2004 Chevy Trailblazer RPM Gauge

Our RPM gauge is not functioning properly, It seems to be frozen. My husband washed the car on Friday at one of those car wash places that you pull your car in and the machine washes around the car. He did the undercarriage wash also. But unfortunately the temperatures that night that he washed the truck dipped below freezing. The temperature have not climbed above freezing yet. We do not have a garage to park our truck in so it sat outside exposed to the temperatures. I notice Saturday morning that the RPM gauge was setting between 2 - 3, even when the car was just running idle in park. Throught Saturday and Sunday the gauge progressively went higher. Now it is to the point it is beyond the number 7 that it registers. The main thing is that the engine is not running at that high of RPMs. What would you suggest to get the gauge to eventually lower to it's normal rate? When he washed the undercarriage could that have frozen water of a sencor or something to make the gauge read the RPMs as it is?