Flame Sensors – All Gas Furnaces

What is the CORRECT procedure to diagnose and clean a flame sensor? 

The flame sensor does not contain any moving or functional parts.  The flame sensor is energized with AC voltage, which is rectified to DC in the presence of flame.  The DC current flow is measurable in micro amps (uA).  A minimum micro amp measurement is required to continue the heating cycle.  On the 33” gas furnaces, 3.7 uA is a typical measurement, with 1.5 uA resulting in a “low flame signal” flash code, and measurements less than .5 uA result in interruption of the heating cycle. 

On many models, the flame current may be read at the flame current pad built into the furnace control board. The control board converts the uA measurement to DC voltage (1 uA = 1 volt DC, see attached illustration).  

Alternately, on furnaces that do not have a flame current pad, the micro amp measurement may be read by placing a meter set to DC Amps in series with the flame sensor (see attached illustration). 

If the flame signal is not adequate for flame proof, check the following:

  • A dedicated ground wire must be connected to the furnace
  • All wiring connections must be tight
  • Manifold pressure must be correct per the furnace rating plate
  • The flame must be impinging on the flame sensor
  • The flame sensor must be clean and free of oxidation. If necessary to clean, use extra fine steel wool.  Cleaning the flame sensor is considered normal maintenance, and is not covered under warranty.  If corrosion or contamination is severe enough that cleaning with steel wool will not improve the sensors performance, the flame sensor must be replaced.
  • Porcelain on the flame sensor must be intact.  If the ceramic insulator is cracked, the flame sensor must be replaced.
  • Check to see if there is voltage at the sensor.  Without a call for heat, there should be voltage (approximately 90 to 110 volts) between the sensor and ground.  The sensor is always looking for flame and is always energized.  If no voltage output is found, check the wiring to the board or replace the board if no voltage output is present.

 Flame Sensor Measurement attachment


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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26 Responses to Flame Sensors – All Gas Furnaces

  1. Glenn says:

    Are you saying the board has voltage output, “replace the board if no voltage output is present” ?

    • Check the flame sensor connection at the board — if no voltage there and you have voltage present at the input (24 VAC & 110 VAC) then yes — replace the board. The flame sensor is always powered looking for flame that is or isn’t suppose to be there depending on the mode it is in.

  2. Dean says:

    Hi. Had a call today on a year old TM9V. Error code. Gas valve energized on no call for heat. Place call inducer starts then board energizes gas valve then stops on fault. Intermittent problem and wouldn’t do it everytime. I determined control board. Checked everything. But didn’t check flame sensor to ground. Could the sensor be part of problem??

  3. Dean says:

    Thanks for the reply. I checked the sensor and it was fine. It was the control board

  4. dean shetter says:

    dm120n20n2aaa3d. serial noc9635530, receiving frequent calls on no heat,go to fault codes, flashing 6, indicating ignition failure, ive cleaned the flame sensor, got 3.3 microamp reading, pulled and cleaned the burners, pressure switch closing, pulling .55 wc. gas pressure 3.5 wc on high fire, 2.0 wc on low fire . it gets reset, it will run for 2 to 3 days and retrip, sometimes 4 timnes a day, ive got 30 + years in the heating trade, for the first time, im stumped, got any ideas?

    • when the burners fire, does the .55 IWC on the pressure switch drop? Is it “high pressure” gas on the INLET side? If it is, are you sure the regulator is functioning properly (see my post on Gas pressure regulators) and is “locking down and not letting the pressure climb above 11 IWC?
      While running, is it maintaining 3.5 IWC high fire?

  5. Frank says:

    Hope you can help. I have a York YP9C080B12MP12CA that has been acting up lately. It is a secondary furnace for the lower level and does not get much use. Last heating season the clear condensate drain pan formed cracks and was leaking for a bit before being noticed. I replaced it with the newer black condensate pan and all was well with the exception of the appearance of some corrosion within the furnace. Tried to fire it up this season and it would start the ignition process, light for a few minutes and quit. When the flame went out it was producing 4 red flashes then when it would go into lockout it and produce 11 red flashes (blower motor would continue to run). I removed the ECM motor and bench tested it and everything checked out fine. The motor was running and modulating fine prior to the burner shutting down. The 3 amp fuse is good so I replaced the limit switch (S1-02541321000) with no change. I pulled the pressure switches and operated them by applying a small amount of pressure and vacuum to both of them. After doing this the burner stayed on and actually produced heat for some time but was producing 4 amber flashes while heating but eventually the burner would go out and it would produce 4 red flashes and go into lockout with 11 red flashes (blower would continue to run). I assumed bad pressure switches and replaced both pressure switches (S1-02435929000 & S1-02541321000). With the new pressure switches installed nothing noticeably changed. I assumed there was only one more piece to puzzle so I replaced the pressure sensor (S1-03109198000). Now the burner lights and produces heat until it satisfies the thermostat, but 4 amber flashes and after the thermostat is satisfied the blower does not shut down (ever) unless the power is turned off. If the power is cycled off and back on, the blower stays off until the thermostat calls for heat and the cycle starts all over again. After the thermostat was satisfied and the blower still running, I disconnected and reconnected the wire at the flame sensor a few times and the blower motor began to modulate and eventually turned off. Curious, I tried it again and was only able to get the blower to modulate way down but not off. Note: during this time the draft inducer motor was not running unless I grounded myself to the furnace (got a shock) LOL. Is it possible that a dirty or bad flame sensor is causing the 4 amber flashes and the blower motor to continue to run after the call for heat is satisfied? If not do you have any suggestions?
    Thank You

    • the furnace you have is a full modulating furnace. It does not operate like conventional furnaces. If you have not been trained on this furnace, you need to get trained or call someone who is trained.
      the diagnostic flash codes you are experiencing have nothing to do with pressure switches or flame sensors. 4 RED 11 RED all have to do with the unit tripping on the limit switch. 4 Amber is reduced firing rate due to high temperature rise. Your blower may not be properly set up. You could be over fired. you could have dirty filters / dirty coils restricting air flow. Your bonnet temperature sensor could be out of calibration. All of these will give you the 4 RED/AMBER and eventually the 11 RED.

