Comprehensive Diagnostic Guide for OBD-II Code P0187

Code shorthand: P0187
Full descriptor (manufacturer-agnostic): Fuel Temperature Sensor B Circuit Low Input
Source note: The Open Source OBD2 code definitions list this as . Wikipedia's OBD-II articles classify P-codes as Powertrain/OBD-II DTCs monitored by the PCM.

What This Code Means

• According to the standard OBD-II framework, powertrain codes (P-codes) are used by the PCM to monitor engine/driveability parameters. A P0187 indicates the PCM has detected a fault in the Fuel Temperature Sensor B circuit with a low input signal.
• In practical terms, the fuel temperature sensor B circuit is reporting an abnormally low voltage/current to the PCM (below the expected minimum). This can be caused by a faulty sensor, wiring issues (short to ground, open circuit, damaged insulation), poor/loose connectors, or PCM input issues. The exact interpretation may vary slightly by vehicle architecture and OEM naming (some vehicles may refer to "B" as the secondary/fuel-temperature sensor, or as a second sensor in a dual-sensor arrangement).

Symptoms

• Malfunction Indicator Lamp (MIL) illuminated.
• Driveability issues may appear: hesitation, rough idle, or transient poor acceleration.
• Possible abnormal fuel trim behavior or fuel economy changes reported by the driver.
• In some cases, there may be no obvious symptoms beyond the MIL and the stored code.
  Note: Symptoms can be subtle and fuel system compensation logic may mask deviations in some driving conditions.

Probable Causes

• Wiring/connector issues in the B circuit (most likely)
  • Damaged insulation, chafed wires, corrosion at the sensor connector, loose or damaged pins.
  • Short to ground or intermittent open circuit in the B sensor circuit.
    Probability range: ~40-60%
• Faulty fuel temperature sensor B itself
  • Sensor drift, internal thermistor fault, or failure to produce a valid signal.
    Probability range: ~20-35%
• PCM input or signal conditioning fault
  • Input leakage, ADC issue, or software calibration oddity.
    Probability range: ~5-15%
• Related sensor OR system interactions (less common)
  • If the vehicle uses two temperature sensors (A and B), a fault in the related circuit or cross-talk could trigger P0187 depending on OEM logic.
    Probability range: ~5-10%
• Other external/system faults (rare)
  • Grounding issues, battery/alternator voltage irregularities affecting sensor reference, or failed relay/voltage supply to the sensor circuit.
    Probability range: ~0-5%

What you'll need (tools and references)

• Scanning tool with live data/Freeze-Frame capability and the ability to log sensor data (fuel temperature sensor B circuit value, voltage, and the Fuel Temperature Sensor B circuit ground reference or reference voltage).
• Digital multimeter (DVOM) or data-logging scanner to measure:
  • Sensor reference voltage (often 5V) and signal voltage at the sensor connector.
  • Continuity/short to ground in the sensor circuit.
  • Resistance/temperature characteristics of the sensor if applicable (per service data).
• Access to vehicle wiring diagrams for the Fuel Temperature Sensor B circuit (pinout, reference voltage, ground, and signal wiring).
• Optional: back-probing tool for non-invasive circuit monitoring; infrared thermometer if the vehicle has a thermal diagnostic path.
• Safety gear: eye protection, gloves, and fuel-system safety precautions (fuel pressure relief, no ignition sources).

Diagnostic flow (step-by-step)

1) Confirm and contextualize

• Read the code(s) with a scan tool. Note any other DTCs stored or pending (especially related fuel, temperature, or sensor circuits).
• Confirm the vehicle is in a stable environmental state (engine off, key on, transmission in park/neutral as applicable) before continued testing.
• Check for service information on the specific OEM regarding P0187 (some manufacturers document multiple "B" sensor configurations).

2) Inspect for obvious causes

• Visually inspect the Fuel Temperature Sensor B connector and wiring harness:
  • Look for broken insulation, exposed conductors, corrosion, or loose connectors.
  • Check for signs of heat damage, oil/gasket leaks, or rodent damage.
• Reseat the sensor connector firmly; listen for a click when connected.
• If accessible, inspect the sensor's mounting and environment (excessive vibration, exposure to moisture, etc.).

