Engine hot ac turned off on my 2008 gmc sierra. any suggestions

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nternal combustion engines run on heat. Chemical energy in the fuel is transformed into thermal energy when the fuel burns, which produces mechanical energy to push the pistons, spin the crankshaft and drive the vehicle down the road.
As efficient as today's engines are, they still waste a LOT of the heat energy they produce. The average gasoline engine is only about 22 to 28% efficient. That means over two-thirds of the heat produced by each gallon of fuel either goes out the tailpipe or is soaked up by the engine itself. Diesels squeeze a little more bang out of each buck's worth of fuel with efficiently ratings of 32 to 38%, but even that leaves a lot of waste heat that must be managed and carried away by the cooling system.
Ironically, the hotter an engine runs the more efficient it becomes. But there's a limit because aluminum pistons and heads can only get so hot before they start to soften and melt. The same goes for cast iron. Engineers have been tinkering with exotic ceramic materials and metallic-ceramic alloys in an attempt to build high temperature, super efficient engines. They've realized some significant gains but ceramics are still too expensive for everyday applications.
HOW HOT IS TOO HOT?
Most engines today are designed to operate within a "normal" temperature range of about 195 to 220 degrees F. A relatively constant operating temperature is absolutely essential for proper emissions control, good fuel economy and performance.
A 50/50 mixture of water and ethylene glycol antifreeze in the cooling system will boil at 225 degrees if the cap is open. But as long as the system is sealed and holds pressure, a radiator cap rated at 15 psi will increase the boiling temperature of a 50/50 coolant blend up to 265 degrees F. If the concentration of antifreeze to water is upped to 70/30 (the maximum recommended), the boiling temperature under 15 psi of pressure goes up to 276 degrees.
So does this mean a cooling system with a maximum concentration of antifreeze in the coolant (70%) can run as hot as 276 without boiling over? Theoretically yes -- but realistically no. The clearances in most of today's engines are much, much closer than those in engines built in the 1970s and early 1980s. Piston-to-cylinder clearances are much tighter to reduce blowby for lower emissions. Valve stem-to-guide clearances also are closer to reduce oil consumption and emissions, too. Plus, many engines today have aluminum heads with overhead cams. Such engines don't handle higher than normal temperatures well, and are very vulnerable to heat damage if the engine gets too hot.
Anytime temperatures climb beyond the normal range, the engine is running in the danger zone.
CONSEQUENCES OF OVERHEATING
If the engine overheats, the first thing that will happen is a gasoline engine will start to detonate. The engine will ping and start to lose power under load as the combination of heat and pressure exceed the octane rating of the fuel. If the detonation problem persists, the hammer-like blows may damage the rings, pistons or rod bearings.
Overheating can also cause preignition. Hot spots develop inside the combustion chamber that become a source of ignition for the fuel. The erratic combustion can cause detonation as well as engine run-on in older vehicles with carburetors. Hot spots can also be very damaging and burn holes right through the top of pistons.
Another consequence of overheating may be a blown head gasket. Heat makes aluminum swell almost three times faster than cast iron. The resulting stress can distort the head and make it swell in areas that are hottest like those between exhaust valves in adjoining cylinders, and areas that have restricted coolant flow like the narrow area that separates the cylinders. The typical aluminum head swells most in the middle, which can crush the head gasket if the head gets hot enough. This will cause a loss of torque in the gasket allowing coolant and combustion leaks to occur when the head cools.
Overheating is also a common cause of OHC cam seizure and breakage.
Wait, there's more. If the coolant gets hot enough to boil, it may cause old hoses or an age-weakened radiator to burst under the increased pressure. Pistons may swell up and scuff or seize in their bores, causing serious engine damage. Exhaust valve stems may stick or scuff in their guides. This, in turn, may cause valves to hang open which can damage pistons, valves and other valvetrain components. And if coolant gets into the crankcase, you can kiss the bearings and bottom end of the engine goodbye.
A HOT warning lamp should never be ignored. Though a few high tech cars like Cadillacs with the Northstar engine can disable cylinders to "air-cool" the engine and keep it running at reduced power in the event of coolant loss, most engines will suffer serious damage if they overheat. So advise your customers to stop driving at the first sign of overheating. Turn the engine off, let it cool down and try to find and fix the cause before risking further travel.