Mixing conventional coolant, that’s ethylene gycol based, with the DEX-COOL that GM vehicles are filled with before leaving the manufacturing plant will significantly reduce the length of its service interval from 5 yrs/150,000 miles to 2 yrs/30,000 miles.
In order to solve this issue, the cooling system needs to undergo two cycles of being flushed immediately.
Refill the system with DEX-COOL and drinkable water in a 50/50 mixture.
No other additives or inhibitors are necessary when DEX-COOL is appropriately used.
Note: Chevrolet Aveo and Pontiac Vibe are not manufactured with DEX-COOL so the above does not apply to those vehicles. You can always check for manufacturer recommendations before filling or flushing your coolant.