Heater fires up and runs for about 5 min and then cuts off. Reset flame saftey switch and it runs 30 sec and cuts off.I bypassed therostat and replaced all fuel lines. It is a 125,000 btu serial # S- 206868. What do I try next /

Travis, you did good with the description of what the heater actually is and not what the title says you have.

Try cleaning the photo eye sensor at the combustion chamber and make sure it seats back into holder completely. Use like a Q-tip to just lightly wipe any dust off the eye. If that doesnt do it, bypass the photo cell by connecting those wires or terminals leading to the eye. Your just completing the circuit by doing this. If it still does the same shut down, then your flame out safety control is like bad. (its the circuit breaker you keep pushing back in when it pops out shutting the heater down.) I do have them for sale here at the house in WNY. Travis, you did good with the description of what the heater actually is and not what the title says you have. Try cleaning the photo eye sensor at the combustion chamber and make sure it seats back into holder completely. Use like a Q-tip to just lightly wipe any dust off the eye. If that doesnt do it, bypass the photo cell by connecting those wires or terminals leading to the eye. Your just completing the circuit by doing this. If it still does the same shut down, then your flame out safety control is like bad. (its the circuit breaker you keep pushing back in when it pops out shutting the heater down.) I do have them for sale here at the house in WNY.