The U3571 code indicates a problem with the circuit that supplies power and data to the coolant temperature sensor located on the stack outlet (often the rear of the engine). It means the vehicle’s control module cannot read a valid signal from that sensor.
The sensor may be damaged, corroded, or have failed internally, producing no voltage or an erratic signal.
Corroded, broken, or disconnected wires or a poor connector contact can interrupt the sensor’s circuit.
A blown fuse, bad ground, or voltage drop in the sensor circuit can prevent the sensor from operating.
Physical blockage or debris around the sensor location can damage the sensor or its wiring.
While rare, a malfunctioning ECM may not interpret the sensor signal correctly; this is usually considered after the sensor and wiring have been ruled out.
Replacing the ECM is rarely necessary. The code is almost always caused by the sensor itself or its wiring. A module replacement should be considered only after the sensor and wiring have been confirmed to be functioning correctly.
Because the sensor that reports coolant temperature to the gauge is not providing a valid signal, the gauge cannot display the correct temperature.
The car may run, but engine performance and emissions may be affected, especially during warm-up. It is best to address the fault promptly.