That time of year to discuss engine coolant There are several different types of coolant Good old ethylene glycol the old standby and works great but has a short life span worse in newer aluminum engines(drain and refill every 2 years)Extended life great for aluminum engines and has a longer life span but GM has expressed some concerns about how long that is ( drain and refill every 3 years)Notice I said drain not flush The so called power flush at most chains ends up to be a drain and refill a few do it correctly but this is rare Drain and refill is a simple task you can do yourself the hardest part is being sure you have all the air removed and bleeding if necessary. As a general rule if the heater is blowing hot the system is OK and on most newer vehicles there is a bleed screw in the area of the stat. Worse case scenario leave the cap off overnight and usually the air will bleed out of its own accord.Last but not least dont check the coolant for strength only but also have the condition checked yearly. Dont count on it to look bad as many times it will look fine and be worn out.Last but not least check the parastic voltage of the coolant. Using a lab scope or a good Vohm meter go from the coolant to ground the voltage present will surprise you check it clean so you have a reference point also consider a seperate ground from the rad