P0304 Cylinder 4 Misfire: Symptoms, Causes and Diagnosis
When a scan tool reports P0304, the engine computer has detected that cylinder 4 is not firing the way it should. The code tells you where the problem is, but it doesn't say which part has failed. Knowing what the code means, how serious the symptoms are, and how a careful diagnosis works can save money and help protect both the engine and the catalytic converter.
What P0304 Means and What It Cannot Tell You
P0304 stands for cylinder 4 misfire detected. A misfire happens when the air and fuel in a cylinder don't burn properly, so that cylinder doesn't deliver the push on the piston the engine expects. Most engine computers watch crankshaft speed closely. When one cylinder's power stroke causes a brief slowdown, the software counts it as a misfire. Once those events pass a set threshold, the check engine light comes on and the code is stored.
The code can't name the cause. Good combustion needs a strong spark at the right moment, the right amount of fuel, a correctly metered air charge, and a cylinder that seals well enough to build compression. A problem with ignition, fuel delivery, an air leak, or mechanical sealing can all set the same P0304. That's why other stored codes, such as lean or injector circuit codes, and the conditions recorded when the code set are so useful for narrowing the search.
Symptoms and When to Stop Driving
A cylinder 4 misfire often shows up as a rough idle, shaking you can feel through the steering wheel or seat, hesitation when accelerating, or a noticeable loss of power, especially under load. The check engine light usually comes on with these symptoms. Some drivers also notice worse fuel economy or a raw fuel smell from the exhaust. Intermittent misfires are harder to catch because the engine may run smoothly between episodes, leaving little to feel even though the code keeps coming back.
A flashing check engine light needs immediate attention. It usually means the misfire is severe enough that unburned fuel could overheat and damage the catalytic converter. If you see it, pull over when it's safe, turn the engine off, and arrange help rather than driving on. Severe shaking, a major loss of power, or a strong fuel smell are also reasons to stop safely. A steady light is less urgent, but it doesn't prove the vehicle is safe to keep driving.
How to Identify Cylinder 4 on Your Engine
Before diagnosing anything, confirm which cylinder is actually number 4. That takes the exact year, make, model, and engine, because manufacturers number cylinders differently. Depending on the layout, cylinder 4 might sit at the end of a short inline engine, near the middle of a longer one, or on either bank of a V-type engine where numbering may run down one side or alternate between sides. A generic diagram taken from a similar vehicle can easily point to the wrong cylinder.
Cylinder number is not the same as firing order. Firing order is the sequence in which the cylinders ignite, so the fourth cylinder to fire isn't necessarily cylinder 4. Counting visible coils, plug wires, or injectors by eye is also unreliable, because wiring can cross the engine and some parts are hidden under covers or intake components. If covers, limited access, or doubts about the exact engine make the location unclear, check vehicle-specific service information or have a technician confirm it.
Possible Causes Behind a Cylinder 4 Misfire
On spark-ignition engines, the ignition system is one area to test. A spark plug that is worn, cracked, or fouled with oil or carbon may not light the mixture reliably. An ignition coil with internal breakdown can produce a weak or missing spark, sometimes only under load or when hot. Damaged connectors, chafed wiring, or a poor ground can cut the signal to the coil. Fuel delivery is another possibility: a clogged or sticking injector, an injector wiring fault, or a problem in its control circuit all need testing before blame is assigned.
Air leaks and mechanical problems can set the same code. A leak at the intake runner or gasket serving cylinder 4 can let in unmetered air and make that cylinder run leaner than the others. Wider problems, such as low fuel pressure or a general vacuum leak, can affect every cylinder but still show up first in one of them. Lost compression from a poorly sealing valve, worn rings, or a head gasket problem can also cause a misfire, though P0304 alone doesn't prove internal engine damage.
Safe Observations and Professional Diagnostic Tests
Before anyone clears the code, record the stored and pending codes and the freeze-frame data. Freeze-frame data is a snapshot of readings like engine speed, load, and temperature at the moment the code set, and clearing codes erases it. Also note whether the misfire happens on cold starts, at warm idle, while accelerating, or in wet weather, and whether it started after recent repairs or a fill-up. Owner checks should stop at looking, with the engine off and cool, for loose connectors, damaged wiring, or disturbed hoses.
Unplugging parts on a running engine, testing for spark, and opening the fuel system should be left to professionals. A technician can use misfire counters, which many scan tools show for each cylinder, to see how often cylinder 4 misfires and under what conditions. Swapping a coil or plug to another cylinder and seeing whether the misfire follows it helps identify ignition faults. Injector and wiring tests, smoke testing for leaks, and compression or leak-down testing narrow things further. Any manufacturer-specific code that appears alongside P0304 should be checked against the exact vehicle's service information.
Repairing the Confirmed Fault and Checking the Result
The repair should follow the test results. If testing finds a chafed wire, the wiring gets repaired. If a smoke test shows a leaking intake gasket, that leak gets fixed. A coil, plug, injector, or mechanical fault is only addressed once testing confirms it. Replacing parts based on P0304 alone is a gamble that can add cost and leave the real cause in place. Any repair should follow the vehicle's own procedures and specifications, with a professional doing the work whenever it involves hard-to-reach parts, fuel pressure, or internal engine components.
Checking the result matters as much as the repair. A successful fix should give a smooth idle, normal acceleration, and no return of the original symptoms under the conditions that used to trigger them. The cylinder 4 misfire counters should stay at or near zero during a proper test drive, and a technician may recheck related data or repeat earlier tests. Clearing the check engine light only erases the record. It doesn't prove the problem is gone, and if the code comes back, the diagnosis isn't finished.