P0304high severityDo not driveP0304 Code: Cylinder 4 Misfire Detected
Written in plain English and reviewed by the eli5cars editorial team · Last reviewed June 2026
What it means in plain English
Your engine fires each cylinder in a specific order to create smooth, even power. P0304 means cylinder number 4 is not firing reliably — it's misfiring. On a 4-cylinder engine, that's 25% of your power gone; on a 6-cylinder it's about 17%. You'll typically feel a noticeable shake, rough idle, or sluggish acceleration. The ECM detected enough missed combustion events to store this code and turn on the check engine light.
⛔ Do not drive with this code active
This fault can cause immediate mechanical or safety damage. Have the vehicle towed or inspected before driving.
Most likely causes — ranked
#1 Worn or fouled spark plug in cylinder 4
most likelyFix: Pull the cylinder 4 plug and inspect it. A worn electrode, abnormal deposits (black carbon = rich mixture, white = lean or overheating, oily = oil consumption), or a cracked insulator all cause misfires. Replace the plug and clear the code. Because plugs wear together, replacing the full set is often recommended at this stage.
#2 Failed ignition coil on cylinder 4
likelyFix: The ignition coil converts 12V battery power into the high-voltage spark needed to ignite the air-fuel mixture. A coil that's cracking internally delivers an inconsistent spark. Perform the swap test: move the cylinder 4 coil to another cylinder. If the misfire code moves to that cylinder, the coil is bad. Replace it.
#3 Clogged or failing fuel injector on cylinder 4
likelyFix: A dirty injector sprays fuel unevenly, causing the cylinder to run lean and misfire. A stuck-open injector floods it. Add a high-quality fuel injector cleaner to the tank as a first step. If the code returns, have the injector tested and replaced. The cylinder 4 injector is often accessible without major disassembly.
#4 Low cylinder compression (worn piston rings, burned valve, or head gasket)
possibleFix: If the ignition and fuel components check out, perform a compression test. Low compression in cylinder 4 compared to others points to mechanical engine damage. A leakdown test narrows it down further — air leaking past the rings, intake/exhaust valves, or a head gasket. These repairs require engine disassembly.
Check engine light on? Get a free code scan
If your check engine light is on, most auto-parts stores (AutoZone, O'Reilly, Advance) will read the code for FREE — no purchase required, just walk in. Bring the code back here and look it up at eli5cars.com/obd2 for the plain-English explanation.
Note: a scan only helps when there's an active fault code. For purely mechanical symptoms (noises, vibrations, leaks), a scan may show nothing — the suspects listed above are your starting point.
Want to read the codes yourself?
First, the free option: AutoZone, O’Reilly, and Advance Auto Parts read your codes for free — no purchase needed. That’s the cheapest way to get the code to look up here. If you’d rather have your own so you can check anytime (and clear a code after a fix), the Veepeak OBDCheck BLE is an inexpensive, reliable plug-in reader that pairs with a free phone app and works on any car sold since 1996.
See the Veepeak OBD2 scanner on AmazonAffiliate link — we may earn a commission at no extra cost to you. Details.
Frequently asked questions
Can I drive with a P0304 code?
Only to get it repaired. A misfiring cylinder pumps raw gasoline into the exhaust system, which can overheat and melt the catalytic converter's internal substrate — turning a $150 repair into a $1,200 one. Don't commute on P0304 until it's fixed.
How do I know if it's the coil or the plug causing P0304?
Do the swap test. Move the cylinder 4 coil to a known-good cylinder position, and also move cylinder 4's spark plug to another position. Clear the codes and drive. If P0304 becomes P0301 (or another number), the component you moved is the culprit. AutoZone or O'Reilly can scan for free to confirm which code came back.
My car shakes at idle and P0304 is set — is that related?
Yes, directly. With cylinder 4 not contributing power, the engine's rotating assembly runs out of balance. The shake you feel at idle and low RPM is the engine compensating for a dead cylinder. Fixing the misfire eliminates the shake.
Does P0304 always mean the catalytic converter is damaged?
Not necessarily. If you catch and fix the misfire quickly, the cat is likely fine. Extended misfiring — days or weeks of driving — is what damages it. If after fixing the misfire you see a P0420 or P0430 code appear, the cat may need evaluation.