Hello,
I have a RUUD UGDA-10EA-AR gas heater which sometime works fine but at times has issues. Scenario of fault below:
Temperature at 62°, set point 72° with digital control unit
The heater comes on and heats maybe to 66° and stops, even though the thermostat control unit states the "heater is on". The heater will not automatically restart. I have to manually put the set point below the current temperature to have the control unit turn the heater off and then set it back to the desired temperature again. At that time the heater will turn on again.
I have replaced the digital thermostat control unit and the Ignition Control Unit (HS780 34PL-308A). Still have issue.
Can anyone point me in the right direction for a resolve.
Thank you,
Jeffery

Does the furnace have a control board which blinks a code? Note: There may be a viewing window in the blower door through which you can see the blink code.

No, the control board is in the gray box at the bottom of the unit with no view inside unless I take it apart. I have a pic, but do not see a way to attach it here.
It works fine if it only has to maintain the current temp, but has issue when has to increase the temp several degrees.

Thanks,
Jeffery