Did service man clean combustion parts and check for loose wires, proper venting etc?
In other words, is the tankless up-to-date on yearly required maintenance?
Did you check outlet for power?
Is the unit properly grounded?
Is the unit vented properly, and set for correct altitude?
Is the thermostat set too high?
What does the manual say about maintenance?
It certainly says to clean water filter frequently, and clean combustion parts yearly, and bleed air from gas line, and delime yearly.
Is your water heater exposed to more than 11 grains hardness in water that will cause failure due to clogged parts?
Does service man have a service manual for your specific water heater?
Does anyone have a service manual for your water heater? Probably not, which means service man will be guessing at one thing and then guessing at the next thing.
Problem could be PC board failure.
Similar to failed motherboard on computer.
Let's be frank:
Do you want the cheapest solution?
Or do you want to chase around trying to find why a complex computer chip connected to electric outlet, electric sensors, gas valve, water valve, water line, gas line, and gas burner with vent to outside has decided to stop working?
Cheap solution, Buy tank-type gas water heater for $350 at local box store. Just get a simple one off the shelf. Don't worry about warranty, because it means nothing.
Select Whirlpool, AO Smith, American, State, Reliance, Craftmaster, Kenmore because these are made by same company and can be repaired and maintained by homeowner, and service manuals are available, and any plumber knows how to fix em..
Avoid Rheem, Ruud, Richmond, GE, for all same reasons except in reverse.
Read the manual, perform the required maintenance.
Keep the air intake clean.
Install water heater yourself same day, turn up the temperature wait, 1/2 hour for tank-full of hot water, and take hot shower.http://waterheatertimer.org/How-to-install-gas-water-heater.html
If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7