I have a ducane heater I been using it for about 7 years now with no trouble, This year the batteries went dead in the thermostat I changed them about 1 month later when I needed the heater, went to turn the heater on and it did not work, I checked the LED
Light is red blinking 3 times, any idea what it can be?

Model MPGA075B3B
Ser. 746831040720551002

Thank you