P0304 means the engine control module detected a misfire on cylinder 4. It does so by watching the crankshaft position sensor: each cylinder's power stroke should speed the crank up a little, and a cylinder that does not contribute shows as a momentary slowdown. P0300 is the random or multiple misfire code; P0301 through P0312 name the cylinder. The number is the cylinder's position in the manufacturer's numbering, not its position in the firing order and not "the fourth from the front" on every engine.
Severity: moderate, high if the light flashes. Can you keep driving? Only if the light is steady. A flashing check engine light means the misfire rate is high enough to dump unburned fuel into the catalytic converter and overheat it, and the correct response is to stop; see flashing versus steady. A steady light with a slight roughness at idle can be driven to a shop.
Find cylinder 4 first
- Inline four-cylinder: cylinder 4 is at the transmission end on nearly all engines.
- V6 and V8, GM and most Chrysler: odd cylinders on one bank, even on the other, numbered front to back; cylinder 4 is the second cylinder on the even bank.
- Ford V8: cylinders 1 to 4 on the passenger bank front to back, so cylinder 4 is the rearmost on that side.
- Subaru flat-four: 1 and 3 on the passenger side, 2 and 4 on the driver side; cylinder 4 is the rear driver-side cylinder.
Get it wrong and the swap test below tests the wrong cylinder.
The swap test
Because the code names a cylinder, the fastest diagnosis is to move a suspect part to another cylinder and see whether the code moves with it:
- Clear the code and note the misfire counter for each cylinder on the scan tool.
- Swap the cylinder 4 ignition coil (or plug wire) with cylinder 3's. Drive until the misfire returns. P0303 now means the coil is bad; still P0304 means it is not the coil.
- Repeat with the spark plug, then with the injector.
- If the code never moves, the cause is in the cylinder itself: compression, a valve, a head gasket, or a vacuum leak at that cylinder's intake runner.
A relative compression test with the scan tool (cranking, all coils disabled) or a mechanical gauge finishes the job when the swap test comes up empty.
What the bulletins say
- Honda 2017 to 2018 CR-V (Service Bulletin 19-037, June 2019): P0300 through P0304 or P0172 with a whirling noise from the engine; Honda extended the powertrain warranty to six years, unlimited miles, for camshafts, rocker arms and spark plugs damaged by the underlying condition. If a CR-V of those years shows P0304, check the VIN against that bulletin before paying for anything.
- GM V6 and V8 pushrod engines 2002 to 2025 (19-NA-219, November 2024): misfire with a chirp, squeak, squeal or tick is a diagnostic tip toward a valvetrain component, not ignition.
- Toyota Highlander Hybrid 2017 to 2019 (T-SB-0148-19): P0301 through P0306 corrected by reprogramming the engine computer and replacing the ignition coil, under the federal emissions warranty.
- 2023 Dodge Hornet 2.0 L turbo built January 28 to March 9, 2023 (TSB 18-079-23): P0300 through P0304 caused by a spark plug cracked during installation.
- 2025 Ram 1500 3.0 L Hurricane built on or before January 15, 2025 (TSB 08-031-25): misfire codes fixed by an ignition system control module software update.
- 2019 Ram ProMaster City 2.4 L (18-068-19): misfire codes addressed by a PCM flash.
The pattern: a spark plug or coil in most cases, a valvetrain or software cause on specific engines. Check bulletins for the exact vehicle before the swap test on any of the vehicles above.
Causes, in the order they are found
Spark plug (worn, fouled, cracked) · ignition coil or plug wire · fuel injector (clogged or dead) · vacuum leak at cylinder 4's intake runner or injector seal · low compression from a burnt valve, worn rings or a head gasket · a collapsed lifter or worn cam lobe on that cylinder · a software calibration on the engines above.
Frequently asked questions
Can I drive with a P0304 code?
If the check engine light is steady and the engine only feels slightly rough, yes, to a shop. If the light flashes, stop; the misfire is damaging the catalytic converter.
Is P0304 always a spark plug or coil?
Most often, but not always. The swap test tells you in ten minutes: move the coil to another cylinder and see whether the code follows. If it stays on cylinder 4 through coil, plug and injector swaps, the cause is compression or a vacuum leak at that cylinder.
Which cylinder is number 4?
On an inline four it is the one nearest the transmission. On a V engine it depends on the manufacturer's numbering; on GM and Chrysler V6 and V8 engines it is the second cylinder on the even-numbered bank, on a Ford V8 it is the rearmost on the passenger side.
How much does it cost to fix P0304?
A spark plug or coil is inexpensive. An injector is a moderate part. A valve or head gasket repair is major. The swap test costs nothing and decides which.
Related codes
- P0300: random or multiple misfire. P0301 to P0308: other cylinders.
- P0201 to P0208: injector circuit codes.
- P0351 to P0358: ignition coil primary or secondary circuit.
- P0420: catalyst efficiency, the eventual consequence of a misfire that was driven on.
The family is covered in more depth in P0300 misfire codes; the code number itself is explained in how to read an OBD2 code number.