GM ds not install TPMS devices in spare tires. I had a flat tire in snowy conditions 100+ miles from the nearest place for assistance. After putting on the spare tire the DIC (driver information center) displayed no air in tire. Thats because a TPMS dsnt exist in spare tires. I started driving a my four wheel drive turned off. I turned it back on and after a very short distance it turned off again. Since the vehicle determined I didnt have air in one of the tires (spare previously mentioned) the four wheel drive shouldnt work to prevent damage. I needed the four wheel drive in the snow so I had to proceed at approximately 25 mph. This is a safety issue for not only my vehicle but others on the roadways. I contacted the dealer that had no idea about the problem and neither did the national GM customer support center. There is a tool that tells the vehicle where each tire is located so that is low tire pressure occurs the driver would know which tire needs attention. If a TPMS was installed from the factory and a flat occurred, spare installed then the four wheel drive would function. If a flat subsequently occurred the driver would still be alerted but wouldnt know tire was involved. Visual check would find it. Either this or have GM provide an override so the trip could be continued safely. I believe that this situation applies to any four wheel drive vehicle not just GM and should be resolved if the NHTSB would initiate an investigation.