The Jandy/Laars LX and LT have no standing pilot burner.
Do not use the heaters to maintain pool or spa water temperature below 70F.
Fault - Shorted H2O Sensor
The H2O sensor is the water temperature sensor.
Thermistor test = Temp sensor test Step 1 measure resistance. If resistance is infinite, stop test, sensor has failed. Step 2 Come to terms with being 50 bucks lighter.
Seriously, if your temp sensor is ok, you should get an ohms reading somewhere between 7.6K (around 88 degrees) and 11.9K (70 degrees).
If your panel is reporting a SHORTED sensor, it could also be chewed wire INSULATION between sensor and board! A severed or chewed through wire would give you an OPEN fault.
You can check if you replace the sensor with a resistor that has a value that would correspond to valid temp and see if the heater fault goes away, and the heater fires. Then you could confirm a bad sensor. Example: if you were to get a 10.0K ohm resistor, you should get a temp reading on the heater display of 77 degrees.
If you are getting a SHORTED fault, disconnecting the sensor from the board should result in a OPEN fault. Or at least the shorted fault goes away, verifying that the board is ok. But if you remove the sensor from the board and you still get the shorted fault, the board would be suspect.
The sleeve the H2O sensor slides into is like a clear "shrink tubing" on the sensor. You may or may not get it on a replacement sensor. It acts to keep temp uniform throughout the length of the sensor.
VERY IMPORTANT: Disconnect power from the heater, disconnect the sensor, then power the heater back up. This will ensure the fault is cleared, the heater will reboot, and re-check for new faults.
Fault - High Limit, HL
That sensor opens when the water temp exceeds the rating of the switch. Hot water opens the switch, heater shuts down, switch cools off and closes, heater re-fires and begins to heat again until the switch opens again.