P0304 Code: Cylinder 4 Misfire Detected
Cylinder 4 is misfiring. Its end or rear position makes water, oil and intake leaks common culprits.
At a glance
Severity: Moderate
- Meaning
- Cylinder 4 Misfire Detected
- System
- Ignition & Misfire · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $90–$4,500
- DIY difficulty
- Moderate
P0304 means the engine computer keeps detecting a missing power stroke from cylinder 4. Where that cylinder sits changes what usually goes wrong. On a transverse four-cylinder it is the end cylinder next to the transmission. On a rear-wheel-drive four-cylinder it is the last cylinder, right under the cowl, where water from a clogged cowl drain and heat soak hit hardest. On V8 trucks it varies: on Ford's 4.6/5.4 V8 it is the rear passenger-side cylinder near the firewall, and on GM small-blocks it is the second cylinder on the passenger side and one of the cylinders shut off by Active Fuel Management. Coils and plugs are still the usual cause, but look for water in the plug well, a leak at the end intake runner, and on GM trucks a failing lifter.
Can I keep driving? With a steady light, drive gently and only as far as the shop. Avoid towing and full throttle. A flashing light means the catalytic converter is overheating, so stop. A ticking noise along with the misfire can mean a lifter problem that gets worse, so don't put the repair off.
How to read P0304
- P Powertrain
- 0 Generic (SAE)
- 3 Ignition / misfire
- 04 Specific fault
Symptoms
- Shake at idle and a stumble when pulling away
- Misfire that is worse after rain or a car wash (water in the plug well)
- Ticking or tapping from the top of the engine (lifter or valvetrain cause)
- Check-engine light on, flashing under load
- Reduced power and gas mileage
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Failed coil or coil boot on cylinder 4, often from water or oil sitting in the plug well
- Worn or cracked cylinder 4 spark plug
- Vacuum leak at the end intake runner, intake gasket or nearby PCV connection
- Clogged or leaking cylinder 4 fuel injector
- Valvetrain problem: collapsed lifter, worn cam lobe or burned valve on cylinder 4
- Chafed harness or corroded connector at the back of the engine
Diagnosis step by step
Locate cylinder 4 and inspect its plug well
Confirm the cylinder numbering for your engine, then pull coil 4 and shine a light down the well. Rust, standing water or oil in the well point you straight at the cause. Clear the cowl drains or fix the valve cover leak, or the new coil will fail too.
Tools: Flashlight, Screwdriver
Smoke-test the intake, focusing on the end runner
End runners and the ends of the intake gasket are common leak points, especially on plastic manifolds that have gone through many heat cycles. Push smoke into the intake and watch the cylinder 4 runner, PCV hoses and vacuum fittings near it.
Tools: Smoke machine
Swap coil 4 with another cylinder
Move coil 4 to another cylinder, clear the code and drive. If the misfire follows the coil, replace it along with the boot. If it stays on 4, the coil is cleared.
Tools: Screwdriver, OBD-II scanner
Replace or inspect plug 4
Check it for cracks, carbon tracking and gap wear. If the plug well had water in it, look closely at the porcelain for a thin black track line, which shows where the spark was jumping to ground.
Tools: Spark plug socket, Torque wrench
Check for a valvetrain problem with a vacuum gauge and a compression test
At idle, a vacuum gauge needle that drops sharply at a regular rhythm suggests one valve isn't opening or sealing. Follow up with a compression test on cylinder 4. On pushrod V8s, a cylinder with good compression that still misfires, plus a tick, points to a collapsed lifter or a worn cam lobe.
Tools: Vacuum gauge, Compression tester
Check injector 4
If everything above passes, confirm injector 4 clicks evenly and has a resistance similar to the other injectors. Check its connector for corrosion, since the back of the engine collects water and grime.
Tools: Multimeter
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace cylinder 4 coil and boot | Yes | $90–$300 |
| Replace spark plugs (full set) | Yes | $120–$450 |
| Replace intake manifold gasket | No | $200–$600 |
| Replace cylinder 4 fuel injector | No | $200–$600 |
| Replace lifters and camshaft (V8 with Active Fuel Management) | No | $2,000–$4,500 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Chevrolet
On GM 5.3L and 6.2L V8s with Active Fuel Management (Silverado, Tahoe, Suburban), cylinders 1, 4, 6 and 7 are the ones that shut down while cruising, and their special lifters are a known failure point. A P0304 with a tick, especially if compression is fine, should be checked for a collapsed lifter before you buy coils. Oil consumption in those cylinders can also foul plugs.
Field data
| Ford | 110 | ||
|---|---|---|---|
| Honda | 80 | ||
| Kia | 15 | Sorento, Optima, Sedona | |
| Hyundai | 13 | Elantra, Santa Fe Sport, Sonata | |
| Jeep | 12 | Wrangler Unlimited, Grand Cherokee, Cherokee | |
| Dodge | 9 | Grand Caravan, Charger, Durango | |
| Chrysler | 7 | Town And Country, Pacifica | |
| Chevrolet | 6 |
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
A P0304 with the check-engine light on will fail an OBD emissions test. If the cause is a lifter or cam on a V8, clearing the code won't hold, because the misfire comes back within a few miles.
Frequently asked questions
Why does my cylinder 4 misfire get worse after rain or a car wash?
Water is getting into the cylinder 4 plug well, usually through a cracked coil boot seal or from a clogged cowl drain above the rear of the engine. Blow the well dry, replace the damaged boot and fix the water path.
Can a lifter cause P0304?
Yes. A lifter that has collapsed or lost contact with its cam lobe keeps a valve from opening properly, so the cylinder can't breathe and misfires. Look for a tick from the top of the engine and good compression paired with a stubborn misfire, a combination common on V8s with cylinder deactivation.
Is cylinder 4 hard to reach?
On most transverse four-cylinders it is easy, at the transmission end under the coil cover. On rear-wheel-drive cars and trucks it can be tucked against the firewall, and some coils need the air intake or other parts moved out of the way first.
The code came back a week after a new coil. What did I miss?
Most often it's water or oil still getting into the well and killing the new boot, or a plug that was also cracked. If the well is dry and the plug is new, move on to the intake leak and valvetrain checks.