Are you handy with a multimeter? Both AC and DC circuits are used in Noritz units. The Technical Data Sheet taped to the inside front cover of the units will summarize the voltage checkpoints for all the components, amp draws from the flame rod assemblies and resistances on the gas valve solenoids and hi limit switches
AC Checkpoints
power supply to circuit board = 120VAC
igniter spark = 120VAC
DC Checkpoints
remote controller = 15VDC
quick connect = 15VDC
water control valves = variable 1-15VDC
flow sensor = 15VDC
gas valve = 90-120VDC
Amp Checkpoints
flame rod rectification > 1 micro amp (10^-6) when flame is present, < 0.45 micro amps when no flame is detected
Resistance Checkpoints
gas valves = 1220-1500 ohms
high limit switches < 1 ohm
thermistor resistance < 1 ohm