HOW TO CHANGE HEATTING SENSORS IN 2000 INFINITY I30T

There may be an issue with the thermostat in the engine not keeping the operating temperature up. Some newer cars, especially with aluminum blocks, have trouble holding heat and will not blow very warm air until you are moving and on the gas.
Even if you have a temperautrue gauge on the dash panel, it is not sensitive enough to show much more than changes int emperauture around 10 degrees farenheit before you can see e movement in the needle. It is meant to show operating normal and near overheating and then overheating tempertures. So, don't depend on it to give you an accurate reading. You can purchase in infra-red thermometer for less than $20 at auto parts stores and they come in handy. Point it at the thermostat housing and you will get a good reading. Try comparing it with the stock gauge so you can see how accurate the changes in the needle are.
Let me know if you need any further assistance.

Assuming coolant level is full and free of air pockets? Any work done to car prior to this problem? Sounds like a coolant flow problem with heater core. This could be caused by partial clogging of core or other item in system or possibly weak water pump?

And when you say heater quits do you mean air stops blowing or heat goes to cool? And how does your temp gauge behave during these changes?