Misfire

📖 YOUCANIC Automotive Glossary

A misfire occurs when the air-fuel mixture in a cylinder fails to ignite or burns incompletely during the power stroke, resulting in lost power, rough running, and increased emissions. The ECU detects misfires by monitoring crankshaft acceleration — each cylinder should contribute an acceleration pulse during its power stroke, and the absence or weakness of that pulse indicates a misfire. Misfires are categorized as: Type A (severe — catalyst damaging, triggers flashing MIL), Type B (emissions exceeding 1.5x threshold, triggers steady MIL), and Type C (detected but below threshold).

Misfire DTCs include P0300 (Random/Multiple Cylinder Misfire) and P0301-P0312 (specific cylinder misfires). The three causes are ignition (bad coil, plug, or wire), fuel (dead injector, low fuel pressure, lean condition), and mechanical (low compression from worn rings/valves, head gasket failure). Freeze frame data reveals misfire conditions — cold misfires suggest ignition, misfires under load suggest fuel or mechanical. The YOUCANIC UCAN-II’s misfire counter data and power balance test help pinpoint which cylinder and cause.