This heater has worked fine for 4 years, but now won't stay lit, and displays an "IF" code. The heater lights normally then shuts down after a few seconds. I've done a lot of research so far and checked quite a bit. Here is what I've done so far:
I checked the gas pressure. I have about 7" on the supply side not running, 5" while running, and 2" (exactly on spec.) on the manifold side when running. So I don't see any gas supply problem.
I checked the orifices. I found #4 slightly blocked by spider webs, and cleaned that out. No change. All other's looked fine.
I pulled the burner tubes and checked each one. No blockages or debris.
I checked voltage to the gas valve. 24v, and the gas valve opens immediately when voltage is applied.
I removed the top of the heater, no blockages.
Following some other advice I saw online, I blocked half the holes on the inducer fan with tape. No change, removed the tape.
With the top removed, I can see through the heat exchanger, the flame looks fine (for the few seconds it's on.) and the igniter glows bright. I don't think there is any problem with the flame itself or the igniter.
I cleaned the flame sensor. No change.
After all the above, I replaced the flame sensor. No change.
At this point, I guess the control board is next. But I hate to throw parts at it. Is there anything I missed? Any other tests I can do before buying parts?