# P0304: Cylinder 4 Misfire Detected **Verdict:** Drive to a shop this week Ignition and misfire - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $100-$2,500 - **Urgency:** This week - **Most likely:** Spark plug or ignition coil on cylinder 4 - **DIY?:** Steps 1-3 ## Quick answer P0304 means cylinder 4 is misfiring. On a four-cylinder engine, cylinder 4 is usually at the end of the engine opposite cylinder 1. A failing coil or plug is the most likely cause, but if those test fine, a compression test can catch a valve or head gasket problem early. ## What does P0304 mean? On an inline four-cylinder engine, cylinder 4 is the last cylinder, at the end opposite cylinder 1 and usually nearest the transmission. On V engines its position depends on the maker: it is second on the passenger side on GM and Chrysler V8s, the rear cylinder of the passenger-side bank on Ford V8s, and the front cylinder of the driver's side on Ford V6s. On Subaru boxers it is the rear cylinder on the driver's side. The computer pins a misfire to cylinder 4 by timing crankshaft speed dips against the firing order. A high misfire rate flashes the light right away to protect the catalytic converter; a lower rate usually has to repeat on a second drive cycle before the light stays on. A steady cylinder 4 misfire that new ignition parts do not cure deserves an early compression test, because burnt valves and head gasket leaks show up the same way. ## What are the symptoms of P0304? - Check engine light on or flashing - Rough idle and shaking - Power loss when accelerating - Poor fuel economy - Possible ticking or puffing sound if a valve is leaking ## What causes P0304? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Failing ignition coil on cylinder 4** (most common). The cylinder 4 coil sees the same heat and vibration as the others, and coils commonly fail one at a time with age. The misfire usually begins under load or in damp weather before becoming constant. Swap it with the cylinder 3 coil, clear the code and drive; if the code changes to P0303, the coil is bad. 2. **Worn spark plug in cylinder 4** (common). Plugs worn past their interval need more voltage than the coil can easily supply, so they misfire first under hard acceleration or when climbing hills. A cracked insulator, oil fouling or the wrong gap does the same. Pull the cylinder 4 plug and compare it with the cylinder 3 plug; a wider gap or heavier deposits point to it. 3. **Faulty cylinder 4 fuel injector** (less common). A cylinder 4 injector that is clogged, sticking or not getting its electrical pulse leaves the cylinder lean, with the misfire usually worst at idle. Engines that sit unused or run on poor fuel gum up faster. A shop checks the pulse with a noid light, runs a balance test or swaps the injector with cylinder 3. 4. **Low compression from a burnt valve or head gasket** (less common). A burnt exhaust valve, tight valve clearance or a head gasket failing near cylinder 4 causes a steady misfire that does not follow the coil or plug. Coolant loss, white exhaust smoke or bubbles in the coolant reservoir point to the head gasket. Engines that have overheated are most at risk. Compression and leak-down tests confirm it. 5. **Wiring problem to the coil or injector** (rare). The cylinder 4 coil and injector connectors are often at the end of the harness, where wires can chafe on brackets or turn brittle near the exhaust manifold. A broken wire cuts the signal intermittently, so the misfire may come and go with engine movement. Wiggle-test the harness while watching misfire counts, and look for a chafed spot. ## How do you diagnose P0304? 1. Confirm which cylinder is number 4 on your engine using a diagram. 2. Swap the cylinder 4 coil with cylinder 3; if the code becomes P0303, replace the coil. 3. Inspect and swap the spark plug next. 4. Check the cylinder 4 injector with a noid light or by swapping it. 5. Run a compression or leak-down test if the misfire stays on cylinder 4, and check for coolant loss. ## How do you fix P0304, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace the cylinder 4 ignition coil | $150-$350 | Quick repair on most engines. | | Replace spark plugs | $120-$450 | Replace the full set. | | Valve or head gasket repair | $1,200-$2,500 | Only if a compression test shows a mechanical problem. | Full price breakdowns: [ignition coil replacement cost](/cost/ignition-coil-replacement), [spark plug replacement cost](/cost/spark-plug-replacement). ## Is it safe to drive with P0304? > **Drive to a shop this week.** With a steady light, drive gently and get it checked soon. If the light flashes or you notice coolant loss or white smoke, stop driving. ## Will P0304 fail an emissions test? An OBD-based inspection fails a vehicle with P0304 and the light on, however smoothly the car seems to run. A misfire caused by a leaking valve or head gasket can also raise tailpipe hydrocarbons where sniffer tests are still used. Once the cause is fixed and the code cleared, the readiness monitors need several days of driving to finish. ## After the repair: clearing P0304 Clear the code and drive, watching the cylinder 4 misfire counter if your scan tool shows it. If valve or head gasket work was done, also watch the coolant level and exhaust for white smoke over the first few weeks. A stable coolant level, a smooth idle and no returning code mean the repair succeeded. ## How to prevent P0304 - Replace spark plugs on schedule, including the end plug that is