P0304 Code: Cylinder 4 Misfire and the Free Fixes

A P0304 code is the OBD-II diagnostic trouble code (DTC) for cylinder 4 misfire detected: the engine computer counted too many strokes in cylinder 4 where the fuel and air didn't burn. You usually meet it as a check engine light, steady, or flashing while the engine shakes.

It's also typed PO304 with a letter O, or P304, and some scan tools show it as P030400; all of them mean cylinder 4. It can arrive alone or with P0300, the random misfire code, and the difference matters.

Below is what the code measures, where cylinder 4 is, how to tell a bad coil from a bad cylinder, when to stop driving, which carmakers pay for the repair and what the parts cost on the three models drivers ask about most. It draws on the carmakers' own bulletins, six owner's manuals and the 338 federal complaints that name the code.

What does P0304 mean?

It means the engine computer counted enough misfires in cylinder 4 to raise emissions or, at worse levels, to damage the catalytic converter. The computer doesn't hear a misfire: it watches how evenly the crankshaft speeds up, and a cylinder that doesn't fire leaves a gap in that rhythm.

California's OBD II rules require the computer to name the misfiring cylinder. Since the 2005 model year, a maker may store the one-cylinder code when more than 90% of the misfires are in that cylinder, and must store a multiple-misfire code, P0300, when two or more cylinders each have more than 10%.

So on a 2005 or later car, P0304 alone usually means nearly all the misfires are in cylinder 4, and P0304 with P0300 means more than one cylinder may be misfiring, or the two codes came from different drives. How the counting works, and why the lamp flashes on some misfires and not others, is on the P0300 page.

What you notice depends on how often the cylinder misses. In the 338 federal complaints naming P0304, owners describe:

• Lost power or hesitation.63 complaints, mostly when accelerating; jerking when accelerating covers how that feels and what else causes it.
• Shaking or vibration.48 complaints, the engine running unevenly with one cylinder not doing its share.
• A flashing check engine light.49 complaints, the sign of a misfire heavy enough to damage the catalytic converter.
• Stalling.39 complaints mention the engine dying.
• A rough idle.23 complaints, the shake at a standstill.
• A fuel smell or a failed emissions test.5 complaints each, the least common signs.

Where is cylinder 4?

Wherever that engine's maker numbered it, so the answer for your car is in its service information, not in a rule of thumb. The owner's manual rarely helps: of the six we read, only the 2012 F-150's prints a firing order, the sequence the cylinders fire in, which isn't the same as where they sit.

One carmaker document does place it: Chrysler's bulletin for its 3.6L V6 tests cylinders 2, 4 and 6 for leaks and replaces the left cylinder head if one leaks 25% or more. That only makes sense with cylinder 4 on the left head, alongside 2 and 6.

On any other engine, ask the shop to point at the coil it means before anything is unplugged. Counting coils from the front of the car is a guess, and a guess is how the wrong coil gets swapped.

What causes a cylinder 4 misfire?

Usually something that feeds that one cylinder: its spark plug, its coil or its injector. Past those is the cylinder itself, and the carmakers' bulletins name several ways it fails.

1. Spark plug

Worn, oily or wet with coolant, a plug can't light the mixture. Oil in the plug well usually comes from a leaking valve cover gasket or plug tube seal, and valve cover gasket replacement cost covers that job.

2. Ignition coil

Most engines have one coil per cylinder, sitting on the plug, which is what makes the swap test work.

3. Fuel injector

No fuel, no fire. Kia's bulletin for some of its 2.5L engines replaces the direct injectors, damaged by ethanol in the fuel.

4. Coolant in the cylinder

A head gasket letting coolant past on Honda's 1.5L turbo, hybrid and newest 2.0L engines, which Honda now covers. On some Ford Escape, Edge and Fusion models it's coolant intrusion on the 1.5L and 2.0L EcoBoost, which the P0301 page covers.

5. Worn valvetrain

The parts that open the valves, a lifter, cam lobe or rocker, usually with a tick. GM names it on its V8s and 2.7L turbo and Chrysler on its 3.6L V6, and their bulletins say who pays.

6. Water or carbon

Water down the No. 4 plug hole on the 2017 to 2021 Civic Type R, and carbon on the intake valves of the 2018 and 2019 Fit.

7. Engine software

Honda updates the engine computer on the 2018 to 2022 Accord 2.0L for misfire codes including P0304.

8. Wiring or fuel delivery

When the cylinder passes a compression test, GM's misfire guide points at an ignition or fuel delivery issue, such as the wiring to coil 4 or fuel pressure. Owners rarely name either: wiring comes up in 5 of the 338 complaints and fuel pressure in 2.

