When diagnosing a faulty ignition coil, you’re essentially checking whether it can produce the required high voltage for the spark plugs and whether it’s electrically sound. Here’s a detailed, step-by-step guide to check a coil sensor (ignition coil) for faults:
1. Safety First
- Turn off the engine and remove the key.
- Disconnect the battery negative terminal to prevent accidental sparks.
- Wear insulated gloves if working near high-voltage parts.
2. Identify Your Ignition Coil Type
Ignition coils come in three common types:
| Type | Description |
|---|---|
| Coil Pack | Multiple coils for multiple cylinders; plugs connect directly to coil |
| Distributor Coil | Single coil feeding distributor rotor and spark plugs |
| Coil-On-Plug (COP) | Each plug has its own coil mounted on top |
Note: Testing method may slightly vary depending on type.
3. Visual Inspection
- Check for cracks, burns, or carbon tracking on the coil housing.
- Inspect the connector and wiring harness for corrosion, loose pins, or broken wires.
- Smell for burnt odor, which indicates overheating.
4. Primary Resistance Test
Purpose: Checks the coil’s low-voltage winding for continuity.
- Set a multimeter to the ohms (Ω) setting.
- Locate the primary terminals of the coil (small pins on the coil connector).
- Measure resistance across the primary terminals.
- Typical resistance: 0.4–2 Ω (varies by manufacturer).
- Compare with the vehicle service manual specification.
- Reading infinite → open circuit → faulty coil.
- Reading very low / 0 Ω → short circuit → faulty coil.
5. Secondary Resistance Test
Purpose: Checks the high-voltage winding from coil output to primary terminal.
- Keep the multimeter on ohms (Ω).
- Measure resistance from coil output terminal (where spark plug connects) to one of the primary terminals.
- Typical resistance: 6,000–15,000 Ω (6–15 kΩ), varies by model.
- Compare with manual; a reading too high or too low indicates a bad coil.
6. Spark Test (Live Test)
Purpose: Checks if coil can produce spark under actual operating conditions.
Method 1 — Spark Tester:
- Connect a spark tester between coil output and ground.
- Crank engine or turn key to “ON.”
- Observe spark:
- Strong, blue spark → coil is good
- Weak or no spark → coil faulty
Method 2 — Old School (High Risk):
- Remove spark plug, connect it to coil, and hold against engine ground.
- Crank engine and look for spark.
⚠️ High voltage risk! Only if proper insulation used.
7. Check with OBD-II Scanner
- Modern engines with coil-on-plug monitor coil performance via misfire detection.
- Look for DTC codes like:
- P0300 – Random misfire
- P0301–P0308 – Cylinder-specific misfire
- Misfire codes can point directly to a faulty coil.
8. Swap Coil Test (Practical Method)
- Swap the suspected coil with a good coil from another cylinder.
- Clear DTC codes and run engine.
- If misfire moves to the other cylinder, the coil is definitely faulty.
9. Other Things to Check
- Voltage at coil connector: Ensure coil is receiving proper 12V from ignition circuit.
- Check ignition module / ECU signal: If coil is fine but spark is missing, problem may be upstream.
Summary — How to Test a Faulty Coil
| Test | What You Check | Normal Result | Faulty Indicator |
|---|---|---|---|
| Visual Inspection | Cracks, burns, corrosion | Clean, intact | Cracks, carbon tracking, burnt smell |
| Primary Resistance | Low-voltage winding | 0.4–2 Ω | Open or shorted |
| Secondary Resistance | High-voltage winding | 6–15 kΩ | Too high or too low |
| Spark Test | Spark production | Strong blue spark | Weak / no spark |
| OBD-II Codes | Misfire detection | No misfire codes | Cylinder misfire codes |
| Swap Test | Cylinder performance | Misfire stays | Misfire moves → bad coil |
💡 Pro Tip: Always compare measurements with the manufacturer’s specifications; resistance values vary by car make and coil type.
Also, a coil may fail intermittently when hot, so testing cold and warm can reveal hidden faults.
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