3) Electrical measurement: sensor circuit on-vehicle check

• With key ON (engine off) or as specified by OEM, back-probe the Fuel Temperature Sensor B circuit at the connector:
  • Measure the signal voltage to ground (and reference voltage to 5V if present). Note whether signal is abnormally low (near 0V) or fluctuating.
  • Measure the reference voltage and ground integrity (look for voltage drop across grounds).
• Measure resistance or impedance of the sensor (cold and at ambient temperature, if the sensor type supports this test per service data). Compare to OEM spec.

4) Circuit continuity and resistance test

• With the ignition OFF, perform continuity tests along the Fuel Temperature Sensor B circuit from the PCM/ECU side to the sensor connector.
• Look for open circuits, shorts to ground, or shorts to power.
• Inspect for wiring harness damage along routing paths (hot pipes, sharp edges, engine movement areas).

5) Functional test and data correlation

• With PCM/ECU powered and the engine running (or as allowed by OEM drive cycle), observe live data:
  • Note whether the Fuel Temperature Sensor B voltage/signal is within expected range for current fuel temperature and whether it reads abnormally low.
  • Check related sensors and parameters (ECT/MAF/engine load) to see if there is cross-sensor misinterpretation or software compensation that could explain a low input reading.
• If available, perform a controlled temperature change (carefully) to see sensor response (e.g., ambient temperature change or coolant/warm-up effect if the sensor is affected by fuel temperature only).

6) Sensor replacement trial (if wiring and PCM appear OK)

• If the circuit and wiring test clean, and the sensor signal remains abnormally low, consider replacing the Fuel Temperature Sensor B (or the OEM-recommended equivalent) and re-test.
• After replacement, clear all codes and perform a drive cycle to confirm the code does not return.

7) PCM/ECU considerations

• If all tests indicate the sensor and wiring are sound, but the code persists, suspect an ECU input issue or software calibration in rare cases.
• Involve the OEM service information if the vehicle is beyond generic repair scope.

8) Post-repair verification

• Clear codes and perform a complete drive cycle to ensure the DTC does not reappear under normal operating conditions.
• Monitor live data for a sustained normal range in the Fuel Temperature Sensor B signal under varying ambient and fuel temperatures.
• If the vehicle has emissions testing or OBD readiness checks, confirm readiness monitors are set.

Safety and risk considerations

• Fuel system safety: depressurize the fuel system per factory procedure before disconnecting or back-probing fuel lines or sensors. Keep sparks, flames, and hot surfaces away.
• Electrical safety: disconnect the battery when performing heavy wiring repairs or sensor replacement in some scenarios; follow OEM guidelines to prevent PCM damage.
• Avoid introducing moisture or contaminants into the sensor connectors; use proper dielectric grease if recommended by OEM after connector work.
• Record all diagnostic steps, data, and test results for traceability and future service needs.

Common Repairs

1) Repair or replace damaged wiring or connectors in the Fuel Temperature Sensor B circuit; restore proper ground reference.

2) Replace the Fuel Temperature Sensor B if wiring tests are clean and sensor signal remains abnormally low after reassembly.

3) Assess and address PCM/ECU input concerns if sensor and wiring tests are conclusive and other DTCs suggest a PCM fault.

4) Re-verify with a drive cycle and confirm no reoccurrence of P0187.

Interpreting OEM variability and cross-references

• The P0187 designation is a standard interpretation of found in the provided Open Source definition. OEM implementations may label or sequence dual temperature sensors differently; consult the vehicle's service information for exact sensor naming, wiring diagrams, and reference voltages.
• General OBD-II guidance indicates DTCs are used by the PCM to monitor powertrain parameters and emissions-related functions; a fault in a sensor circuit can impact driveability and emissions readiness.

Documentation and next steps

• If you're diagnosing for a customer or internal use, document:
  • Vehicle year/make/model, engine type, and sensor circuit details.
  • All measured values (sensor voltages, resistances, continuity readings).
  • Wiring condition notes and any connector modifications.
  • Repaired components and the final drive cycle results.
• After repair, confirm the vehicle passes any required readiness tests and re-check for additional DTCs that may have been masked by the original fault.

Summary

• P0187 is a Fuel Temperature Sensor B Circuit Low Input DTC. Use a systematic approach: verify wiring/connectors, measure sensor signal and reference, check for shorts to ground or open circuits, test sensor resistance if applicable, and ultimately replace the sensor if the circuit is clean and the signal remains abnormally low. Always clear codes and perform a drive cycle to validate a successful repair, while following all safety precautions around the fuel system and electrical work.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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