It sounds like a faulty gauge, this is a pretty common issue for Trailblazers. Here's a link with more information about these GM gauge problems. And where you can purchase replacement parts on Amazon if you want to do the repair yourself.
Trailblazer Speedometer
Dr Speedometer Repair Kit Includes 6 Qty Six Brand...
Amazon com Dr Speedometer Repair Kit...
Stepper motor failure in instrument cluster. visit my website www.joedatech.comi have a mail in service for your convenience. or call me if you live in az.
SOURCE: Speedometer/ Fuel Gauge & Tachometer not working
check for loose connection at the instrument cluster.
SOURCE: 2004 dodge neon speedometer stopped working
Try giving it a good rap on dash above speedo, see what happens, and get back to me, it may have same problem as my old 95' neon did.
SOURCE: speedometer and tachometer stopped working
If the speedo isn't working properly, the first thing to check is the output speed sensor wires on the rear right side (near the bar code) of the tranny case. Follow the wires and make sure it hasn't been cut, pinched or shorted.
If all is well there, take the output speed sensor off. Watch out cause you'll get wet with tranny fluid (Dexron 6). (Be ready to plug it up.) If there are no metal chips or debris on the magnet inside, check it with an ohmeter on the two pins to see what the reading is. It should be around 6 to 10 ohms. If it's not steady, get a new one. Make sure the pins are clean on the connectors.
Apply some white or build grease around the shaft and "O" ring and re-install. Don't overtighten. The bolt is a "tap-tite" and should need only about 5 or 6 ft/lbs torque. (Run it in and give the bolt a quarter turn.)
SOURCE: Both speedometer & tachometer not working, both reading wrong.
"The Fix" works...I would have not believed it untill I did it.... Start the car in reverse and about 5 times later it reset the speedometer back to zero.....wow!
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