Made 23 week of 1991 /natural draft/ hot surface ignitor with crossover tube pilot .initial symptom was a growling noise ,that could be felt when touched ,from robert shaw control board.the growling stopped and furnace would no longer light . Replaced control board with, universal roger white board. Now, thermostat calls for heat, transformer has 110v in and 24v out to board. Only getting 20v reading at gas valve. Furnace goes through 3 cycles of... HSI glows brite ,crossover pilot lights for 1-2 seconds then goes into lockout code at control board .there is no visible flame sensor on this model [HSI/flame sensor in one unite?] trying hard to do this myself, just need some help. Humble thanks for any tips.