Shut downs with new heater might be defective PC board.
Other issues could be associated with improper installation.
Low gas pressure caused by undersized gas meter or water in the gas line, undersized gas line to unit (requires 1" gas line to within 10 feet and then 3/4"), wrong type of gas line (cannot use flexible gas line), improper vent (for example too long horizontal run or undersized vent pipe or shared vent with furnace or other gas appliance), not enough incoming fresh air supply, nearby exhaust fans or vent fans stopping proper draft on tankless, temperature set too high for cold incoming water supply (install tempering tank to help preheat water),..
http://waterheatertimer.org/Tempering-tank.html
Other causes can be clogged water filter located where cold water enters tankless, or clogged water line, or crossover caused by bad single-handled faucet or bad crossover valve on recirculation system, improperly sized recirculation system. These problems can mean less water flow through tankless, so burner shuts down, and unit throws error code because sensor indicates water flow is still moving but inadequate to trigger burner
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/Water-heater-recirculation-system.html
Gene
Water Heater Timers Save Money If you need further help, I’m available over the phone at
https://www.6ya.com/expert/gene_9f0ef4df2f9897e7
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