If you are relying solely upon the dash gauge to indicate coolant temperature, don't. That "gauge" is more of a trim package than anything which might be construed as an instrument. If you can verify the relative accuracy of the temperature gauge by comparing the CTS scan data (or sensor resistance) then you might have a problem.
Coolant temperature will almost always increase with an idling engine since the water pump is not moving nearly as much liquid, there is no ram air to cool the radiator since the vehicle is not moving over 30MPH, and with an automatic transmission, frictional heating of the transmission oil from constant slip (leaving the selector in DRIVE but not moving) will add significantly to cooling system load, since the transmission oil is cooled by the engine cooling system. (That's also one of the fastest ways to a transmission failure).
If you discover that the dash gauge is close to correct, and there is actually a problem, you may need to investigate air flow as tigger indicated, and coolant flow and condition. The condenser and radiator are highly exposed to road debris, and should be cleaned off so that heat transfer is possible. Check for obstructions between the condenser and radiator as well. All that dust, grime, insects, leaves, plastic shopping bags, etcetera can accumulate both ON the front of the condenser and BETWEEN the condenser and radiator.
Make sure the fans are operating. If the coolant temperature is 235°F or higher and both fans are not operating, there may be a control problem.
Make sure the coolant level is correct, and there is no air in the system. If the coolant is more than 5 years old, flush the cooling system, neutralize the cleaner, rinse, and refill with fresh coolant mixture. If you have switched away from the original DexCool coolant or have added anything other than OAT coolant to top off the system, you need to flush every TWO years.
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