I have a MasterTemp 400 that I have been troubleshooting since last year. I've check different foruns and looks like a common issue, but I have yet to see a resolution. Perhaps you can help. As soon as I turn the heat on, I see a 75 reading. It 'heat' blinks to call for heat and then steady light. It Ignites, air coming out of the exhaust. After about 5 secs it shutts off and temp starts counting up fast to 124 and then goes back to 75 and do the same cycle offer and offer again. No error codes on the front and no LED indicator lights on the back.So far, I replaced the thermar regulator, thermistor, a broken and just before opening the pool for the season, I replaced the bupass manifold. I found a rock in fhe by pass manifold so I thought I have resolved the issue. I was wrong. I also checked the blower. It was clean. No nest in there. Wires are intact. I'm getting frustrated with this. I don't want to buy and replace the control board because it's expensive and it might not be the problem. Filter is clean since I just opened the pool. The pressure is 20-25 psi, pump is running at 3450 rpm about 85gpm.