The most common cause is a failing spark plug or ignition coil on cylinder 4, especially in modern engines with coil-on-plug ignition systems.
When a misfire is detected specifically on cylinder 4 (often labeled as P0304 in OBD-II systems), it points to a disruption in the ignition, fuel delivery, air intake, or mechanical health of that cylinder. This article outlines the leading causes and the typical diagnostic steps used by technicians to identify the root problem.
Leading causes of cylinder 4 misfire
Below are the suspects most commonly implicated in a cylinder 4 misfire, drawn from diagnostic data and repair guides across mainstream automotive service literature.
- Worn, fouled, or incorrectly gapped spark plug in cylinder 4 reducing ignition energy or preventing reliable combustion
- Faulty ignition coil or coil-on-plug module supplying insufficient energy to cylinder 4
- Clogged or malfunctioning fuel injector for cylinder 4 causing an improper air–fuel mixture
- Vacuum leaks or intake manifold gasket leaks affecting air intake specifically around cylinder 4
- Low compression due to worn piston rings, valve issues, or a head gasket leak affecting cylinder 4
Because misfires can stem from several distinct systems, verifying the root cause usually starts with inspecting the ignition components on the affected cylinder, then moving on to fuel delivery, air leaks, and compression as needed.
Diagnostic approach to cylinder 4 misfires
What technicians look for
The following steps outline a typical diagnostic workflow used by professionals to distinguish ignition, fuel, and mechanical causes for P0304.
- Check the stored trouble codes and live data for cylinder-specific misfire indicators, including misfire counters and related sensor readings.
- Inspect or swap the cylinder 4 spark plug; test or swap the ignition coil for cylinder 4, and observe if the misfire moves to another cylinder (which would point to the coil or plug).
- Test fuel delivery to cylinder 4 by measuring injector resistance, listening for injector operation, and confirming rail pressure and fuel trim data.
- Inspect for vacuum leaks or intake manifold leaks around cylinder 4 that could alter the air–fuel mixture.
- Perform a compression test on cylinder 4 to assess mechanical health (follow up with a leak-down test if needed).
- Check electrical wiring and connectors for cylinder 4, and correct any loose or damaged wiring or harness issues.
Depending on the findings, repairs may involve replacing a worn spark plug, swapping or replacing an ignition coil, cleaning or replacing a faulty injector, repairing vacuum leaks, or addressing compression issues.
Summary
For cylinder 4 misfires, the most common cause is an ignition-related fault on that cylinder—typically a failing spark plug or a faulty ignition coil—followed by fuel delivery issues, air leaks, or mechanical compression problems. A systematic diagnostic approach helps pinpoint the exact cause and guides effective repair.