    • Jim says:

      I agree you should really get a qualified technitian there. There are so many reasons this could be happening from the flame sensor(minor repair) all the way up to a cracked heat exchanger(big problem). Be safe and call someone who knows what they are doing its not worth your life. Flame sensors can be removed and cleaned gently with light sanding because sometimes a film will build up on them causing the rectified dc to go so low that it shuts down the furnace. Again though it could be other issues. Good luck!

  6. meandher57 says:

    Hoping someone can help me figure out an ongoing issue. To backtrack… for the past decade we have had a mid efficiency gas furnace, drawing indoor air. Over the winter 2 years ago, we started having flame sensor issues and the technician had to come about every 2 months to clean the flame sensor. No cause was ever found. Last year, we had a brand new high efficiency furnace installed by Climate Care. Outside air only. Within two months of operation, the flame sensor needed cleaning. They extended the exhaust vent pvc in case it was flowing back into the intake. Two months later… it had to be cleaned again. This time they added a loop, or trap, to the drain, and cleaned the sensor again. No issues during the summer, of course, or doing the pre-winter checkup. I had them come back after 2 months of operation, and sure enough the flame sensor needs cleaning. (according to the technician, it had gone down from 3.9 whatever… to just over one). Everything has been adjusted… tightened…cleaned… and the only thing the manufacturer has left to suggest is that the burner be replaced – which is on order. The back room in which the furnace lives, has no open containers, no cat litter, no paint thinner, etc. Outdoors, there is nothing in the vicinity of the intake that could corrupt the fresh air. The dryer exhaust is on the opposite side of the house. Hoping someone might have a suggestion that I could ask the technician to look into. The only commonality between the old and new furnace is the gas… which is supplied by Union Gas. Any brilliant thoughts?

    • Since it appears that you have eliminated the possibility of contaminated combustion air, the only thing left is the GAS.. I would suggest that you have your utility check for possible contamination.
      Also, keep in mind that just because you are using outside air now for combustion that the air could contain pollutants even though they do not bother you.

      • meandher57 says:

        Well we just had the gas company over… they say there are NO problems with the gas. Hoping that the new burners being ordered make a difference. I don’t think our backyard air could be any different than our neighbours who apparently have no issues. Duct cleaning is scheduled in a couple of weeks although I understand that can’t possibly affect the flame sensor. It was on the suggestion list from the technician so we will proceed.

      • meandher57 says:

        Well the technician spent time yesterday installing a new burner on our Climate Care (Made by York, I believe) modulating furnace. He also checked the gas line for oil.. checked all the electrical connections… and cleaned the flame sensor yet again. The impression he left me with, was that the retailer would not send a service rep back to check the state of the flame sensor under the terms of the warranty, as the manufacturer feels there is NOTHING wrong with the furnace. Union Gas says there is nothing wrong with the gas. Neighbours with HE efficiency furnaces have no issues so it can’t be the air. The ducts all get cleaned next week, and then we are pretty well on our own, apparently. I can’t be the only one in the world with this issue! lol

      • If the sensor has to be cleaned often there has to be a source of contamination whether that is from inside air, outside air, or the gas.
        If the furnace is a 2 pipe system with all the air for combustion from outside, then it has to be the gas or the burner compartment is inot sealed. Dirty duct work should not have a great impact on the sensor getting dirty.

      • meandher57 says:

        If the sensor has to be cleaned often there has to be a source of contamination whether that is from inside air, outside air, or the gas.
        If the furnace is a 2 pipe system with all the air for combustion from outside, then it has to be the gas or the burner compartment is inot sealed. Dirty duct work should not have a great impact on the sensor getting dirty.

        Well I was hopeful… but the technician ended up replacing the flame sensor this morning. Readings had slid down again after only 3 1/2 weeks. So the manufacturer says ‘no defects’. the gas company says ‘no issues with gas’ (based on their brief inspection). And the outdoor air quality is not impacting HE gas furnaces located beside us, and across from the road. Has anyone ever had an issue with city gas affecting one house but not the one next door? And if so, how do I convince Union Gas to look any further into it?

      • You could have moisture and/or dirt in your meter that is not in your neighbors.
        Are ALL the openings to the furnace sealed so no inside air can get in? Is the burner compartment sealed from the blower compartment? Blower is negative pressure and, if no sealed, can pull air into the burner compartment causing the sensor to corrode.
        These are the most common problems we see

  7. Dane says:

    Hope you can help me.
    I am currently servicing a Southbend Convection Gas Oven. I was wondering if I could ask for your assistance? I changed the flame sensor on the oven because it was producing low microamps even though there was an output of 11-12 volts. I do not have the specifications for this equipment. After changing the flame sensor, there was no improvement of the micro amps. After reading this post, you stated that the flame sensor has 90 -110 volt output. Would different board have different outputs and microamsp? Is 11-12 volt output really low?

    • I am not familiar with your equipment. Different manufacturers use specific controls with different power to the flame sensor. York has used both line and low voltage power to the sensor.
      You need to contact your manufacturer for their specs.
      You should also check to make sure your equipment has a good ground. That will affect the microamp

  8. Dane says:

    Thank you for the response.
    When you mention low voltage power for the york system, is it 24 volts?

  9. Daryl York says:

    This blog has been very helpful. Than you for providing it!

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