dead-coil pattern.
- The misfire follows the coil to its new cylinderThe coil is the fault. Replace it. If the engine has real mileage on it, the other coils are on the same clock.
- The misfire stays on cylinder 4The coil is fine. Look at the coil connector and the harness around it next — chafed and rodent-damaged connections at that plug are a common second answer — then pull the plug.
Read the plug you pulled, and check the boot for carbon tracking.
Black and sooty means it has been running rich. White or blistered means lean or too hot. Oil-wet points at valve seals or rings. Fuel-wet points at an injector holding open. A carbon-tracked boot arcs to the head and imitates a dead coil for the price of a new boot. On an older engine with a distributor, the cap, rotor and plug wires belong in this step too.
If spark is accounted for, go to fuelling, then to compression.
An injector balance or flow test comes first because it is quicker. Compression test second: a cylinder more than about 15% below its neighbours is mechanical, and no ignition part will fix it. A vacuum leak on this one runner sits alongside these — a leak big enough to affect every cylinder would be setting P0300 instead.
A flashing check-engine light means the misfire is putting raw fuel into the catalytic converter, which can destroy it quickly. Stop driving as soon as it is safe to. A steady light with a mild misfire is less urgent, but stop anyway if it shakes hard, loses power, overheats, or makes a noise it was not making before.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Rough idle
- Engine shake
- Check engine light
- Power loss
Likely causes
- Failed ignition coil (cylinder 4)Most common
- Damaged coil connector or harness at cylinder 4
- Worn spark plug or carbon-tracked boot (cylinder 4)
- Clogged or leaking fuel injector (cylinder 4)
- Low compression on cylinder 4
- Vacuum leak at the cylinder 4 intake runner
Read the full diagnostic procedureP0304 means cylinder 4 is misfiring.
P0304 means cylinder 4 is misfiring.
The 10-15 minute coil swap drill is the same
pull cylinder 4's plug, read it, swap its coil with an adjacent cylinder, clear codes, and confirm whether the misfire moves. On a 4-cylinder engine, cylinder 4 is the cylinder furthest from the timing-belt/chain end and often runs slightly hotter and leaner because of intake-runner length and exhaust heat soak from the manifold collector. On a transverse V6 (Honda J-series, Toyota 2GR, Chrysler Pentastar), cylinder 4 typically lives on the front bank closest to the radiator, which means it's also the easiest cylinder to access — useful when the swap test points back to it. If the coil-swap and plug-swap both fail to move the misfire, run a compression test, then a leak-down test if compression is borderline (15-20% leakage past the rings is wear; >25% leaking out the exhaust is a burned valve).
Vehicle-specific patterns2002-2006 Toyota Camry/Highlander with the 2AZ-FE 2.4L is famous for P0304 from oil consumption — the piston rings glaze, oil enters cylinder 4 speci…
- 2002-2006Toyota Camry/Highlander with the 2AZ-FE 2.4L is famous for P0304 from oil consumption — the piston rings glaze, oil enters cylinder 4 specifically because of its position relative to the PCV pickup, and fouls the plug repeatedly; Toyota issued a TSB for ring replacement on affected VINs.
- 2011-2016Chevy Cruze/Sonic with the 1.4L turbo (LUJ/LUV) frequently throws P0304 from a cracked intake manifold or failed PCV diaphragm causing a lean condition that hits cylinder 4 hardest.
- 2007-2013Mazda 3/CX-7 with the 2.3L DISI turbo throws P0304 from carbon-fouled VVT solenoids retarding cam timing under load.
Estimated repair$40 (coil/plug) to $3,500+ (ring job on the 2AZ-FE).
Look up another code
P0304 — frequently asked
What does a P0304 code mean?
Cylinder 4 misfire.
What causes a P0304 code?
Common causes: Failed ignition coil (cylinder 4); Damaged coil connector or harness at cylinder 4; Worn spark plug or carbon-tracked boot (cylinder 4); Clogged or leaking fuel injector (cylinder 4); Low compression on cylinder 4; Vacuum leak at the cylinder 4 intake runner.
What are the symptoms of a P0304 code?
Typical symptoms: Rough idle; Engine shake; Check engine light; Power loss.
Is it safe to drive with a P0304 code?
Depends on the light. Flashing: stop as soon as it is safe to — raw fuel is reaching the catalytic converter. Steady, and it only feels rough: a short drive to a shop is usually fine. Either way, stop if it shakes hard, loses power, overheats or makes a new noise. Estimated repair: $30-$400 — swap the coil to a quiet cylinder before buying parts.
How do I diagnose a P0304 code?
The ECU has already told you which cylinder. That makes the fastest honest test a swap: move one part to another cylinder and see whether the fault moves with it. It identifies the fault instead of guessing at it, and it costs nothing. 1. Photograph the coils before you touch anything, and note which is which. 2. Move the cylinder 4 ignition coil to a cylinder that is not misfiring, put that cylinder's coil on 4, clear the code, and run it until the misfire returns. 3. Read the plug you pulled, and check the boot for carbon tracking. 4. If spark is accounted for, go to fuelling, then to compression.
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