What would cause my speedometer not to work on my 2001 ford windstar?

Sometimes the speedometer works fine and sometimes it goes up to 20/25mph. Then stops there. All other gauges work fine. I took it to the shop and they replaced a sensor and the whole new speedometer console thing and it still went up to 20/25 mph. And stopped so they put my original parts back on so I didn't have pay for them. They told me to take it to ford dealership. Any ideas will b appreciated Thanks

You didn't say what sensor was changed. The VSS (vehicle speed sensor) started to go bad on our 1998 Ford Windstar and the speedometer sometimes worked and sometimes the needle would bounce. The sensor is prone to failure due to heat from the exhaust pipes that run directly under the transmission where the sensor is located. In a vain attempt to fix this design problem, Ford engineers placed a thermal blanket between the sensor and the exhaust pipe to help protect the sensor but the sensor fails eventually from the heat.