A vehicle's engine cranks normally but fails to start. A spark test confirms no spark at any of the spark plugs. Fuel pressure is within specification, and injector pulse is observed using a noid light. What is the MOST likely cause?
- AA defective Crankshaft Position (CKP) sensor
- BA faulty Mass Air Flow (MAF) sensor
- CA clogged catalytic converter
- DA faulty fuel pump relay
Show answer & explanationAnswer & explanation
Correct answer: A. A defective Crankshaft Position (CKP) sensor
The engine cranks, fuel pressure is good, and injector pulse is present, indicating the PCM is receiving a basic engine speed signal and trying to fuel the engine. However, the absence of spark at all cylinders points to a failure in the ignition system's primary control. The Crankshaft Position (CKP) sensor provides the essential signal for the PCM to determine engine speed and piston position, which is critical for timing spark delivery. If the CKP signal is absent or incorrect, the PCM will not know when to fire the ignition coils, resulting in no spark.
Why the other options are wrong
- B. A faulty MAF sensor would primarily affect fuel mixture and engine performance, but it would not typically cause a complete absence of spark at all cylinders during cranking.
- C. A clogged catalytic converter would cause severe engine performance issues but would not prevent spark from occurring during cranking.
- D. A faulty fuel pump relay would prevent fuel delivery, which is contradicted by the 'fuel pressure is within specification' statement.
CKP Sensor & Spark
The Crankshaft Position (CKP) sensor provides the primary signal for engine speed and piston position, which is essential for the PCM to time ignition spark.
- No CKP signal usually means no spark and often no injector pulse.
- Crucial for both starting and running the engine.
- Its failure often results in a crank-no-start condition.
Memory trick: If it 'CRANKS' but has 'NO SPARK', the 'CKP' sensor leaves a mark.