How do you drain the coolant out of a 2002 yukon

this is it

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Gm cars and trucks use dex cool coolant.

2 possibilities. - Possible heater core leak - does it smell like anti freeze/coolant or windshield draining into cabin area.

dex-cool

Look on the passinger side firewall.Where the fire wall starts to curve down under the body you will see a little black rubber hose.Usualy under your AC reciver/dryer.Round aluminum wrapped in black insulation.Well hope this helps.

Knock sensor is located on side of engine block screwed into a block coolant drain, your best access is from underneath the vehicle. Coolant will run out when you unscrew it so be sure the engine is cool. Just unplug the wire unscrew the sensor, screw in the new one and plug the wire back in. Also be sure to top off the radiator if you loose any coolant.

it has gone bad replace it

Make sure that your coolant is full. That is usualy the cause.When you are at idle there is not sufficent coolant to be pushed thru the heater core.

Your description may actually be your answer, so to speak. You mention 'generally as engine is cooling or if coolant is added too quickly'. It sounds to me like the reservoir is over-filled.
When the engine is cooling, coolant tends to flow back to the reservoir (plastic 'jug' where you add coolant). A bad radiator cap or one that was replaced with too low a PSI rating can easily allow too much to fill the reservoir and and occasionally overflow it. Most reservoirs use a rubber tube to drain excess coolant so it may appear to 'leak' near the front left wheel or left side of engine.
Replace your radiator cap. (did they pressure test the cap too?)
Make sure you check your owner's manual for the proper PSI. My sources show 15-16psi, but that sounds a little high to me. Also, make sure you have the correct (50/50) anti-freeze/water mixture.
After you replace the cap, make sure the reservoir has the right amount of coolant and isn't over-filled. (To 'fill cold' line) Drive the engine until hot, then park the truck and see if the problem is fixed. Good Luck!

Sorry but most newer radiators don't have a petcock. I just changed mine by removing the lower radiator hose.Most newer cars only hold a gallon or so of antifreeze anyway. If you have five year coolant (dexcool) DO NOT MIX WITH ANY OTHER BRAND regardless of what the bottles tell you. If you mix red coolant with green it will turn into a radiator and heater core clogging glop.