I have a 2006 Monte Carlo SS. A few months ago the Service Tire Monitor System came on. The Left rear tire reading was blank. I took it back to the dealer and they said the sensor was bad and replaced it. Just recently the Service Tire Monitor system warning has come back on with the right rear tire reading blank, but other times it is left rear blank again. Is this just the sensor in both wheels or is it something else? Any help would be appreciated. Thanks.