My Laars LX 400,000 natural gas heater...the control panel is frozen, so I can not turn up and down the heat. It stays at 84 degrees

SOURCE: My teledyne Laars Telstar Model TGS-50 spa heater

Without doing more troubleshooting i would have to say that it is the main board (the board with the knobs on it) Take it out and see if there is any corrosion on it.

SOURCE: hi limit switch code

mine were corroded, but passed a continuity test. removing the corrosion opened a hole in one of the cases. $30/ea to replace (parts only). then i read how the brass 'screw' they contact with are prone to corrosion (salt pool), not listed in the parts manual, but are available as part# R0383200 (another $6/ea). not a happy jandy/laars/teledyne owner.

SOURCE: Where is the Air Flow switch on a Jandy LX natural

The air flow switch is in the upper panel on the inside of the heater. There are gray hoses plugged into it.

SOURCE: Spa will not heat more than 85 degrees or so

on the back of the board is the bottom for seting the max temp follow the intruction from the manual or get one at jandy.com

SOURCE: Pool Heater wont light

NO NO NO the heater is design to work on propane or natural gas only