Continuous Water Heater Issues

[Continuous Water Heater Issues]

Thanks for the response.  Makes sense but why does opening the pressure release valve allow it to work?

[Continuous Water Heater Issues]

I have an Atwood tankless water heater (model OD-5001CW) with a strange problem.  All of a sudden, it quit heating water.  With water flowing, I could hear the igniter clicking but it didn’t ignite.  In troubleshooting.  I took the pressure regulator off the hose.  Still nothing.  I then opened the pressure relief valve on the water heater and it instantly lit.  Pressure relief valve closed, no ignition.  Pressure relief valve open, ignites immediately.  Any thoughts as to next steps/resolution would be appreciated.