ASE G1 Auto Maintenance and Light RepairEngine SystemsHard
A technician is diagnosing a vehicle that cranks normally but does not start. They have confirmed good spark and adequate fuel pressure. When checking for injector pulse using a noid light, they observe no flashing. Which of the following is the MOST likely cause?
- ALow engine compression
- BClogged fuel injectors
- CDefective crankshaft position sensor
- DFaulty fuel pump relay
Show answer & explanationAnswer & explanation
Correct answer: C. Defective crankshaft position sensor
If there's no injector pulse (noid light doesn't flash) despite good spark and fuel pressure, it indicates the engine control module (ECM) isn't commanding the injectors to fire. The crankshaft position sensor is crucial for the ECM to determine engine speed and position, which is necessary to time both spark and fuel injection. Without this signal, the ECM won't know when to fire the injectors.
Why the other options are wrong
- A. Low engine compression would prevent the engine from starting even with spark and fuel, but it would not prevent the ECM from sending an injector pulse if the other conditions were met.
- B. Clogged injectors would still receive a pulse; they just wouldn't spray fuel effectively. The noid light tests the electrical pulse, not fuel delivery.
- D. A faulty fuel pump relay would prevent fuel pressure, which was already confirmed good.
No Injector Pulse Diagnosis
Troubleshooting a no-start condition where fuel injectors are not receiving electrical signals to open, despite other conditions like spark and fuel pressure being present.
- Indicates the ECM is not commanding fuel injection.
- Often points to a missing engine position signal (crankshaft/camshaft sensor).
- Can also be caused by ECM failure or wiring issues to injectors.
Memory trick: No 'PULSE' means the 'CRANK' sensor's gone 'MISSING'.