I have a 2003 Chevy Trailblazer with a malfunctioning tachometer and speedometer. What is the likely part I need to replace?

There is no dedicated fuse for speedometer only. There is voltage and ground for the instrument cluster.
The vehicle speed sensor sends a signal to pcm concerning speed then the pcm sends signal to instrument cluster for speedometer.

Check speed sensor or a fuse

i comes frm ur distrubuter cap it has a wire that run too it make sure the wire is ther

This is a pretty common issue for Trailblazers to have. 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

Hi Norton,

What has happened is your speedometer and tachometer drive motors are failing inside your silverado instrument cluster. I see this very often in GM trucks, to fix the problem you need to have your instrument cluster rebuilt.

Check out this article that talks more about it http://drspeedometer.com/chevy-silverado-instrument-clusters-why-they-quite-working

Thanks,
Fred

The speedometer and tachometer problem on your Trailblazer instrument cluster is very common on the 03-06 models. It's caused by failing stepper motors on the cluster's circuit board and can be fixed. Check out this page for more on how to get it fixed - Trailblazer tachometer problems

The trailblazer / envoy vehicles have instrumentation problems. A sweep test with a scan tool will likely reveal a problem with the tachometer in your instrument cluster. It can be repaired by any number of dedicated repair facilities or the entire cluster can be replaced. I heard that there is a recall of these, but do not know for certain. It might be worth a call to a chevy dealer to see if it is.

On the 03-06 Trailblazer instrument clusters this is a common problem. It's caused by failing stepper motors on the cluster's circuit board and it can be repaired. Check out this facebook group dedicated to the problem - My Trailblazer Gauges Are Driving Me Crazy!

"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!

I know that on my 98 cirrus lxi, we blew the PCM thats the powertrain control modul while trying to jump start it, should be located near the fuse box near the fender