Mini Max NT LN Heater will not stay lit. Has power and Service light on. TSAT, AUX, and PRESS are on. After it cycles and starts, HEAT light comes on and Service light goes off. After a few seconds it sounds like the flame goes out and heater goes back to service light and no heat. On the back of control panel TFUSE and HITEMP lights are lit. PV light comes on as Heat comes on. It will repeat this cycle every minute or so. I have replaced multiple parts, but cannot resolve the issue. Please help!!??