A vacuum leak near cylinder 4 is often blamed too, but no carmaker document we read names one for P0304, and only 2 complaints mention one. Treat it as a check, not as a known cause.

Can you drive with a P0304 code?

Gently and not far, and not at all once the coolant is falling. A flashing light means a misfire heavy enough to damage the catalytic converter, and the owner's manuals for the three models drivers ask about most each say what to do:

A steady light means a lighter misfire, and Honda's manual still says to avoid high speeds and get the car inspected immediately. On an engine losing coolant into a cylinder, driving on also drains the system that stops it overheating.

The flashing check engine light guide covers what the blink means on any code, and bad catalytic converter symptoms covers what a long misfire does downstream.

How do you tell the coil from the cylinder?

Swap the coil first, because it's free and it splits the causes in two. Move coil 4 to another cylinder, clear the code and drive: if the code comes back as the other cylinder's, the coil is bad, and if it comes back as P0304, the plug is next.

Which coil is number 4 depends on how the maker numbered that engine, so have a shop or a parts counter point it out before you unplug anything; where cylinder 4 is explains why.

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Plugs can be tested the same way, and one 2022 Accord owner, in complaint 11550704, quotes the dealer's paperwork doing just that: cylinder 4 swap spark plugs to known working plug misfire gone. A swap isn't a carmaker procedure, though, and at a shop the makers start with the cylinder:

• GM: compression first.Its misfire guide, for cars the normal diagnosis hasn't solved, runs a relative compression test first; a failed one leads to a leakage test and the valvetrain, and a pass points at spark or fuel.
• Toyota: compression, then spark, then fuel.Its cold-start misfire bulletin checks compression, then the plugs and coils, then the injectors' resistance.

A relative compression test compares the cylinders electronically while the engine cranks. A leakage test pumps air into one cylinder and measures how much escapes, and where; both are shop work.

Does Honda cover a P0304 head gasket?

On many recent Civic, CR-V and Accord models, yes, if the dealer finds coolant in the cylinders. On August 26, 2026 Honda extended its head gasket warranty to 10 years from the original purchase or 150,000 miles, whichever comes first, for overheating, coolant loss, misfire or codes P0300 through P0304.

Eligibility is checked by VIN, the car's 17-character vehicle identification number, and the models are:

• Accord.2018 to 2022 with the 1.5L turbo, every 2023 to 2026 model, and the hybrids. Not the 2018 to 2022 2.0L turbo.
• Civic.The 1.5L turbo from 2016 on, including the Si, the 2025 and 2026 2.0L with the continuously variable (CVT) automatic, and the 2025 and 2026 hybrid. Not the 2016 to 2024 2.0L.
• CR-V.2017 to 2026 with the 1.5L turbo, and the 2020 to 2026 hybrid. The 2.4L isn't listed.

The dealer's check is specific: pressure-test the cooling system at 18 psi, pumped in with a tester, then remove all four plugs and look into each cylinder with a borescope, a camera on a thin probe, for coolant coming past the gasket. If there's none, Honda's bulletin says it doesn't apply and the normal diagnosis carries on.

Cars already past 10 years get 6 months from the date of Honda's letter, which the company expected to start sending in early October 2026. For the oldest, the 2016 and 2017 Civic and the 2017 CR-V, Honda's bulletins for those carry an expiry date of May 31, 2027.

Other Honda bulletins name P0304 too, and two of them carry longer coverage:

• 2018 and 2019 Fit.Carbon on the intake valves; the cleaning and a software update are covered for 10 years or 150,000 miles.
• 2013 to 2015 Pilot, 2013 to 2017 Odyssey.Worn piston rings let oil foul the plugs. The 2025 extension runs 10 years or 150,000 miles from purchase, and a window that covered every one regardless of age or mileage closed on January 31, 2026.
• 2017 to 2021 Civic Type R.Water from the hood scoop pools in the No. 4 plug hole; a new valve cover, plug and coil, under the normal warranty.
• 2018 to 2022 Accord 2.0L.An oil change and an engine computer update, under the normal warranty.

What do Ford, GM, Jeep and Toyota say about P0304?

Each has bulletins that name it, and most point past the plug and coil, to moisture, lifters, a cylinder head or valve clearance:

If the misfire comes with a tick, GM's diagnostic bulletin for its pushrod V6 and V8 engines says the noise won't stop when a shop cuts the injectors, and it runs at camshaft speed, half the crankshaft's. That points at a worn cam lobe, a lifter or a valve, which new plugs and coils won't touch.

