Circuit Description

PCM uses Crankshaft Position (CKP) sensor to determine position and RPM of engine at various degrees of rotation. PCM can then base misfire diagnostics on the principle that crankshaft velocity will increase as each cylinder contributes its power input. Under certain conditions, PCM will perform a series of tests to determine misfire percentage on a cylinder based on crankshaft velocity variation. The malfunction Indicator Light (MIL) will flash if an active catalyst damaging misfire occurs and will stay on when misfire is present after 2 consecutive ignition cycles. On automatic transmissions, torque converter is disabled momentarily to determine if misfire was due to rough road condition.

DTC will set if random misfires have been detected after a series of tests when:

• More than 12 misfires counts are recorded.
• Engine speed at 469 or 6400 RPM.
• ECT between 20-254°F (-7°C to 123°C).
• Fuel level greater than 10 percent.
• Engine running for at least 5 seconds.
• Ignition voltage between 9-17 volts.
• No TP, CKP, IAT, MAP, ECT, O2S, fuel trim lean/rich, knock, CMP, VSS, idle speed, ECM memory, TCC, and PCM long term memory DTCs have been set.