Either the gauge itself is bad or it is getting a bad signal from sender.
I would contact Chevy service and see if there are any recalls or TSB's on this problem.
GM had some problems with eratic readings, gauge problems with the Trailblazer/Envoy Instrument Panels.
A friend of mine just had the IP replaced on an 03 Envoy because of eratic speedometer readings under a recall.
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Thursday, January 15th, 2009 AT 9:45 AM