U0677 means the engine or body control module can no longer communicate with the secondary fuel-level sensor (Sensor B). This is a network/communication fault rather than a fuel-level accuracy fault, so the module cannot read or validate the sensor's signal. It commonly triggers the check-engine light and can cause the fuel gauge or distance-to-empty display to behave erratically.
The sensor's internal electronics, float mechanism, or bus transceiver can fail, causing it to stop responding to module requests. This is the most common root cause for a lost-communication code directed at a single sensor.
Open, shorted, pinched, or chafed wires between the sensor and the control module interrupt the data and power lines. Corrosion, water intrusion, or backed-out pins at the sensor or module connectors produce the same result.
If the sensor loses battery voltage, ignition voltage, or a reliable ground, it cannot power up and communicate. This can be caused by a blown fuse, bad splice, loose ground eyelet, or high-resistance connection.
On LIN- or CAN-based fuel-level sensors, a fault elsewhere on the same network branch, a missing termination resistor, or excessive bus noise can isolate Sensor B from the module. Other modules on the same bus may set related U-codes.
The PCM, BCM, or fuel-pump driver module could have an internal communication driver fault, but this is uncommon and should only be considered after the sensor, wiring, power, and ground have been ruled out.
Usually no. A 'lost communication' code for one specific sensor most often points to the sensor itself, its connector, or the wiring. PCM replacement for U0677 is rarely the correct fix and should only be considered after the sensor circuit is fully diagnosed.
Generally yes, but the fuel gauge for the affected tank may be unreliable. You should verify actual fuel level manually until the fault is repaired to avoid running out of fuel unexpectedly.
Sensor B is a secondary fuel-level sensing unit. It may be used in dual-tank systems, as a redundant sender, or to monitor a separate fuel reservoir or chamber depending on the vehicle design.
When the control module loses communication with the sensor, it no longer receives valid fuel-level data. It may default to an empty or fixed reading, or disable that portion of the gauge until communication is restored.