Drivers notice the fuel gauge acting like a dead battery. The needle may stay at “E,” jump between full and empty, or the display may show no fuel level at all. Simultaneously, the low‑fuel warning light often illuminates even when the tank still holds fuel, and the estimated range on the instrument cluster disappears. Because the fuel‑level information is missing, the vehicle’s trip computer cannot calculate mileage‑to‑empty, which may trigger a generic “check engine” or “instrument cluster” warning on some models. These symptoms appear suddenly and persist until communication with the sensor is restored.
The control module that manages fuel‑level data (often the instrument cluster or body control module) stores the sensor’s calibration and communication parameters. If the module’s firmware becomes corrupted—through a failed flash, power surge, or improper re‑programming—the CAN‑bus messages that request sensor data may never be answered, producing U0677.
Fuel Level Sensor B is wired to the vehicle’s high‑speed CAN network. A broken wire, corrosion at a connector, or a short to ground can interrupt the data frames. Because the network relies on precise voltage levels, even a minor impedance increase can cause the module to register a loss of communication.
Some manufacturers integrate the sensor driver circuitry directly into the control module rather than using a separate external driver. If that internal driver fails—due to moisture intrusion, component fatigue, or overheating—the sensor’s raw resistance signal never reaches the module, resulting in a lost‑communication code.
The sensor circuit draws a small constant voltage from the module’s supply rail. A weak or fluctuating supply (often caused by a failing battery connection or a grounding issue) can prevent the sensor from powering up, leading the module to flag a communication loss.
Connect a professional scan tool, read U0677, and note any accompanying codes (e.g., other communication faults). Freeze‑frame data may show vehicle speed or engine load at the moment the fault was set, helping to rule out transient conditions.
Using a multimeter, measure voltage at the sensor’s power pin (typically 5 V) while the ignition is on. Confirm a solid ground continuity (≤ 0.1 Ω). Low voltage or high resistance suggests a wiring issue rather than a module fault.
Most scan tools can request live data from the fuel‑level sensor. If the tool shows “no response” or “invalid data,” the problem lies in the network path. Inspect the harness for pinched wires, damaged insulation, or corrosion at the connector near the sensor and the module.
Disconnect the sensor plug and inspect the pins for corrosion, bent terminals, or debris. Clean with electrical contact cleaner and reseat firmly. A poor connection often produces intermittent U0677 occurrences.
If wiring and connectors are sound, re‑flash the module that hosts the sensor driver (instrument cluster or BCM). Use the manufacturer’s latest calibration file; this restores correct sensor IDs, timing, and error‑handling logic. After flashing, clear codes and perform a road test to confirm the gauge returns to normal.
When re‑programming fails to restore communication, the module’s internal driver is likely damaged. Replace the module with a VIN‑matched unit, then have it programmed to the vehicle’s specifications. This step ensures the new module speaks the correct CAN‑bus protocol and includes the proper sensor calibration.
After repair or replacement, run a full system scan, verify that U0677 is cleared, and monitor the fuel gauge for at least 30 minutes of driving. Confirm that the low‑fuel warning and range estimate function correctly.
Typical costs
Service Recommendation: Most issues related to this fault are diagnosed and corrected through inspection, wiring repair, and calibration rather than module replacement. For modules not typically replaced through aftermarket suppliers, diagnosis and repair should be performed by a certified automotive technician with access to factory service information and tooling.