Turn the heater on. The fan will come on for about 30 seconds, then you will hear a click, and the heater should fire. At that moment, which ever thermostat you are using (pool or spa) hold that button down for ~10 seconds. Display should change from water temp to exhaust temp. Normal temps should be in the range of 290-350. 350-500 is considered high and you should look for a...
1) Sooted exchanger(external) due to improper gas pressure or lack of proper venting. either supply of fresh air or exhaust, or both. Heater is being starved of air and the resulting air/fuel mix is too rich (fat).
2) Calcified exchanger (internal) due to improper water chemistry. Note: Not necessarily right now, but at some point, and for a while. This wont happen over night.
3) Flow or velocity issue. Water is moving too slowly through the exchanger, gathering or absorbing too much heat, and is not carrying it away quickly enough to keep the heater at a workable temp. In this case, you may have a valve closed down, but not fully closed, or may be wanting to save a little more money on your electricity bill, so you turned down your VS pump as low as you can get away with. You will need to turn it back up to correct this !
Thanks for the update.