P0304: Cylinder 4 Misfire Detected
Misfires concentrated on cylinder 4. A cylinder-specific code points away from shared causes (fuel pressure, MAF) toward components dedicated to that cylinder — coil, spark plug, injector, compression. Verify by swapping the suspected coil/plug to a different cylinder; if the code follows, the part is bad.
What P0304 means
P0304 is a generic OBD-II powertrain code in the misfire & ignition group. In plain English: the engine computer counted combustion events that didn’t fire properly.
- Check engine light
- Usually on
- Emissions-related
- Yes — can fail inspection
- Limp mode possible
- Not typical
- Drive cycle to clear
- Required
Get it fixed
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Symptoms
- 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
Common causes
- 01Ignition coil for cylinder 4 failedMost likely
- 02Spark plug for cylinder 4 worn or fouledMost likely
- 03Cylinder 4 injector cloggedPossible
Can I drive with P0304?
Only gently, and not if the light is flashing.
A flashing check engine light means an active misfire that is dumping unburned fuel into the exhaust. Stop driving and have it towed or repaired; continuing can melt the catalytic converter.
Why we rate it urgent
- Misfires can overheat and ruin the catalytic converter
- Turns on the check engine light
- Emissions-related: will usually fail an OBD inspection
- Needs a drive cycle to confirm a repair
Severity is our rule-based rating from the code’s flags in OBDex and the system it affects, not an inspection of your car. If the check engine light is flashing, or you notice overheating, low oil pressure, or brake or steering problems, stop driving and get help.
How to diagnose P0304
- 1
Confirm the code and save freeze-frame data
Read the codes with an OBD-II scan tool and note every code, not just P0304. Save the freeze-frame data (speed, load, temperature when the fault set) before clearing anything; it tells you when the fault happens.
- 2
Check for related codes first
Look for P0303, P0305, P0302, P0306. Related codes often share a cause; a circuit or sensor code usually needs fixing before a performance code like this one.
- 3
Find the misfiring cylinder
Use the scan tool's misfire counters to see which cylinders misfire. Swap the ignition coil (or plug) from a misfiring cylinder to another one: if the misfire follows the part, you found it. Check for vacuum leaks and fuel trim if all cylinders misfire.
- 4
Test the likely causes in order
Start with the most likely cause and work down: Ignition coil for cylinder 4 failed; spark plug for cylinder 4 worn or fouled; cylinder 4 injector clogged. Test before replacing parts.
- 5
Check for recalls and service bulletins
Look up recalls for your exact vehicle with its VIN, and ask a dealer about technical service bulletins (TSBs). Manufacturers sometimes issue software updates or extended warranties for known faults.
- 6
Clear the code and verify the repair
After the repair, clear the code and complete a full drive cycle (cold start, idle, steady highway cruise, deceleration). The code is fixed when the related readiness monitor completes without it returning.
P0304 repair cost estimate
Estimate, not a quote. Labor times are typical ranges for this code from OBDex, and $125/hr is an assumed rate. Shop rates and labor times vary by vehicle and region. Diagnosis time may be billed separately.
- DIY difficulty
- Easy
- Good DIY job?
- Yes, with a scan tool
Basic hand tools; a common first DIY job.
Parts and systems involved
- Ignition coil
- Spark plug
- Fuel injector
P0304 FAQ
What does P0304 mean?
P0304 means “Cylinder 4 Misfire Detected”. Misfires concentrated on cylinder 4. A cylinder-specific code points away from shared causes (fuel pressure, MAF) toward components dedicated to that cylinder — coil, spark plug, injector, compression. Verify by swapping the suspected coil/plug to a different cylinder; if the code follows, the part is bad.
Is P0304 serious? Can I keep driving?
We rate P0304 as urgent severity. Only gently, and not if the light is flashing. A flashing check engine light means an active misfire that is dumping unburned fuel into the exhaust. Stop driving and have it towed or repaired; continuing can melt the catalytic converter.
What causes P0304?
The most common causes, from most to least likely: Ignition coil for cylinder 4 failed; Spark plug for cylinder 4 worn or fouled; Cylinder 4 injector clogged. Test before replacing parts, because several causes can set the same code.
How much does it cost to fix P0304?
Typical labor is about $40–$500 at an assumed $125/hour shop rate, plus parts. That is an estimate, not a quote: labor times and rates vary by vehicle and region, and diagnosis may be billed separately.
Can I fix P0304 myself?
DIY difficulty is easy. Many owners can handle it with a scan tool, a multimeter and basic hand tools. Always confirm the cause before buying parts.
Will P0304 clear itself?
Once the fault is fixed, the check engine light usually goes off by itself after the system passes its self-test over several drive cycles. Clearing the code with a scan tool resets readiness monitors, so an inspection station may turn you away until a full drive cycle is completed.