RobH
Member
The original issue transmission thermal bypass valve (thermostat) for the MYC (6l80E) and MYD (6L90E) transmissions maintains the transmission fluid at around 185F. The Service Bulletin No.: 21-NA-199 (search online for the current version) calls for installing a 70C/158F Thermal Bypass Valve to keep the transmission cooler. You may not have one of these two six speed transmissions.