My speedometer currently reads 90mph. My 2004 Trailblazer is in the driveway, in park. As I go down the road, the reading on the speedometer will increase, but never go back down to zero, no matter how fast I go. If I disconnect the negative battery cable for a few minutes, the reading on the speedo will go back down to zero and act normal. Sometimes it will work great for a few weeks, sometime it won't last an hour. Very grateful for speedo on smartphone, but just want to permanently fix this. I'd like to finally put away the socket wrench I use to disconnect the battery. Any ideas? THANK YOU in advance.

 

It sounds like a bad speedometer to me. Have you looked for one at a salvage yard?

 

Look on Amazon and Ebay. Lots of kits to replace the gauge cluster stepper motors. Other wise, used from junkyard.

 

Thank you very much both of you. I will definitely try that.