I have an M199 Munchkin boiler that fails to light. When powered up, it will repeatedly purge, fire the igniter but gas valve does not open. Gas pressure is normal, and hot water heater on same gas manifold with standing pilot working fine. Control board and gas valve have been replaced along with low voltage wiring harness. It should be throwing faults after 4 tries but keeps on trying. Yesterday, tech thought he found bad ground connection and the system operated completely normally for about 12 hrs. CO2 when running on spec. This has the 926 controller with Vision option that resets HW set point with outside air. It is correctly reading OA temp (d5 code) and HW setpoint (d11) is per the reset schedule (e.g. 40 deg OA --> 149 deg HW set point). System when running modulates combustor to hold that schedule set point just fine as zones cycle to satisfy T-stats.

When it fails to light, ionization probe looks clean and has 110 AC on it, spark looks normal on igniter, but something is inhibiting the control board from opening the control valve. What is very strange is not seeing any fault codes even after many tries at ignition. HTP factory has so far not suggested anything that is helpful as suggested replacements do not fix problem. The fact that it is intermittent is troubling, but with new wires and all connections double checked, not sure where else to look.

Anyone with ideas?