P0304 Code: Cylinder 4 Misfire Detected

A P0304 code means the PCM caught cylinder 4 not pulling its weight. The crankshaft position sensor feeds the PCM a detailed picture of crank acceleration and deceleration. When cylinder 4 fires, the crank should speed up. When it does not, the PCM counts that as a misfire. Enough misfires in a given window and you get the code.

The diagnostic approach is identical to any cylinder-specific misfire — isolate whether the problem is spark, fuel, or compression. But cylinder 4 has some quirks depending on engine layout that are worth knowing about.

Why Cylinder 4 Specifically?

On inline 4-cylinder engines, cylinder 4 sits at the end of the block — furthest from the water pump on many designs. That means it often runs hotter than the other three cylinders. Heat kills ignition coils faster, and it can lead to pre-ignition or detonation that damages pistons and rings over time. On V6 and V8 engines, cylinder 4 placement varies by manufacturer, but the same principle applies: figure out if it is spark, fuel, or mechanical.

Common Causes of P0304

Ignition System Failures

• Ignition coil failure: COP coils on cylinder 4 of inline-4 engines tend to fail early due to heat soak from the exhaust side. Internal insulation breaks down and the coil arcs internally instead of at the plug gap.
• Spark plug issues: Check the gap with a feeler gauge — do not eyeball it. OE spec is usually 0.028–0.044 inches. Also look at the plug tip color: tan/light gray is healthy, black is rich, white is lean, oily is burning oil.
• Coil connector corrosion: Heat and moisture corrode the connector pins over time. Pull the connector and inspect — green or white crusty buildup on the terminals means high resistance in the circuit.

Fuel Delivery Failures

• Injector failure: Use a stethoscope to listen for click at the injector body. No click means no pulse. Verify with a noid light that the PCM is sending the signal. If you have signal but no click, the injector is stuck or the coil inside is open — check resistance with a DVOM. Most port injectors spec at 12–16 ohms.
• Wiring fault: Trace the injector harness from the connector back to the main engine harness. Look for chafed spots, melted insulation near the exhaust, or pulled pins at the connector.
• Low fuel pressure: While this usually affects all cylinders, a weak pump under load can cause the last injector in the fuel rail to starve first. Check fuel pressure at idle and under snap throttle — spec varies by vehicle but most port injection systems run 35–65 PSI.

Mechanical Failures

• Low compression: Burned exhaust valve, head gasket breach, cracked ring land. Run a compression test — minimum 120 PSI, no more than 10% spread between cylinders.
• Intake valve carbon (GDI): On direct injection engines, fuel does not wash the back of the intake valves. Carbon builds up and prevents proper sealing. Common on Hyundai/Kia GDI engines, BMW N54/N55, and Ford EcoBoost after 50K miles.
• Cam lobe wear or lifter collapse: If a hydraulic lifter bleeds down or a cam lobe is wiped, the valve barely opens. Check for a ticking noise that changes with RPM.

Step-by-Step P0304 Diagnosis

Step 1: Read All Codes and Freeze Frame

Document every code — stored, pending, and history. Check freeze frame data for the P0304: note engine RPM, load, coolant temp, and fuel trim at the time of the fault. If the misfire happened at 3,200 RPM under load, that is a different animal than a steady idle misfire.

Step 2: Swap Test — Coil and Plug

Move the cylinder 4 coil to cylinder 2 (or any convenient adjacent cylinder). Move the cylinder 4 plug at the same time if you want to test both. Clear codes and run the engine for a few minutes — idle and some light revs. If the misfire code follows to the new cylinder, you found the bad part. If P0304 stays, move on.

Step 3: Test the Injector

With the engine running, use a stethoscope or long screwdriver pressed to the injector body. You should hear steady clicking. If silent, backprobe the connector with a noid light or scope the injector waveform. You are looking for a clean voltage drop from battery voltage to near zero as the PCM grounds the injector. A lazy or erratic waveform means a wiring or driver issue.

Step 4: Run a Compression Test

Disable ignition and fuel. Crank the engine through at least 4-6 compression strokes per cylinder. Record the readings. Compare cylinder 4 to the rest. If cylinder 4 is low, do the wet compression test — add a tablespoon of oil, retest. Compression rises = ring issue. No change = valve or head gasket issue.

Step 5: Cylinder Leak-Down Test

For a more precise picture, run a leak-down test. Apply 100 PSI of shop air to cylinder 4 at TDC on the compression stroke. Acceptable leakage is under 15-20%. Listen at the tailpipe (exhaust valve leak), intake (intake valve leak), or adjacent cylinders and radiator (head gasket leak).

Step 6: Check Scan Data PIDs

Monitor these PIDs during a test drive: misfire counters per cylinder (Mode 6), short-term and long-term fuel trims, O2 sensor voltages, and calculated load. If fuel trims are normal and the misfire only happens under load, think mechanical. If trims are way off and the misfire is constant, think fuel or air.

Common Diagnostic Mistakes

• Replacing the coil without swapping first. A $35 coil is cheap, but if the real problem is a burned valve, you just wasted time and parts.
• Not checking for TSBs. Many engines have known cylinder 4 issues. Toyota 2AZ-FE engines, for example, had oil consumption problems that caused misfires on specific cylinders. Always check TSBs for your specific engine.
• Overlooking the wiring harness. On cylinder 4 of inline engines, the harness runs right past the exhaust manifold. Heat damage is common and creates intermittent opens in the coil or injector circuit.
• Skipping the visual inspection. Pull the coil and look down in the plug well. Oil pooling around the plug boot means a leaking valve cover gasket — that oil causes the coil boot to swell and the spark to arc to ground instead of across the plug gap.

Confirming the Fix

Clear codes, warm the engine fully, and perform a thorough test drive including idle, cruise, and moderate acceleration. Monitor Mode 6 misfire data — cylinder 4 should be at zero or single digits across all RPM ranges. Two consecutive drive cycles with no MIL confirms the repair.

If you are seeing misfires across multiple cylinders alongside P0304, read our P0300 random misfire guide — the root cause may be systemic. And if cylinder 3 is also acting up, check the P0303 guide for additional insight.