I have an old carrier 80 comfort gas furnace heater. I am not sure how old that is since I got a new house and I am a new homeowner. I have started to do DIY fixes lately. The AC broke down last year and it was under warranty and it took 4 weeks for the repairman to schedule and repair my AC. I did a DIY fix for error 21 (secondary voltage), I changed the blade fuse and it worked ($3 automotive fuses). Now I have understood decoding error codes and thanks to this forum and YouTubers in helping me out. [u][b]Problem:[/b][/u] My heater has stopped working lately. I never used it for a while and it was working a week ago. After the heat is on on thermostat, the blower keeps running (auto mode) for long hours with no heating. I had a bad filter which I removed it and installed a new one. Thereafter, I see the error code as 33 and after few minutes it changes to 13 and is stuck there for indefinite hours. Decoding the errors, I check various sensors. The end wire connections (quick disconnect) on switches are crimped so it was difficult to take out them and measure continuity. Anyways, i checked for two things (1) Continuity with the end wires still connected (2) End voltages while the blower was running I have checked: (1) Main limit switch which has a special case behind the gas regulator (2) Two manual reset limit switches which are on side brackets of four flame burner (either side) (3) Draft safeguard switch which is on the exhaust Flue gas I checked for continuity - ALL GOOD I checked for voltages across the end connection and checking for one end by grounding the other end of meter - ALL SHOWS 0.0 V (This doesn't change even after clicking manual red limit switches) It do not have good knowledge of their wiring but my intuition says these safety switches much be in series so any switch can break the entire circuit. It looks like the safety circuit is not getting 24V supply since Blower ON test showed 0 V across all switches. Since Error 33 goes to error 13, i kept the flower running for 3-4 hours and haven't seen any changes in its error status.( I through it should auto-reset after 3 hours) To clarify, the ignitor never gets hot, no flames, and the inducer motor also don't run as well. I am stuck at this point and need your expertise to Troubleshoot from this point. [i]Sorry for the long post, I am new to this forum and this is my first post.[/i]

My furnace has honeywell control board, gas valve, and ignitor module. I have replaced the thermostat, spark ignitor, spark wire, and an hvac guy changed the ignition module and adjusted the pilot. In addition to the new parts, I have tested that the inducer and its pressure switch, and the gas valve - which all work properly as far as I can tell. The issue is still this: Thermostat call for heat Air clearing fan kicks on Few seconds later ignitor starts Pilot lights just for a second then the induction fan and ignitor stop The induction fan immediately starts again (I checked the voltage through the fan pressure switch and the fan starts and stops so quickly that the switch never gets time to disengage - so voltage remains through it) It continues this on and off cycle for 1 to 8 times before it finally stays lit and operates normally - and this has been going on for about 2 months. Thanks for any help you can provide.