My speedometer works but my digital odometer does not work on my 2003 old alero

The odometer calculates and stores the mileage information within the processor inside the gauge cluster. If the mileage reading is not properly stored and updated then repair or replacement of the gauge cluster will be required to correct the condition.If the speedometer works then the VSS is producing a signal and the ECM is passing the signal along to the instrument cluster, so this only leaves the cluster as the possible faulty component. If replacing the cluster and you want the mileage to show correctly then the cluster would need to be programmed.

Try checking the fuses

This is one guy's advice: Replace the speedo head, that should take care of the problem."

This site is also interesting because the guy said
General Motors cars had instrument cluster problems from 2003 through 2006. When I researched instrument cluster problems on the Internet what I found raised concerns in me. Many of those vehicles had problems that began with the instrument cluster, but eventually led to the engine refusing to start.
(an electronic instrument cluster, digital instrument panel or digital dash for short, is a set of instrumentation, including the speedometer, that is displayed with a digital readout rather than with the traditional analog gauges. Many refer to it simply as a digital speedometer.)

http://www.instructables.com/id/Erratic-Instrument-Panel-on-GM-Cars/