U0670 means a control module (usually the PCM) can no longer communicate with or receive valid data from the fuel temperature sensor. Depending on the vehicle, this may be a dedicated sensor signal fault or a loss of serial-data communication on a LIN/CAN subnet. The code sets when the expected temperature message or voltage is missing, intermittent, or out of protocol for a calibrated period.
The sensor itself can fail open, shorted, or internally silent on the data bus, causing the module to report loss of communication. This is the single most common root cause for this specific U-code.
Chafed, corroded, backed-out, or moisture-wicked pins in the sensor harness—especially near the fuel tank—can interrupt power, ground, or signal lines and produce a communication-loss code.
A blown fuse, bad splice, or corroded ground can keep the sensor from waking up or transmitting, which the PCM interprets as lost communication rather than a simple out-of-range value.
If the sensor communicates over a serial bus, bus interference, incorrect termination, short-to-power/ground on the data lines, or a failing intermediate module can break the message path.
A corrupted calibration, failed internal communication transceiver, or damaged PCM input can mimic a sensor communication loss, but this is genuinely uncommon and should only be considered after the sensor, wiring, and bus are ruled out.
Usually yes for normal driving, because fuel temperature has limited immediate effect on engine operation. However, you should diagnose it promptly to avoid masking other faults or affecting evaporative-emissions diagnostics.
No—PCM failure is one of the least likely causes. Most U0670 cases are resolved by repairing wiring, cleaning or replacing the connector, or replacing the fuel temperature sensor itself.
When communication is lost, the PCM may display a default, fixed, or blank value. Once the sensor circuit is restored, the live data should return to normal.
Yes, on many vehicles the fuel temperature sensor is built into the fuel pump module or sender assembly, so a fault in that assembly or its connector can set this code.