My Pentair heater fires up (heat comes on) for about 30 seconds and then shuts off. I have replaced the flame sensor and temp probe. The air flow switch is working, but I bypassed it just to be sure. The temperature limit switches are in their normally open position. I can't read the error code on the control panel because some of the led's are out (it has been that way for many years). The error light inside the unit in flashing one time, which (according to the label) is an air flow problem. I think the only air flow sensor in the air flow switch. Any thoughts or ideas? Thanks.