Just had the water pump, tstat, and a few other things replace on my 2000 gmc sierra 2500 (6.0L). Got a low coolant light so I topped it off with some autozone universal 50/50 (claims to mix with any color) because that'* what was in the truck so i figured that'* what the system was filled with after the work done.
Just found out it had dexcool in the system the whole time. I've been running this mixture for months now without issue, just keep getting a low coolant light and topping it off, and then forget about it.
It still runs cool and just fine, but I just need to know how to properly go about this. Guy at the parts store told me to just drain the radiator and then fill it up with dexcool but I think I need to flush the entire system. Should I expect to find sludge when I flush it? Also if anyone has a link or advice on how to flush the entire system, that'd be great.
Any thoughts are appreciated. Thanks

Drain it all out, use that flush ****, fill with water, run engine for a few minutes, drain it all out, fill back with water, run for a few, drain again, then top off with 50/50.

Use a flush product with it? And should I worry about getting the engine hot enough for the tstat to open so that the block will get flushed as well?

I'd call it good myself, unless you really want to get the Dexcool out.

. . . and if you mix your own 50/50 then use distilled water.

To get the engine so hot that the cold flush will get the thermostat open would make the engine unusable. I always pull the thermostat and upper/lower radiator hoses, flush, then put it back together again and fill with 50/50.

My 2001 YukonXL 5.3 (so nearly the same engine and year) got its first coolant flush (that I know of) and first water pump/hoses/thermostat replacement somewhere in the 290,000'*. Prior to that, I was topping it off for the leaking water pump shaft seal for months. I know it was orange Dexcool when I bought it in the 180,000'*, and was mostly green when I drained it to swap the pump etc. I've observed nothing bad about the coolant ever, except that the Dexcool in the reservoir smelled a bit old and musty when the water pump leak started.

I ended up draining and filling the system in the late 290,000'* after a couple of months of topping it off again for what ended up being a cracked radiator tank. Now it'* definitely green coolant.

I use Prestone BTW. I buy what they now call "Concentrate" and mix 50/50 with distilled water. Saves a few bucks, and usually I outsource the mixing activities to a kid.

. . . and I've never flushed it. I took a tank off the old radiator and there was almost no buildup.

It'* been running fine with the mix... i assume there'* a small leak that burns off on the engine, I can't see any dripping underneath the truck and I can't smell any antifreeze burning. I'm just worried that the mixture is causing some kind of sludge that is getting blocked up and causing the sensor to read that it'* low. I know that'* unlikely and probably not possible, especially because it'* running cool below 210, even when it says it'* low. I'm at college right now and don't have access to my driveway and drain pans, etc. So I was looking for the easiest option that isn't going to trash this coolant system and new water pump.

How many miles on it?

My bet: radiator tank has a small crack somewhere sturdy. At this age with a lot of miles and heat cycles this is most-likely in my book.

I've never seen sludge build up from modern antifreezes. Olden days like `80'* and earlier all kinds of fun things would grow inside cooling systems. Often, if the fun thing you found was sludgy or chunky it was because someone had added a lot of the primitive stop-leak solutions of the era to either ensure their work to get the car out the door or to stop a leak they didn't want to pay a lot to find and fix. Back then the gaskets sucked and the radiators found all manner of reasons to rot.

Not sure if you already know this, but just in case: On your truck the low-coolant sensor is in the reservoir, not the radiator like a lot of cars. In other words, in your truck it is announcing "low coolant, you should top it off soon", and in the cars that have it halfway down the radiator it is announcing "too late, you're (more likely than not) screwed".

I think easiest option is just keep topping it off with universal coolant. Eventually, the ratio of universal to Dexcool will have almost no Dexcool in there. Later, when you find the leaking radiator tank (or whatever it is) you'll have to dump it all anyways. Then fill it with universal and enjoy the next hundred thousand miles.

It has just over 160k on it. Ill see if I can see a small crack on the radiator or something, but that'* a good guess. I just want to be sure that adding this 50/50 universal isn't going to turn into sludge when it mixes with the dexcool (guess it wouldve already by now). I suppose I'll just keep adding the universal until the leak gets so bad I need a new radiator and can then flush it and run universal or dex only.

Coolant is coolant. The only reason why there are manufacturer specific coolant is for 1 so they can make the money and 2 to make sure you are getting everything out of the parts that the coolant is touching. Manufactures put certain things in their coolant to make sure your radiator, pump and thermo gets its full life cycle. That'* it. Nothing more. If you want to mix it then mix it. Your car is going to be cooled by whatever you put in even water. Just be sure you are paying attention to what type you are putting in. If you put the cheap 50/50 in then you will not get 100k out of it and you need to drain around 50k. If you get lifelong coolant then 100k protection you get and drain and refill.

Then there is a difference if you get concentrate and dilute with water out a facet or distilled. Distilled is the best way to go but if all you have is a hose then guess what that'* what you have and you're still going to have a cooled engine. Just know the chemicals in regular water can corrode the radiator but that will take a while to happen.

That'* nice, recommend to someone that straight water is ok to use as coolant.

Anyone else thinks that I was recommending straight water for cooling an engine?