Temperature gauge past "H", air from heater comes out cold.
Once the engine warms up, then the TG needle goes past H, it can be after 15 minutes or 45 minutes running. Heater air cools from very warm. Sometimes it is running normal, you stop at an intersection and the needle begins to inch to H. After take off, it may go back to normal or not. Once is on H, it is unlikely to come down. I have notice that by stepping on the gas on neutral, heater air warm up for a few seconds and then cools down. I open the hood and you can "feel" the engine is very hot, and the water/antifreeze reservoir is full. Can not see any leaking.
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The dash gauges are notoriously inaccurate. But it is possible that the thermostat is stuck open. Does the upper radiator hose get hot after engine warmed up? Also feel both heater hoses-when engine is warm, both should be hot-indicating the coolant is circulating through the heater core. If heater hoses are hot, and you still have cold air, the temperature blend door on the heater case may not be operating.
Try a new thermostat and see if engine warms up faster and better.
If there is cold air coming out the vents than its not a thermostat. The thermostat gullets the temperature of the engine not the temp of the heater which bypasses the thermostat.
If there is never any heat out the vents than there are two possibilities. The coolant level is to low. The first thing you will notice if the coolant level gets to low is the lack of heat out t
Here's the way I usually approach problems such as this. First I'll hook up a scan tool that allows me to see the actual engine coolant temperature is [not just what the gauge says] as the vehicle is being driven down the road at highway speeds. The coolant should be near the 192 - 200 degree range to get the best heat out of the heater. If the coolant temp is below that level then the amount of heat you'll get will drop according. IF the coolant is reaching full temp but the air coming from the heater isn't very warm then you could have a restricted heater core or a temp control door in the ductwork that isn't working properly and is allowing cold air to sneak past the heater core. Usually if you have a water pump impeller problem you'll also have an overheating problem as the pump often isn't able to move enough water through the radiator to keep the engine cool. Hope this gives you a starting point and good luck!
Here's the thing. The ECU will shut down AC if the engine temperature is too hot. If the coolant temperature sensor is malfunctioning (lying to the ECU) telling it that the temperature is hotter than it actually is, the computer will turn the AC OFF.