Pool and Spa
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If it's displaying 126 it's telling you it think the water temp is 126 so it won't fire because it thinks the water is too hot. Are there any diagnostic LED's light on the back of the control board?
You have to remove the control panel to get to see the back of the board.
Take the top of the unit off
Sounds like you have a bad temp sensor
Just turn the unit off, disconnect the leads and unscrew it as long as the heater is at the high point of the system. If the heater is lower than the pool or spa then you need to isolate the heater. There should be valves to do so.
Do you know where it's located? do you have the manual?