What does P0304 mean?
The engine control module detected repeated misfires specifically in cylinder 4.
Quick Decision Guide
- Worn or fouled spark plug in cylinder 4
- Failing ignition coil on cylinder 4 (often worse under load)
- Clogged or leaking fuel injector on cylinder 4
When to stop driving
A flashing check-engine light on P0304 means cylinder 4 is actively misfiring — reduce driving and inspect promptly to protect the catalytic converter. Stop driving if you feel severe shaking, smell fuel, see temperatures climbing, or lose power under acceleration. A steady light with only mild roughness usually allows cautious short-distance driving to a safe place for inspection, but P0304 should still be diagnosed within days.
Reviewed by ratchit Automotive Solutions for clarity and inspection-first safety framing. Not a substitute for professional diagnosis.
Repair Decision & Cost Guide
Use this guide to decide what to inspect first, what it may cost, and when to move from DIY checks to professional diagnosis.
P0304 points to a misfire condition, not a guaranteed failed coil or plug. Isolate the failing cylinder before buying parts.
DIY vs. Shop
Can you keep driving?
Best first DIY check
Don't replace this first
When to seek a shop
Recommended next action
Estimated Repair Cost
Inspection cost
$0–$25 DIY scanner
DIY parts cost
$5–$120
Inspect first before replacing parts.
If the vehicle is unsafe to drive, stop driving and seek professional help.
Shop diagnostic cost
$80–$150 (typical shop diagnostic fee)
Shop repair cost
$40–$500
Diagnostic uncertainty: This code points to a system or condition, not a guaranteed failed part. Inspect before replacing.
Cost variation: Costs vary by vehicle, location, part quality, labor rate, and the confirmed root cause.
Inspect Before Replacing
These checks are cheap, fast, and rule out the most common causes before spending money on parts.
- 1Note when the misfire is worst (idle, load, cold start) — this narrows what to inspect first.
- 2Scan for companion codes such as lean codes or fuel-trim issues that would change the plan.
- 3Pull and read the cylinder 4 spark plug — look for fouling, oil, or a widened gap.
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ratchit AI Insight
· P0304What this usually means:P0304 narrows the misfire to a specific cylinder, but the cause still needs to be confirmed before replacing parts. Misfire codes do not automatically mean spark plugs or coils are bad — air/fuel, vacuum, fuel delivery, and mechanical issues can all trigger the same code.
Common mistake: Many drivers replace all the spark plugs and coils first. That sometimes fixes it, but if the real cause is a vacuum leak, an injector problem, or low compression, the misfire returns and the parts money is wasted.
Inspect before replacing parts
- 1Check for a flashing check-engine light — a flashing CEL during a misfire is a high-risk condition for the catalytic converter.
- 2Swap the suspect coil/plug to a known-good cylinder and re-scan to see if the misfire follows.
- 3Inspect for obvious vacuum leaks, cracked intake boots, and disconnected PCV hoses.
- 4Review fuel trims and injector behavior before assuming an ignition part is bad.
Confirming the cylinder and the system (ignition vs. air/fuel vs. mechanical) before buying parts usually keeps the repair in the $20–$200 range instead of replacing a full ignition set.
Safety threshold: If the check-engine light is flashing, stop driving as soon as it is safe — continued driving in this state can damage the catalytic converter.
AI-assisted guidance reviewed against ratchit's inspection-first safety rules. Codes are clues, not confirmed diagnoses — always verify before replacing parts.
Understanding P0304 in detail
P0304 flags cylinder 4 as the cylinder repeatedly missing power strokes. A useful clue with P0304 is *when* the misfire is most noticeable, because that changes the likely root cause. A misfire that shows up mainly at idle usually points to a vacuum or intake leak, a weak injector at low pulse widths, or a fouled plug. A misfire that appears under load or hard acceleration usually points to a weakening coil, worn plug gap, low fuel pressure, or a compression problem that only reveals itself at higher cylinder pressures. A cold-start misfire that clears as the engine warms up often points to a cracked coil, moisture in a plug well, or an injector that leaks or sticks when cold. There is no universal 'cylinder 4 always fails first' pattern — the correct next step depends on the plug reading, when the misfire occurs, and what other codes are stored. The goal on P0304 is to match the symptom pattern to the right inspection before replacing any parts.
Symptoms of P0304
- Check engine light on or flashing
- Rough idle with noticeable vibration
- Loss of power under acceleration or load
- Engine stumbling on cold start that improves as it warms
- Fuel smell from exhaust
Common causes
- Worn or fouled spark plug in cylinder 4
- Failing ignition coil on cylinder 4 (often worse under load)
- Clogged or leaking fuel injector on cylinder 4
- Damaged wiring or connector at the cylinder 4 coil or injector
- Vacuum or intake leak near the cylinder 4 runner (worse at idle)
- Low compression in cylinder 4 (valve, ring, or head-gasket issue)
How to diagnose P0304
Steps ordered by urgency — tackle Now items first.
- NowNote when the misfire is worst (idle, load, cold start) — this narrows what to inspect first.
- NowScan for companion codes such as lean codes or fuel-trim issues that would change the plan.
- NowPull and read the cylinder 4 spark plug — look for fouling, oil, or a widened gap.
- SoonSwap the cylinder 4 coil with a neighboring cylinder, clear the code, and see whether the misfire follows.
- SoonInspect wiring and connectors at the cylinder 4 coil and injector for heat damage, chafing, or corrosion.
- SoonVerify injector signal and, if possible, swap injectors to see whether the misfire moves.
- LaterIf ignition and fuel both check out, perform a compression or leakdown test on cylinder 4, especially if the misfire is worse under load.
- LaterSmoke-test the intake for vacuum leaks affecting the cylinder 4 runner if the misfire is mainly at idle.
How to fix P0304
Fixes listed in order of likelihood and cost-efficiency.
- 1Replace the cylinder 4 spark plug when inspection shows wear, fouling, or damage
- 2Replace the cylinder 4 coil only after a swap test confirms the misfire follows it
- 3Clean or replace the cylinder 4 injector when signal or flow tests fail
- 4Repair wiring or connector damage at the cylinder 4 coil or injector
- 5Repair vacuum or intake leaks affecting the cylinder 4 runner
- 6Address low compression (valves, rings, or head gasket) if mechanical tests fail
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Recommended tools & parts
These recommendations appear because this issue commonly requires inspection in this system area. ratchit does not recommend replacing parts until the likely cause is confirmed.
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Estimated Repair Cost — P0304
Ranges vary by vehicle, region, and shop. Use as a ballpark — always get a quote.
DIY estimate
$5 – $15
Shop estimate
$40 – $100
DIY estimate
$25 – $70
Shop estimate
$100 – $300
DIY estimate
$40 – $120
Shop estimate
$200 – $500
Cost data is estimated and may not reflect your specific vehicle or location.
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Frequently Asked Questions
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