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DTC codes / U06xx · Network communication / U0671

U0671Lost Communication With Fuel Temperature Sensor B

U0671 is a generic network communication code indicating that a control module on the vehicle's Controller Area Network (CAN) bus has lost contact with the fuel temperature sensor B circuit. The computer detected that expected messages or voltage signals from that sensor are missing or implausible for a calibrated period, so it flags the sensor as offline.

SystemNetwork
Code typeGeneric (all makes)
SeverityLow
Fix difficultyIntermediate
AreaNetwork communication

Symptoms you may notice

  • Illuminated check engine light or service vehicle soon message with no other noticeable drivability issue
  • Rough idle or slight hesitation when the engine is cold, because fuel temperature input is missing
  • Reduced fuel economy as the powertrain control module substitutes a default fuel temperature value
  • Intermittent hard starting or stalling in extreme ambient temperatures
  • Other communication-related warning lights if the fault lies on a shared bus segment
  • Often no symptoms besides the warning light, especially on vehicles that do not use fuel temperature for critical strategies

Common causes, most likely first

  1. Failed fuel temperature sensor B itself, typically an open or shorted thermistor element
  2. Damaged or corroded wiring harness between the sensor and the module that monitors it
  3. Loose, backed-out, or corroded connector pins at the sensor or at the control module
  4. Blown fuse or poor ground shared by the sensor circuit or the module that reports the fault
  5. Faulty control module that is not transmitting or receiving the sensor message correctly
  6. CAN bus wiring fault such as an open, short, or excessive resistance on the data lines
  7. Aftermarket accessory or poor previous repair disturbing the sensor circuit or bus integrity

How to diagnose and fix it

  1. Scan all modules for stored and pending codes, then record freeze-frame data to see when U0671 set
  2. Inspect the fuel temperature sensor B connector and the module connector for corrosion, loose pins, or pushed-out terminals
  3. Check the sensor circuit wiring for continuity, shorts to ground, and shorts to voltage using a digital multimeter
  4. Measure the sensor resistance at known temperatures and compare it to the manufacturer's specification table
  5. Verify the sensor reference voltage and ground at the connector with the key on, engine off
  6. Perform a wiggle test on the harness and connectors while watching live sensor data to catch intermittent faults
  7. If wiring and sensor test good, check for communication bus faults or module software updates before condemning the module

What U0671 does not always mean

  • A U0671 does not automatically condemn the fuel temperature sensor, because a loose ground or a chafed wire on a shared bus segment can produce the same lost-communication symptom.
  • Many vehicles use fuel temperature only for cold-start enrichment and emissions strategy, so the driver may never feel a change and the code can sit stored for months before anyone notices.
  • Technicians often replace the sensor only to find the real fault is a backed-out terminal in the connector that was disturbed during the repair, so pin drag and terminal tension must be checked.
  • A weak or failing battery or charging system can cause modules to drop communication intermittently and set U0671 along with other U-codes, so always verify system voltage first.
  • On some platforms the fuel temperature sensor shares a five-volt reference or ground with other sensors, and a short in a neighbouring circuit can pull the whole reference down and set this code.

Can I keep driving with U0671?

The vehicle is generally safe to drive because the powertrain control module substitutes a default fuel temperature value, but avoid long trips or heavy load until the fault is diagnosed. If the check engine light flashes or the engine runs poorly, stop driving and have it inspected promptly.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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