I need help with a Master Temp 125 natural gas heater. It fires up and runs about a minute then shuts down and AFS lights up on the control board. I checked the gas pressure, it's good. I replaced the Air Flow Switch, Gas shut off sensor, Hi Limit sensor, thermistor, and thermo-regulator. It has a sand filter (TA100) and a 1hp FR whisperflo pump. I've backwashed and the sand isn't old. It has good prime so I don't believe it's a water flow issue. Now what???