Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGauges, Warning Devices, and Driver Information Systems Diagnosis and RepairMedium
A customer complains that the 'Service Engine Soon' light is illuminated, and the fuel gauge reads empty even after filling the tank. A scan tool shows a fuel level sensor input of 0.0 volts. What is the most likely cause?
- AA faulty instrument cluster.
- BA short to power in the fuel level sensor circuit.
- CAn open circuit in the fuel level sensor signal wire.
- DA stuck-open purge valve.
Show answer & explanationAnswer & explanation
Correct answer: C. An open circuit in the fuel level sensor signal wire.
A fuel level sensor typically operates as a variable resistor, where higher fuel levels result in lower resistance and higher voltage, and lower fuel levels (or an open circuit) result in higher resistance and lower voltage. A 0.0V reading indicates an open circuit, preventing any voltage from reaching the PCM, which would then interpret it as an empty tank.
Why the other options are wrong
- A. A faulty instrument cluster might cause an incorrect gauge reading, but the scan tool showing 0.0V indicates a problem at the sensor or wiring, not just the display.
- B. A short to power would result in a high voltage reading, likely showing a full tank.
- D. A stuck-open purge valve is related to the EVAP system and would cause different DTCs, not directly affect the fuel level sensor voltage or gauge reading in this manner.
Fuel Level Sensor Open Circuit
An open circuit in a fuel level sensor signal wire will typically result in a 0.0V reading at the PCM and an 'empty' fuel gauge indication, as no signal can be transmitted.
- Most fuel level sensors are variable resistors.
- Voltage output correlates to fuel level.
- Open circuit prevents voltage signal.
Memory trick: Fuel Gauge Empty, 0 Volts: No 'SIGNAL' means the circuit's 'OPEN' and not 'FULL'.