P0304 Code: Cylinder 4 Misfire Detected

What the P0304 code means

The misfire detection system measures the instantaneous change in crankshaft speed and works out which cylinder had the incomplete combustion. The P0304 code corresponds to cylinder four.

Like every single-cylinder code, it points to an individual component and not a general cause. That makes it easier to solve than a P0300, as long as you follow the right order: spark, fuel, air and compression.

It also helps to note whether the misfire shows up only when cold, only when hot or only under load. A weakened coil tends to fail when it needs the most voltage, which means while accelerating.

On four-cylinder engines, the last cylinder is often the one that takes the most heat. It sits at the end of the block, gets less coolant flow and is frequently closer to the exhaust manifold, which puts its coil and wiring under higher temperatures all the time.

That is why many vehicles repeat misfires on this particular cylinder over the years. It is not bad luck or a defective part: it is temperature. If the code has come back more than once, check the condition of the wiring harness in that area and consider protecting it with a heat sleeve when you make the repair.

P0304 code symptoms

• Check engine light flashing during the misfire.
• Vibration at idle and at startup.
• Jerking when you accelerate and loss of power.
• High fuel consumption.
• Smell of unburned gas.
• Uneven exhaust sound.
• The misfire returns on the same cylinder every so often even though the part was replaced.
• Hardened or cracked wiring insulation in that area of the engine.
• A misfire that shows up sooner on long uphill climbs than on flat roads.

Likely causes of the P0304 code

1. Cylinder 4 ignition coil weakened by heat.
2. Worn spark plug or one with an excessive gap.
3. Clogged injector.
4. Coil or injector connector with an intermittent contact.
5. Vacuum leak at the intake runner for that cylinder.
6. Low compression from mechanical wear.
7. Wiring harness damaged by its closeness to the exhaust.
8. Harness insulation degraded by temperature, with shorts that only appear when hot.
9. Poor cooling of the last cylinder because of a partially clogged passage.
10. Low-quality replacement coil that cannot handle the temperature in that position.
11. Coil connector with a broken retaining clip, which lets it shift slightly and lose contact from vibration.

How to diagnose the P0304 code step by step

1. Swap the coil with the one from another cylinder and check whether the code moves.
2. Swap the spark plug and examine its condition.
3. Check the connector and harness for cylinder 4, which on many engines sits close to the exhaust.
4. Swap the injector if the misfire stays.
5. Watch the misfire counter in live data while you accelerate. A weak coil mostly fails under load.
6. Measure compression as the last step.
7. Examine the wiring insulation in that area. If it is hardened, cracked or brittle, the short appears only when hot and that is the real cause.
8. Check whether the misfire returns after you replace the part. A code that keeps coming back on the same cylinder points to an underlying heat problem, not the component.
9. Check the engine operating temperature. A weak cooling system punishes the last cylinder first.
10. Check the connector retaining clip. If it is broken, it allows a tiny shift that is enough to cause intermittent misfires with engine vibration.

How to fix the P0304 code

• Replace the faulty coil and, if the engine has high mileage, consider the full set.
• Change the set of spark plugs with the manufacturer part number.
• Clean or replace the injector.
• Repair the damaged connector or harness section and route it away from the exhaust.
• Fix the vacuum leaks.
• Take on the mechanical repair if compression is low.
• Protect the harness with a heat sleeve when you repair it, and move it as far from the exhaust manifold as its routing allows.
• Use coils from a recognized manufacturer in this position. Cheap ones do not handle the temperature well and fail again.
• Check the cooling system if the code keeps returning, because an engine running hotter than it should punishes this area first.
• Replace the connector if its retaining clip is broken, instead of holding it with zip ties or tape.

Technical reference data

• Expected compression: 10 to 14 bar (145 to 200 psi), with a maximum difference of 10 percent.
• Ignition coil primary resistance: 0.5 to 2 ohms.
• Typical injector resistance: 12 to 16 ohms.
• A coil that works at idle can fail under load, so always test while accelerating.
• Ambient temperature next to the exhaust manifold: it can exceed 150 degrees Celsius (302 degrees F) while the engine runs.
• Ignition coil primary resistance: 0.5 to 2 ohms, always measured cold.
• Brittle insulation indicates long-term heat exposure, and the whole section should be replaced.
• A connector without a firm retainer causes misfires that only appear with vibration or on long climbs.

Brand-specific notes

On transverse engines, cylinder 4 often sits at the hottest and least ventilated end of the block. It is no coincidence that its coil fails sooner than the others on many vehicles.

If this code has returned more than once on the same vehicle, stop treating it as a bad part and treat it as a heat problem: protected harness, a quality coil and a cooling system in good shape.

Frequently asked questions about P0304