The Service Heater light came on. The fault code is AFS. Blower and burner do not fire. Service heater light comes on a few seconds after the power up sequence on the LED panel. Pool service tech came today and the troubleshooting tree didn't find what was wrong so we are looking where to go next. The heater just stopped working (3.5 years, so just out of warranty - yech) so we don't suspect wiring issues. Neither the blower fan nor the burner come on at all nor do they sound like there is any attempt to do so.
The heater is controlled by a Pentair automation controller.
Here's what we did. All are done with the main power and pump running and with spa heat called for (with the Service light on)
1. Voltage checks. 24V from transformer.
2. Even though the path doesn't take us there disconnected F1 and F2 and the blower does turn on.
3. Checked and replaced the Air Flow Switch which didn't fix it. Tubes are not plugged.
4. He tried to swap the control board (the one for the pushbuttons and status display) but the board didn't look identical (he thought it was a newer model) but the board got fried! All the main connectors keyed correctly but there seemed to be a different layout. So it appears that it is not a direct replacement for this model.