P0304

P0304: Cylinder 4 Misfire Detected — Causes, Fixes & Cost

P0304 means your engine's cylinder 4 is not firing correctly. The ECU detected incomplete or absent combustion in cylinder 4 specifically. It's a pinpointed misfire — unlike P0300 which jumps around — making it easier to diagnose. Like all misfire codes, ignoring it will destroy your catalytic converter and may leave you stranded.

🔧 Common Causes

Faulty spark plug in cylinder 4 (most common)
Bad ignition coil on cylinder 4
Leaking or clogged fuel injector on cylinder 4
Low compression on cylinder 4 (worn piston rings or burned valve)
Vacuum leak near the cylinder 4 intake port
Faulty fuel injector wiring or ECU driver circuit

⚠️ Symptoms You'll Notice

Check Engine Light on, possibly flashing during severe misfire
Rough, lumpy idle — noticeably uneven
Engine vibration or shaking, especially at low RPM
Sluggish acceleration and reduced power
Poor fuel economy
Smell of unburned fuel from the exhaust

Estimated Repair Cost

DIY Cost

$10–$70 (spark plug + coil)

Parts only

Shop Cost

$120–$500 depending on root cause

Parts + labor

DIY Fix Path — Cheapest First

Work through these in order. Most people fix it by step 2.

1

Replace Spark Plug in Cylinder 4

$5–$20

Start here. Pull the plug from cylinder 4 — if it's fouled, cracked, worn, or has abnormal deposits, replace it. This is the fix ~40% of the time and takes under 20 minutes on most engines.
2

Swap & Test Ignition Coil on Cylinder 4

$30–$70

Move the coil from cylinder 4 to another cylinder (say cylinder 2) and clear the code. If the misfire follows the coil to cylinder 2 (now showing P0302), the coil is bad. Replace it. Coils are $25–$60 at the parts store.
3

Clean or Replace Fuel Injector on Cylinder 4

$15–$80

If plugs and coil check out, try a bottle of fuel injector cleaner added to the tank. If the code persists, have the injector professionally flow-tested or swap it (injectors for most vehicles are $20–$50 each).
4

Compression Test on Cylinder 4

$0 (DIY tool rental) – $80

If all the above pass, check cylinder 4 compression. Normal is typically 150–200 psi. Low compression means worn rings or a burned valve — a more involved repair. A wet compression test (add oil to cylinder) can tell you which.

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Frequently Asked Questions

Which cylinder is cylinder 4?

Cylinder numbering depends on your engine. On a 4-cylinder inline engine, cylinder 4 is usually at the opposite end from the timing belt (the "back" of the engine near the firewall). On V6 and V8 engines, the layout varies by manufacturer — consult your repair manual or check a WrenchAI diagnosis for your specific vehicle.

P0304 came back after I replaced the spark plug — now what?

The ignition coil for cylinder 4 is the next suspect. Swap the coil from cylinder 4 to another cylinder and clear the code. If the misfire follows the coil (now showing a different cylinder code), replace the coil. Coils often look fine externally but fail under heat.

Can low compression cause a P0304?

Yes. If a piston ring is worn or a valve is burned on cylinder 4, there isn't enough compression to complete combustion. A compression test is the definitive check — normal readings are 150–200 psi. Low compression on one cylinder while others are normal points to internal engine wear.

Is P0304 an emergency?

If the Check Engine Light is flashing, yes — stop driving. A flashing CEL means the misfire is severe enough to damage your catalytic converter within minutes. A steady light is less urgent, but fix it within a week. Every mile with a misfire risks $800–$2,500 in catalytic converter damage.