Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisHard
A technician is diagnosing a vehicle with an intermittent 'Check Engine' light and a stored DTC P0301 (Cylinder 1 Misfire). Using an oscilloscope, the technician connects to the ignition coil primary circuit for cylinder 1 and observes a waveform that shows a normal firing line, but a very short spark duration (less than 0.5ms) compared to specifications (1.0ms - 1.5ms). All other cylinders show normal spark duration. What is the most likely cause of this misfire?
- AA faulty spark plug in cylinder 1.
- BAn open in the ignition coil primary winding for cylinder 1.
- CLow compression in cylinder 1.
- DA restricted fuel injector in cylinder 1.
Show answer & explanationAnswer & explanation
Correct answer: A. A faulty spark plug in cylinder 1.
A short spark duration on the primary ignition waveform indicates that the coil is having difficulty maintaining the spark across the plug gap. This is commonly caused by a worn, fouled, or excessively gapped spark plug, which increases the resistance in the secondary circuit and makes it harder for the spark to jump, shortening its duration. Other options would affect the firing line or cause completely different waveform characteristics.
Why the other options are wrong
- B. An open primary winding would prevent the coil from firing at all, resulting in no spark duration or firing line.
- C. Low compression would primarily affect the engine's ability to ignite the air/fuel mixture but would not directly cause a short spark duration in the ignition waveform itself.
- D. A restricted fuel injector would cause a lean misfire but would not directly affect the ignition coil's spark duration waveform.
Ignition Coil Primary Waveform Analysis
Using an oscilloscope to analyze the voltage and current patterns in the primary circuit of an ignition coil to diagnose ignition system faults.
- Firing line: voltage required to jump spark plug gap.
- Spark duration: time spark is maintained across the plug gap.
- Worn plugs or high resistance in secondary circuit shorten spark duration.
- Opens in primary or secondary circuits cause distinct waveform anomalies.
Memory trick: Waveform tells the spark's TALE.