First post here.... Been battling my furnace for the past month. Replaced controller board (White Rodgers 21V51U-843), and flame sensor so far. The furnace will light sometimes, but mostly, will retry then go into lockout. I can finally reproduce the exact thing that it does. Now, have to diagnose and repair the issue. I've video recorded it numerous times so I could then check to see how its behaving Here's exactly what happens.... 1) Call for heat 2) Inducer runs at high speed 3) Inducer lowers to low speed 4) Ignitor comes on for its 17 seconds or so 5) Gas valve "clicks" but it fails to light (THIS IS THE ISSUE) 6) About 4 seconds later, the ignitor shuts off, and the system goes into purge mode (this is how the control board is supposed to work) Then it will retry until lockout. The new control board is working correctly as far as I can tell based on timings. So no clue how to proceed. Could be the gas valve I guess, but not sure how to diagnose.

Ok this is a goodman unit I'm not sure of the model it's 70k btu I believe input 80%. Last year This furnace gave me the same problem, I checked the ducting that year and we'll with only one return and in the living room partially covered by a couch I thought well well is it no return air I went and checked the filter and they had 2 filter a corrigated one and a metal mesh type filter Wich was just completely backed up so I removed that one and tried with the single corrugated filter but no dice. it would run with no filter just fine so I bought a new filter the cheapo kind and that got better, by the way I had a thermal probe on the over load just to verify temps, so at this point I moved the couch made sure nothing was blocking the return and it ran fine so, I ended up adding a return away from the couch, also I pulled the burners blew out the heat exchanger cleaned the burners put it all back in, fired it up and checked the gas pressure which was at 3.4 inches of water column and when I installed furnaces before we would set them 3.5 so o figured this was fine. Well it ran all last year with no issues what so ever, but this year they tell me it's starting and stopping so I stop by and sure enough it's over heating, so I do a quick re check also I checked the flue the only thing I have not popped off is the inducer cause it seems to run fine no noise and also the blower hasn't come off cause it seems to run fine I changed the speed to medium, but this year I did check the vents as I forgot last year as it was working and I was tight on time and I do these repairs pro Bono for the older people in my neighborhood. This place only had 5 vents all together 2 big floor vents on the main floor and 3 in the basement. 1 in a bathroom which gets covered by the door when open one in the hall way which vent had an inner screen completely blocked off and the other was fine so I removed the blocked vent and closed the door and we'll the furnace ran. Now I believe the furnace is only 5 or 6 years old and 4 years it seemed to run with no issues so im stumped if this last thing doesn't fix it. Although what was there was I believe a converted gravity furnaces with no blower originally and they added a gas control and blower, etc and then like I said a couple years back they had a technician come in and sell them a new unit, I think rightfully so as they need that behemoth changed out, problem is I don't recall what the rating on the furnace was and if it's il verify when I go back as I read the label but I just can't remember it no matter how I try. 70k seems like a bit big for the house, I mean the house consists of 2 bedrooms and living upstairs and a kitchen a bathroom and a laundry room downstairs. So my question is could this be cause by an over sized furnace? Cause I remember being taught that over sized furnaces short cycle, but I can't rember most specifics from school, but instead what's mixed up in my head is from past working experience and whatever veterans of the field have taught me. thanks for any help