hardest to reach. - Keep the cooling system healthy; overheating is a common cause of head gasket and valve damage. - Check coolant level monthly so a slow head gasket leak is noticed early. - Have valve clearance checked at the listed interval on engines with adjustable valves. - Keep coil and injector wiring clipped away from hot exhaust parts after any repair. ## Codes often seen with P0304 - [P0300](/codes/p0300): Random/Multiple Cylinder Misfire Detected - [P0303](/codes/p0303): Cylinder 3 Misfire Detected - [P0305](/codes/p0305): Cylinder 5 Misfire Detected - [P0420](/codes/p0420): Catalyst System Efficiency Below Threshold (Bank 1) ## Frequently asked questions ### Where is cylinder 4? On most inline four-cylinder engines, cylinder 4 is at the opposite end from cylinder 1, usually nearest the transmission. On V6 and V8 engines its position depends on the maker; Ford V6s put it at the front of the driver's side, for example. Look up a cylinder diagram for your exact engine. ### What if the coil swap does not move the misfire? Then the coil is probably fine, and the plug, injector or compression is next. Swap or replace the plug, test the injector, then run a compression test. A misfire that stays on cylinder 4 through all of that usually points to a valve or head gasket problem. ### Is P0304 serious? It is moderately serious and should be fixed within days. A misfire can damage the catalytic converter, and in some cases it is an early sign of a valve or head gasket problem. Most causes are cheap, but a steady misfire with coolant loss needs attention right away. ### How much does it cost to fix P0304? Most P0304 repairs cost $120 to $450 for spark plugs or a cylinder 4 coil at an independent shop. If a compression test shows a burnt valve or head gasket problem, expect $1,200 to $2,500 or more. That is why confirming the cause before buying parts matters with this code. ### Can a head gasket cause P0304? Yes, a leaking head gasket can cause P0304, especially if it leaks coolant into cylinder 4 or combustion gas into the cooling system. Signs include coolant loss with no visible leak, white sweet-smelling exhaust and a misfire worst on cold starts. Combustion leak and compression tests confirm it. ### Can tight valves cause P0304? Yes, on engines with adjustable valves, clearance that closes up with wear can keep a valve from sealing, causing a misfire and eventually a burnt valve. It often shows first as a rough cold idle. Several Honda engines and some others list valve adjustment in their maintenance schedule. ### Can I drive with P0304? Short, gentle trips are usually fine if the light is steady and the engine runs reasonably smoothly. Stop driving if the light flashes, the temperature gauge climbs, or you see white smoke or lose coolant, because those point to a head gasket problem that driving will make much worse. ### Can I fix P0304 myself? If the cause is the cylinder 4 coil or plug and you can reach it, yes. On most inline four-cylinders it is a one-bolt coil and a plug socket. Injector, valve and head gasket work needs a shop, but an inexpensive compression gauge can help you rule out a mechanical problem first. ### Will P0304 pass inspection? No, P0304 with the check engine light on fails an OBD-based emissions inspection. After the repair, clear the code and let the readiness monitors complete with normal driving. If valve or head gasket work was done, make sure the engine shows zero misfire counts before you retest. ### Why do P0303 and P0304 show up together? When cylinders 3 and 4 both misfire, look for something they share. That could be a head gasket leaking between the two neighboring cylinders, an intake gasket leak near both ports, or a harness section that serves both. A compression test showing both cylinders low points to the gasket. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) ## Other ignition and misfire codes - [P0300 Random/Multiple Cylinder Misfire Detected](https://diagxcar.com/codes/p0300) - [P0301 Cylinder 1 Misfire Detected](https://diagxcar.com/codes/p0301) - [P0302 Cylinder 2 Misfire Detected](https://diagxcar.com/codes/p0302) - [P0303 Cylinder 3 Misfire Detected](https://diagxcar.com/codes/p0303) - [P0305 Cylinder 5 Misfire Detected](https://diagxcar.com/codes/p0305) - [P0306 Cylinder 6 Misfire Detected](https://diagxcar.com/codes/p0306) - [P0307 Cylinder 7 Misfire Detected](https://diagxcar.com/codes/p0307) - [P0308 Cylinder 8 Misfire Detected](https://diagxcar.com/codes/p0308) - [P0316 Engine Misfire Detected on Startup (First 1000 Revolutions)](https://diagxcar.com/codes/p0316) - [P0320 Ignition/Distributor Engine Speed Input Circuit](https://diagxcar.com/codes/p0320) ## Symptoms - [Why is my car idling rough?](https://diagxcar.com/symptoms/rough-idle) - [Why is white smoke coming from my exhaust?](https://diagxcar.com/symptoms/white-smoke-from-exhaust) - [Why does my car hesitate when I accelerate?](https://diagxcar.com/symptoms/car-hesitates-when-accelerating) - [Why does my car shake at idle?](https://diagxcar.com/symptoms/car-shakes-at-idle) - [Why does my car shake when I accelerate?](https://diagxcar.com/symptoms/car-shakes-when-accelerating) ## Repair costs - [Ignition coil replacement cost](https://diagxcar.com/cost/ignition-coil-replacement): $150-$400 - [Spark plug replacement cost](https://diagxcar.com/cost/spark-plug-replacement): $120-$450 --- Source page: https://diagxcar.com/codes/p0304 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.