Worn or faulty spark plug or spark plug wire on cylinder 4. Faulty ignition coil (coil pack), or distributor cap and rotor where fitted. Vacuum leaks. Low or weak fuel pressure, or running out of fuel. Improperly functioning EGR system. Incorrect ignition timing or a defective MAF, crankshaft, camshaft or throttle position sensor (listed by RepairPal). Mechanical engine problems such as poor compression, a burned exhaust valve, a leaking head gasket or other valve problems. Faulty fuel injector or oxygen sensor (listed by OBD-Codes.com; RepairPal calls both common misdiagnoses). Faulty catalytic converter or a defective engine computer (listed by OBD-Codes.com).

Replace the spark plug, plug wire or ignition coil on cylinder 4. Repair vacuum leaks. Repair low fuel pressure or a faulty injector once testing confirms it. Repair EGR, ignition timing or sensor faults found during diagnosis. Repair mechanical problems found by a compression or leak-down test, such as a burned exhaust valve. Replace the catalytic converter if it has failed or been damaged by a long-running misfire.

P0304⚡ Ignition / engine misfireStop and fix promptly

P0304: Cylinder 4 Misfire Detected (Meaning, Causes, Fixes)

Q: What does P0304 mean?

P0304 means the engine computer has detected that cylinder 4 isn't firing properly. Worn spark plugs, plug wires or ignition coils are the first things both sources point to, but vacuum leaks, fuel pressure, EGR faults, sensor faults and mechanical problems such as low compression or a burned exhaust valve can also set it. P0304 is a generic OBD-II code, so it applies to all OBD-II vehicles, though repair steps vary by make and model. RepairPal explains how it's detected: the misfire monitor counts crankshaft sensor pulses, and when cylinder 4 isn't contributing its share (more than about a 2 percent RPM variance between cylinders) it sets P0304 and turns on the check engine light. Above roughly a 10 percent variance the light blinks to warn of a misfire harmful to the catalytic converter. Cylinder numbering differs between engines, so check your service information to find cylinder 4. The sources disagree on some causes: OBD-Codes.com lists faulty oxygen sensors and faulty fuel injectors as possible causes, while RepairPal lists fuel injectors and oxygen sensors among the common misdiagnoses for P0304. OBD-Codes.com also lists a faulty catalytic converter, running out of fuel and a defective engine computer, which RepairPal doesn't mention, while RepairPal adds incorrect ignition timing, vacuum leaks, low fuel pressure, EGR problems and MAF, crank/cam and throttle position sensor faults.

Q: Can I keep driving with P0304?

Stop and fix promptly. RepairPal rates P0304 'Urgent', meaning a malfunction that can cause severe damage to the vehicle. If the check engine light is flashing, RepairPal explains that a misfire harmful to the catalytic converter is happening right now, so ease off, avoid hard acceleration and get it checked as soon as you safely can.

Reviewed 2026-10-22 · Based on 3 cited source(s)

What P0304 means

P0304 is a generic OBD-II code, so it applies to all OBD-II vehicles, though repair steps vary by make and model. RepairPal explains how it's detected: the misfire monitor counts crankshaft sensor pulses, and when cylinder 4 isn't contributing its share (more than about a 2 percent RPM variance between cylinders) it sets P0304 and turns on the check engine light. Above roughly a 10 percent variance the light blinks to warn of a misfire harmful to the catalytic converter. Cylinder numbering differs between engines, so check your service information to find cylinder 4. The sources disagree on some causes: OBD-Codes.com lists faulty oxygen sensors and faulty fuel injectors as possible causes, while RepairPal lists fuel injectors and oxygen sensors among the common misdiagnoses for P0304. OBD-Codes.com also lists a faulty catalytic converter, running out of fuel and a defective engine computer, which RepairPal doesn't mention, while RepairPal adds incorrect ignition timing, vacuum leaks, low fuel pressure, EGR problems and MAF, crank/cam and throttle position sensor faults.

Severity: can I keep driving?

Stop and fix promptly

RepairPal rates P0304 'Urgent', meaning a malfunction that can cause severe damage to the vehicle. If the check engine light is flashing, RepairPal explains that a misfire harmful to the catalytic converter is happening right now, so ease off, avoid hard acceleration and get it checked as soon as you safely can.

Common symptoms

Check engine light on, or flashing while the misfire is happening
Rough running, hesitation or jerking when accelerating
Engine harder to start, or stumbling (listed by OBD-Codes.com)
Rough idle, dying at stop signs, lack of power or lower fuel economy (listed by RepairPal)
Often no adverse symptoms noticed by the driver (noted by RepairPal)

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Worn or faulty spark plug or spark plug wire on cylinder 4
Faulty ignition coil (coil pack), or distributor cap and rotor where fitted
Vacuum leaks
Low or weak fuel pressure, or running out of fuel
Improperly functioning EGR system
Incorrect ignition timing or a defective MAF, crankshaft, camshaft or throttle position sensor (listed by RepairPal)
Mechanical engine problems such as poor compression, a burned exhaust valve, a leaking head gasket or other valve problems
Faulty fuel injector or oxygen sensor (listed by OBD-Codes.com; RepairPal calls both common misdiagnoses)
Faulty catalytic converter or a defective engine computer (listed by OBD-Codes.com)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Read all stored codes and freeze frame data, and record them. RepairPal says to note the engine load, throttle position, RPM and road speed when the code set, and to note any other codes because the misfire may be caused by another fault.
If there are no symptoms, OBD-Codes.com suggests clearing the code and seeing if it comes back.
If the engine stumbles or hesitates, check all wiring and connectors to the cylinder 4 spark plug and coil.
If the spark plugs, wires, cap and rotor are old, replacing them is regular maintenance anyway (OBD-Codes.com).
If your scan tool shows a misfire counter for each cylinder, watch cylinder 4. If not, RepairPal suggests swapping components such as the coil or spark plug with another cylinder to see if the misfire follows the part.
A rotten-egg smell from the exhaust points to a failing catalytic converter, according to OBD-Codes.com.

Typical fixes

Replace the spark plug, plug wire or ignition coil on cylinder 4
Repair vacuum leaks
Repair low fuel pressure or a faulty injector once testing confirms it
Repair EGR, ignition timing or sensor faults found during diagnosis
Repair mechanical problems found by a compression or leak-down test, such as a burned exhaust valve
Replace the catalytic converter if it has failed or been damaged by a long-running misfire

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

The check engine light is flashing
The misfire stays on cylinder 4 after replacing the plug and coil
A compression or leak-down test is needed to check valves, rings or the head gasket
The misfire only happens under certain load or speed conditions and has to be reproduced on a test drive with a scan tool

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0304?

RepairPal rates it 'Urgent'. If the check engine light is flashing, the misfire can damage the catalytic converter, so limit driving and get it fixed soon.

Is P0304 always a bad spark plug or coil?

No. Plugs, wires and coils are the first things both sources list, but vacuum leaks, fuel pressure, EGR faults, sensors and mechanical problems like low compression or a burned exhaust valve can also cause it. Swapping the coil or plug to another cylinder is a cheap way to see if the misfire follows the part.

What's the difference between P0300 and P0304?

P0304 pins the misfire to cylinder 4. P0300 means random or multiple cylinders are misfiring, which usually points to something shared by all cylinders, such as fuel pressure or a vacuum leak.

Sources

Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.