A saga of repair, but successful. The heater would not stay on - run for a few sec, then turn off. Found the diagnostic manual on-line was very helpful and trouble shooting videos too. The first thing was to replace the display board / bezel. No display, and therefore no code. Tried to start the heater, and after trying 4 or 5 times, displayed the dreaded 'IF' code.
Next step - take the easy path, replaced the flame sensor. No luck, same problem. Next step, clean the gas pipe around the ports, no luck, still not staying lit. Now, my troubleshooting is indicating either the ICB or gas valve, and both parts are expensive, so measured the leads coming from the ICB to the GV using a DVM during the ignition sequence and saw that the ICB was sending 24 V to the GV, so I suspected it was the gas valve. I scheduled a pro to service the heater and confirm my diagnosis. The pro said I had to replace the whole unit if the problem was the gas valve, because the GV is unobtainium for my model H400FDN. I got online and found GVs that appeared to be the correct unit for the F400fdN, which is the No Nox model, from multiple sources, so don't really know what the pro was talking about.
So, based on the info above, I removed the GV to see if I could service it. Once it was removed, I could see the jets showed clogging in at least 4 jets with spider webs especially in the jet next to the flame sensor. Removed jets and cleaned the webs out of each and reassembled. Just happened to have some yellow Teflon tape laying around ,so was able to reconnect the GV to the gas pipe and it fired up and is heating at 22 F / hr so far.
Next step - take the easy path, replaced the flame sensor. No luck, same problem. Next step, clean the gas pipe around the ports, no luck, still not staying lit. Now, my troubleshooting is indicating either the ICB or gas valve, and both parts are expensive, so measured the leads coming from the ICB to the GV using a DVM during the ignition sequence and saw that the ICB was sending 24 V to the GV, so I suspected it was the gas valve. I scheduled a pro to service the heater and confirm my diagnosis. The pro said I had to replace the whole unit if the problem was the gas valve, because the GV is unobtainium for my model H400FDN. I got online and found GVs that appeared to be the correct unit for the F400fdN, which is the No Nox model, from multiple sources, so don't really know what the pro was talking about.
So, based on the info above, I removed the GV to see if I could service it. Once it was removed, I could see the jets showed clogging in at least 4 jets with spider webs especially in the jet next to the flame sensor. Removed jets and cleaned the webs out of each and reassembled. Just happened to have some yellow Teflon tape laying around ,so was able to reconnect the GV to the gas pipe and it fired up and is heating at 22 F / hr so far.
