* Spark Plug: Even if you haven't replaced *this* specific plug, it could be faulty. A cracked insulator, worn electrode, or carbon buildup can significantly weaken the spark. Try a brand new plug specifically for that cylinder.
* Ignition Control Module (ICM) or Electronic Ignition Control Unit (ECU): These components control the timing and strength of the spark. A faulty ICM/ECU can cause weak or absent sparks in specific cylinders.
* Crankshaft Position Sensor (CKP) or Cam Position Sensor (CMP): These sensors tell the ECU the engine's rotational position, crucial for precise spark timing. A malfunctioning sensor can lead to erratic or weak sparking.
* Distributor (if applicable): Older vehicles with distributors can have worn rotor caps, internal problems, or a bad pickup coil causing weak sparks to a particular cylinder.
* Wiring Harness: Check the wiring between the ICM/ECU and the coil, looking for breaks, corrosion, or poor connections. A damaged wire can reduce the voltage reaching the coil.
* Coil Pack (if applicable): If your vehicle uses a coil-on-plug system, and you haven't replaced the *individual* coil for the affected cylinder, this is a potential cause. One coil within the pack can fail while others remain functional.
* High-voltage wires: Double-check the spark plug wire is securely connected to both the coil and spark plug (even if it's new). Inspect the wire itself for damage or cracks.
* Computer/ECU problem: While you've checked some things, a fault in the computer that controls ignition timing or strength could be the issue. This would typically require a diagnostic scan tool to identify.
Troubleshooting Steps:
1. Visual Inspection: Carefully examine all wiring, connectors, and components related to the ignition system for that cylinder.
2. New Spark Plug: Replace the spark plug in the affected cylinder with a known good one.
3. Diagnostic Scan: Use an OBD-II scanner to retrieve any diagnostic trouble codes (DTCs) that might pinpoint the problem.
4. Professional Diagnosis: If you're not comfortable working on your ignition system, take your vehicle to a qualified mechanic for diagnosis and repair.
Ignoring a misfire can lead to engine damage, so it's best to diagnose and fix the problem as soon as possible.
