BMW P0304: cylinder 4 misfire detected — Diagnosis and Repair

P0304 indicates cylinder 4 misfire detected. This guide identifies the recording module, distinguishing symptoms and evidence-based tests to run before replacing parts. Start a structured P0304 diagnosis by reviewing the recording module and freeze frame in OBDThink. What Does P0304 Mean? P0304 indicates cylinder 4 misfire detected. This guide identifies the recording module, distinguishing symptoms and evidence-based tests to run before replacing parts. DTC: P0304 Definition: cylinder 4 misfire detected Module: verify with VIN and BMW service data Evidence: freeze frame, module scan, target/actual comparison Note: definition can vary by chassis, engine and software Before moving parts, record cylinder-specific misfire counters and the freeze frame; a cold-idle miss is not the same fault pattern as a full-load breakup. Symptoms • Cylinder 2 can shake the engine in a regular rhythm at idle while the other cylinders remain smooth. • A weak coil or plug may cut out during boost or heavy acceleration, causing a sharp hesitation and a flashing malfunction lamp. • A leaking injector can make the next hot restart rough, leave a fuel smell and wash the cylinder wall. • Low compression may create a persistent miss that does not migrate when the plug, coil or injector is exchanged. Possible Causes Use migration and cylinder evidence to separate inexpensive ignition faults from fuel-system or mechanical work. • A worn, cracked or fuel-fouled cylinder 2 spark plug, or an ignition coil whose insulation breaks down under load. • Oil or water in the cylinder 2 plug well, a loose coil connector or damage in that coil's power, ground or trigger circuit. • A restricted, leaking or electrically faulty cylinder 2 injector; injector calibration requirements depend on the BMW injection system. • An intake-runner or manifold seal leak concentrated near cylinder 2. • Loss of compression from a valve-sealing problem, ring/cylinder damage, valvetrain fault or head-gasket leakage. Diagnosis Steps with OBDThink Keep each swap controlled and change only one variable at a time, otherwise the result cannot identify the culprit. OBDThink is an independent BMW coding and diagnostic mobile application with no affiliation to BMW AG. Depending on the vehicle, adapter and module, use it to save P0304, freeze frame and supported live data; physical measurements and the BMW test plan still prove the repair. • 1. Capture P0304, companion mixture or injector faults, freeze frame and every cylinder's misfire counter before clearing anything. • 2. Inspect cylinder 2 plug condition and well contamination; compare the plug with a neighboring cylinder rather than judging it in isolation. • 3. Move the cylinder 2 coil to a known cylinder where the engine design permits, run the same condition and see whether the counter follows the coil. • 4. If the miss stays at cylinder 2, test the injector circuit and injector behavior using the engine-specific BMW procedure; do not loosen direct-injection fuel lines casually. • 5. With spark and fueling proven, perform relative compression and then a mechanical compression or leak-down test to localize sealing loss. • 6. Fit the confirmed repair, clear the fault and recheck cylinder counters during cold start, idle and the load range from the original freeze frame. Misfire follows coil or plug → ignition component proven Misfire remains at cylinder 2 and injector command is abnormal → test injector and circuit Fuel or oil fouling returns → investigate injector leakage, oil entry or compression Misfire remains after spark and fuel checks → measure cylinder sealing A swap result is stronger than appearance: if the fault moves with the coil, cylinder 2 itself is no longer the primary suspect. Repair • Low cost: Renew the cylinder 2 plug or clean and reseal its contaminated plug well after the cause of contamination is removed. • Medium cost: Replace the ignition coil or repair its connector and verified circuit fault when the misfire follows the component. • Medium cost: Service the cylinder 2 injector only after leakage, flow or electrical testing; apply coding/calibration only where the engine requires it. • High cost: Repair valve, valvetrain, head-gasket or cylinder damage only after compression and leak-down evidence identifies the mechanical loss. After the repair, erase P0304, repeat the freeze-frame operating window and rescan every relevant module. Verification requires stable live data and no returning current or pending fault. Can You Drive with P0304? Do not continue driving with a flashing malfunction lamp or strong shaking; unburned fuel can overheat the catalyst and a leaking injector can damage the engine. For P0304, the cleanest diagnosis is a trail of evidence from counter to swap test to cylinder sealing—not a basket of ignition parts.