Rheems 80+ forced air heater turns off within a minute. If I turn off thermostat for few minutes and turn on again it will start but turns off in a couple minutes. What can I test to find problem?

Try cleaning the flame senser

Check the status light(s), if present. A code label should be attached to one of the front panels, or the circuit board housing.

Can't see any status lights.
Flame sensor's function? Isn't it to turn on the flame so if furnace does start but will not continue beyond few minutes, something is sensing something is getting too hot? A temperature sensor, but where?

Flame senser is just that, it senses the flame. If the flame senser is dirty, the unit will work spuradically.

Try this, remove the covers of the furnace, and then push in the blower door safety switch and let us know if you see any status lights on the control board

Couldn't find any status lights. Have to go to bed now. Will look tomorrow and get back here. Thanks for the suggestions.

Found the flame sensor next to the pilot light thermocouple and cleaned both. Still the same problem. Forgot to mention that draft motor continues with pilot light on but heating gas flames turn off after couple minutes of blowing heat. Can't find any sensor lights. The furnace is over 15 years old so maybe they did not install such then. What else can I test?