Teledyne Laars Lite2 with a thermocouple that is a solid copper tube rather than wires. Not shown on latest drawings. Where can I purchase a new thermocouple.

SOURCE: starting a teledyne laars heater

check for proper millivolt reading( 400-600).it could be 20 different problems and/or components

SOURCE: I need an install manual for teledyne larrs one

try jandy pool products on internet.most manuals are available

SOURCE: laars lite2 comes on then go's off some times a

clean flame sensor with sand paper

SOURCE: teledyne laars series 2 pool heater

First of all, I'm assuming that your heater has a pilot and not self ignition.You will need a jumper wire ( 16" of wire with an alligator clip on each end)attach one end to the terminal on the gas valve that the red wire from the pilot generator is connected to ( pilot gen. is the thing the pilot flame touches).
Now attach the other end of the jumper to the terminal where the wire from the connects to the knob where you set the water temp).
NOTE:
JUMPING BYPASSES ALL BUILT IN SAFETY DEVICES AND IS FOR TEST LEAVE THE JUMPER CONNECTED TO RUN THE HEATER FOR HEATING PURPOSES!!!!

SOURCE: Teledyne Laars LD250 Propane Pool heater cycles on and back off

Do you replace the thermopile or check the milivolts?
Check the Pressure gas
You need replace all of them terminals and check again the milivolts at the gas valve