P0304

Cylinder 4 Misfire Detected

Ignition / misfire Generic OBD-II High

What it means

P0304 means the engine's misfire monitor has detected that cylinder 4 specifically is not firing reliably — fuel isn't burning properly in that cylinder, so the engine runs rough and loses power. Because it's isolated to one cylinder, it usually points to that cylinder's spark plug, coil, or injector. Sustained misfires can overheat and ruin the catalytic converter.

High

Can I keep driving? Limit driving — a misfire on cylinder 4 dumps raw fuel into the exhaust and can overheat the catalytic converter, especially if the light is flashing.

Real-world scan data

Seen in

7,116

scans in the past year

Found in

1.1%

of scans

2.0%

of scans that found faults

Appears alongside · share of P0304 scans

P0300in 82.4% of scans

P0303in 47.4% of scans

P0302in 46.1% of scans

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

Common symptoms

What you'll notice

Check-engine light on (flashing during an active misfire)
Rough idle and noticeable shaking or vibration
Reduced power and hesitation on acceleration
Increased fuel consumption and a possible fuel smell

Common causes

Most likely first

Worn or fouled spark plug on cylinder 4Likely
Failing ignition coil on cylinder 4Common
Clogged or leaking fuel injector on cylinder 4Common
Vacuum leak near cylinder 4's intake runnerLess often
Low compression on cylinder 4 (valve, ring, or head-gasket issue)Less often

How to diagnose & fix

Work through it in order

Read freeze-frame data, then swap the cylinder-4 coil and plug with a neighbouring cylinder and re-scan — if the misfire follows the part, you've found it.
Inspect and replace the spark plug and coil on cylinder 4 first — the most common cause.
Test the cylinder-4 injector (resistance and spray) and check fuel delivery.
Check for a vacuum leak around that runner with carb cleaner or smoke.
If it persists, run a compression/leak-down test on cylinder 4 for mechanical faults, then clear and re-test.

FAQ

Quick answers

Is it safe to drive with P0304?+

Only briefly. A single-cylinder misfire sends unburned fuel into the catalytic converter and can destroy it — if the check-engine light is flashing, drive as little as possible.

Can I just clear P0304?+

You can, but it returns within a few drive cycles until the cause on cylinder 4 is repaired. Fixing the plug, coil, or injector is what clears it for good.

Read & clear P0304 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX