My blazer was stuck in the mud and we had to use 4lo to get it out.because for some reason 4hi was not working. We had to gun it to get it out. The service engine light came on. When I drove it, it was lugging like it was still in 4lo. Took it to the mechanic. Who discovered that I was missing 1 gear and overdrive. However when he ran a diagnostic check on it. there were no codes to be reported. He has manually turned off the service engine light but it will come right back on. He has gone through every inch of the blazer and can not seem to figure out why there are no trouble codes. Has this ever happened to anyone.?can the codes be erased after driving so many miles? any help or ideas would be great so I can get my blazer fixed and head to the dealer to trade it in Would the computer chip affect the transmission in any way? If the chip was going out?