P0187 - Fuel Temperature Sensor B Circuit Low
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
P0187 is stored when the Engine Control Unit (ECU) detects a voltage signal from fuel temperature sensor 'B' that is below the calibrated minimum threshold (typically below 0.2V). This sensor is an NTC thermistor where resistance decreases as temperature increases. A 'circuit low' code usually signifies a short circuit to ground in the signal wire or a sensor failure with abnormally low internal resistance. The ECU utilizes this data to calculate fuel density and adjust fuel trim for combustion efficiency. Incorrect data can lead to poor air-fuel ratios and increased emissions.
Possible Causes
Short circuit to ground in the fuel temperature sensor B signal wire
Faulty fuel temperature sensor (internal electrical failure)
Damaged connector or corrosion in the terminals
Faulty Engine Control Unit (ECU)
Symptoms
Recommended Solution
- Visual Inspection: Inspect the wiring harness for sensor B for chafing or signs of a short circuit. Check the connector for corrosion or bent pins.
- Reference Voltage Test: Unplug the sensor and measure with the ignition on. There should be a 5V reference signal from the ECU.
- Resistance Test: Measure the sensor's internal resistance using a multimeter. Compare with factory specs (e.g., 2000-3000 Ohms at 20°C). If resistance is near 0 Ohms, the sensor is shorted.
- Continuity Test: Ensure the signal wire is not shorted to chassis ground when disconnected at both ends.
- Component Replacement: Replace the fuel temperature sensor if readings are out of specification.
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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