Possible Causes
NOTE: The listed causes may not be a complete list of all potential issues and there may be other causes.
- Faulty spark plug
- Faulty ignition coil
- Intake vacuum leak
- Faulty camshaft position sensor
- Ignition coil harness is open or shorted
- Poor electrical connection in ignition coil circuit
Description
Under CARB/OBD regulations, the Engine Control Module (ECM) must determine if a misfire is occurring and also identify the specific cylinder(s) and severity of the misfire event, and whether it is relevant for emissions or damaging to the catalyst. To accomplish these tasks, the control module monitors the crankshaft for acceleration losses during the firing segments of each cylinder based on the firing order.
The misfire/engine roughness calculation is derived from differences in period duration (T) of individual increment gear segments. Each segment period consists of a 120° angular range of crankshaft angle that begins 78° before top dead center (TDC).
If the expected period duration exceeds the allowed value, a misfire fault for that particular cylinder is recorded in the ECM’s fault memory. Depending on the measured misfire rate level, the control unit illuminates the Service Engine Soon light, may cut fuel to the particular cylinder, and may switch to open-loop lambda operation. All misfire faults are weighted to determine whether the misfire is relevant for emissions or damaging to the catalyst.
How to Repair
Check the “Possible Causes” listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.
Possible Symptoms
- Engine Light On (or Service Engine Soon Warning Light)