The speedometer on my 2003 GMC Sierra quit working, reads zero all of the time when cold and during the initial driving period. Sometimes works after driving a couple of hours but reads 5mph slow vrs GPS. All of the other gauges in the dash cluster work fine. I thought the vehicle speed sensor might be bad but the parts guy at the Chevrolet dealer said 99+% sure the dash cluster is bad. 2003 is notorious for this problem. This is a 4wd, Z71, 5.3 T engine with 116k miles. How can you tell for sure the gauge cluster is bad? Or test the vehicle speed sensor, I think this truck has more than 1 too LOL! Speedometer worked fine before the engine service light came on for a bad O2 sensor (bank 1 first 1). Replaced the O2 sensor and service light went out but now the speedometer does not work. Thanks for any ideas and/or suggestions.