On a GM V8 with Active Fuel Management, cylinder 4 is one of the four the engine shuts off to save fuel, with 1, 7 and 6, and GM describes those lifters collapsing or sticking. That doesn't make cylinder 4 special, since cylinder 1 is in the same set.

On Chrysler's later 3.6L engines, a 2025 tech tip describes worn intake cam lobes and rockers, typically between 10,000 and 60,000 miles; NHTSA lists it for vehicles including the 2018 Wrangler and the 2019 and 2020 Ram 1500. It says outright that it doesn't authorize warranty repairs.

How much does it cost to fix P0304?

Usually $256 to $572 at a shop, CarMD's US averages with labor for a coil, plugs or an injector, and $480 for plugs and coils together. The part alone costs far less, and the box below prices it on the F-150, the Silverado and the Accord.

A cylinder repair is a different order of money, which is why the coverage check comes first. Honda allows its dealers 11.5 hours for the head gasket alone on a 2018 to 2022 Accord 1.5L, and 14.8 with a new head and short block, the engine's lower half, and Chrysler allows 8.2 to 10.2 hours for a 3.6L left head.

A misfire left alone can also mean a catalytic converter replacement, which CarMD puts at $1,511 on average. Out of coverage, head gasket replacement cost covers what that job runs.

How do you fix a P0304 code?

By finding which part failed before buying one, starting with what's free. To fix a P0304 code, work through the 7 steps below, in order:

1. Read and save every code and its freeze frame. The freeze frame is the snapshot the computer saves when a code sets, and Toyota's cold-start bulletin reads it for the engine temperature. P0305 and P0306 beside it on a 2011 to 2013 F-150 3.5L, or P0316, a misfire just after starting, which Ford's coolant intrusion bulletins list on its EcoBoost engines, each points somewhere specific.
2. Check the coolant level. A level that keeps falling with no visible coolant leak points at coolant getting into the cylinder; stop driving and have the system pressure-tested.
3. Run the VIN. A dealer checks open campaigns and warranty extensions by VIN, including Honda's head gasket and Fit carbon coverage. You can also run it through our free VIN checker to see open recalls and owner complaints first.
4. Swap coil 4 with another cylinder's. Clear the code and drive; if the code follows the coil, replace the coil.
5. Pull and read the plug. Wet with coolant, black with oil or sitting in water, it's pointing past itself.
6. Have the injector checked. A shop measures its resistance and runs a balance test; on a 2018 or 2019 Fit, the carbon cleaning comes first.
7. Then test compression and leakage. A cylinder that fails points at a head gasket, a valve or the valvetrain, which is when a warranty extension matters most.

The swap and the plug are yours to do, and from the injector on it is shop work. A quote for a new head with no leakage test or pressure test behind it skips a step GM, Chrysler and Honda all write down, and reading a repair estimate shows what else to check.

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Common questions

Asked most often

What is the most common cause of a P0304 code?

The spark plug or coil on most cars, and coolant getting into the cylinder on 1.5L and 2.0L Ford EcoBoost engines. Ford and Lincoln owners mention coolant in 105 of 127 federal complaints, while across every make spark plugs come up in 88 and coils in 52.

Can a dirty air filter cause a P0304?

Not going by the documents: the air filter feeds every cylinder alike, and no carmaker bulletin we read names it for P0304. A misfire that stays in cylinder 4 points at something only cylinder 4 has, its coil, plug, injector or the cylinder itself.

Why is there a P0304 after changing the spark plugs?

Then the plug probably wasn't the cause. The next checks are the coil, the injector and the cylinder's compression; one 2013 Wrangler's P0304 survived new coils and plugs until a leak test found the cylinder head.

Is a cylinder 4 misfire more common than cylinder 1?

Not in the federal record: 338 complaints name P0304 and 332 name P0301, and 100 name both. Model by model the two run close on some cars and apart on others, from 57 and 57 on the Escape to 19 and 31 on the Odyssey.

Is P0304 covered by warranty?

On some engines, beyond the normal warranty. Honda covers the head gasket on many recent Civic, CR-V and Accord models for 10 years or 150,000 miles, and the 2018 and 2019 Fit's carbon cleaning for the same, checked by VIN.

Prices are ranges compiled in September 2026 for the US market and vary by region, engine and shop rate. Prices from $10 up are rounded to the nearest dollar, so $14.13 reads as $14 and $14.52 as $15. Below $10 they're exact. This article is general information, not advice about your specific vehicle. See the disclaimer.