Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairEasy
A technician is diagnosing a vehicle that exhibits an intermittent misfire on multiple cylinders, accompanied by a P0300 DTC (Random/Multiple Cylinder Misfire Detected). The scan tool shows that the engine's RPM sensor reading momentarily drops to zero or fluctuates erratically during the misfire events. Which sensor is MOST likely faulty?
- AOxygen (O2) sensor
- BManifold Absolute Pressure (MAP) sensor
- CCrankshaft Position (CKP) sensor
- DEngine Coolant Temperature (ECT) sensor
Show answer & explanationAnswer & explanation
Correct answer: C. Crankshaft Position (CKP) sensor
The Crankshaft Position (CKP) sensor is crucial for detecting engine RPM and piston position. Erratic or lost signals from this sensor would directly cause misfires and fluctuating RPM readings, as the Powertrain Control Module (PCM) loses synchronization for fuel injection and ignition timing.
Why the other options are wrong
- A. The O2 sensor monitors exhaust gas oxygen content for fuel trim adjustments; a fault typically affects fuel economy or emissions, not directly causing RPM drops or random misfires in this way.
- B. The MAP sensor measures manifold pressure to calculate air density for fuel delivery, but a fault usually causes incorrect fuel mixtures, not direct RPM drops to zero.
- D. The ECT sensor primarily influences fuel enrichment and cooling fan operation, not directly engine RPM or misfires in this manner.
Crankshaft Position (CKP) Sensor Function
The CKP sensor monitors the rotational speed and position of the crankshaft, providing critical data to the PCM for fuel injection and ignition timing.
- Essential for engine synchronization.
- Generates a signal based on a reluctor wheel.
- A faulty CKP sensor can cause no-start, stalling, or misfires.
Memory trick: Crank, Cam, and Knock keep the engine in sync.