Furnace Troubleshooting (the first step)

I recently had a furnace come in- complaint was that nothing happened- no fan, no heat.. nothing.

Whenever I have a furnace problem, the first step I take is to go to the thermostat. This one had a simple heat only, mechanical thermostat. I pull the cover off, switch my multimeter leads to read amperage and hook up the leads across the thermostat terminals.

On this furnace, I immediately read a 1 – 1.4 amp draw, which dropped quickly to around .2 amps. This told me the time delay fan relay in the furnace was good (on most furnaces built in the past 20 years, this will be a valid test- the exceptions will be new models with “fan control” circuit boards, or models where the board has been retrofitted with a Dinosaur brand fan control board).

After about 40 seconds, the blower came on, and the amperage rose a bit to around .5 amps. After another 45 seconds, the amperage rose again to about 1 amp.

What these readings told me was 1- the relay was good and the furnace had power (the initial high amperage reading was the heating element in the relay heating up), the second amperage jump (after the blower came up to speed) told me that the limit switch and sail switch were both OK, and power was getting through to the circuit board, the last amperage jump told me that the gas valve was getting power, and the circuit board was opening it. (A side point is that the final amperage reading- 1 amp- is the setting that the “anticipator” should be set at on mechanical thermostats).

For that job, I was lucky- the only problem was a bad connection in the thermostat- I repaired that connection and the furnace worked fine, but I hope you can use this method of troubleshooting to diagnose furnace problems (if you have a combination heat and AC thermostat, the same method can be used, but you have to find and break the correct thermostat wire to the furnace, which is usually easiest to find right at the furnace, and will nearly always be one of the two blue wires).


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Furnace Troubleshooting (the first step) by Chris Bryant is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.


469 comments for “Furnace Troubleshooting (the first step)

  1. Kerri
    November 25, 2015 at 2:56 am

    I have a Suburban SF-20F furnace. The blower runs but the ignitor does not come on. You can hear it click like it is trying to light but it never does. How do we check this?

    • November 27, 2015 at 1:53 pm

      You cannot usually hear the ignitor itself on that model, but you can hear the gas valve make a clunk. The SF series really need to be removed for most problems and to access the burner. The main problem with that model that can be easily fixed is a bad or oxidized limit switch. There is an inside removeable cover tha has the limit switch right behind it- often simply unplugging and re-plugging it will fix a “no fire” issue. Unfortunately, if yours is trying to ignite, that probably is not the problem.

  2. Don
    April 6, 2015 at 9:54 pm

    I have a Suburban SF20 rv furnace.. I checked the power and have power to the limit switch.. The fan will not run and the igniter won’t light.. Can you give me some ideas on what the problem might be??.. Everything worked fine in September.. I parked the rv and now the furnace won’t do anything.. I’ve taken the furnace out of the rv.. Do I have to have propane going to the fan to test it??..

    • April 7, 2015 at 12:10 pm

      It all depends on whether you have a fan control board, or the fan is controlled by a separate relay- I’m figuring you have a fan control model. For this model, the thermostat closes, feeding 12 volts to the thermostat input on the board, *and* to the sail and limit switches. The board knows the sail switch should be open, because the fan isn’t running. If the sail switch is closed, and the board is getting power through it and the limit, it will not start the fan, as it knows there is a fault. If the fan does start, if the limit switch is open, it will shut down as well.
      It doesn’t have to have LP to do a basic test, but without it, it should start and try to light 3 times- after the third unsuccessful attempt it will shut down on lockout.

      Hope this makes sense!

      — Chris

      • Ralph Wozniak
        October 19, 2015 at 8:25 pm

        Have same problem with both front and rear furnaces, Tiffin said it is a control board in roof top coleman mod 8535, Cannot find anybody to verify or get a part number. Tiffin said they don’t carry part and have no Part number. When I connect the blue wires at furnace the roof top and furnace both start with thermostat in off position. Puzzled and frustrated, Help, Tiffin said board controls both front and rear furnaces. Ser number of air unit is 010409287.

        • October 22, 2015 at 1:55 pm

          Hi Ralph,
          What thermostat do you have? I assume it is a single thermostat with zones. There are 2 zone control thermostats that I can find- one at this link to a pdf, and the older model at this pdf file, but everything coming on at once suggests that power is being supplied to the wrong wire- if the thermostat is getting its power from the furnace, try switching the blue wires around- one wire from the furnace is hot, the other cold.

  3. brian scarbrough
    January 19, 2015 at 12:00 am

    i have a 2004 Dutchmen 31bh i had 2 wires on my power plug to arc and had to replace my end. Ever since this my heater blower turns on and will not turn off even when i turn thermostat to off. It is blowing cold air the igniter isnt working. Only way i have found to turn it off is pull the 15 amp fuse that works heater and refrigerator. any help is appreciated!!!!

  4. Anonymous
    December 23, 2014 at 3:57 pm

    I still have a situation no-one seems to understand. if the thermostat sits down with the screw tightened down on it furnace will not ignite. if I raise it up with a washer under it , it works. ????

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