This is a Noritz N-069M(OD) tankless water heater. It will try to ignite and sometimes will,but most of the time it won't. I changed out the ignitor and circuit board with no change. It's been in service for about three years.Do I need to clean the heat exchanger and how exactly do I do it? How would I bypass the gas valve to test if its faulty? Any info would be greatly appreciated!