There are several common causes for a speedometer to stop working. Typically, these malfunctions are caused by an issue with a car's wiring, a broken gear in the speedometer system, a speed sensor issue or a faulty engine control unit (ECU).
According to the experts at Seva Call, a faulty speed sensor is the most likely culprit in the case of a malfunctioning speedometer. Often covered under warranty, the speed sensor is located near the front axle and calculates the speed of the vehicle.
Another common and easy-to-fix issue could be a broken gear on the speedometer unit itself. Autos.com suggests that after diagnosing an issue with the speedometer unit, the vehicle owner can purchase replacement components from the dealership where the car was purchased, at certain body shops or on the Internet.
A malfunctioning speedometer may also be due to damaged wiring. Wiring is vulnerable to water damage and can cause fuses to blow. If the problem appears during wet weather especially or if the car has been exposed to excessive water, there is a good chance that this could be the issue.
Lastly, many automobile problems can be traced back to a malfunctioning ECU, which is the main computer that manages many of the car's processes. If the ECU is not working, it is possible that it is preventing the speed sensor from reporting the speed to the speedometer. A broken ECU breaks this link between the two components and could cause an issue.