Tankless troubleshoot- CODE 10 1- inspection of exhaust vent and air intake vent. Exhaust was great, air intake has lint/debris partially covering the intake cover. 2- inspection of burner- burner is clean and in perfect working order 3- inspection of condensation drain, it is working perfectly 4= water flow, with no yearly maintenance, I guarantee the water flow is less than expected. Maintenance will not help 5- gas flow. Only reading 4.6 inches of water column. 7.0 is required