Dirty Flame Sensor Problem

It really is typical this seasonal time of year to note that your furnace is switching on and off two or three times for very brief runs. Then subsequent to the three attempts, the furnace shuts down for about an hour or two. After this wait period, the furnace attempts to start-up again. So why is my furnace not staying on? Maybe or maybe not your flame sensor may perhaps be the problem. Do I have an unsatisfactory flame sensor? Maybe you have a dirty flame sensor. This is very common for furnaces which may have not had a optimization or regular maintenance in the previous Two to five years.

What exactly ought I do regarding a dirty flame sensor?

Get a service technician clean and check your flame sensor. It might require replacing or only a cleaning, depending upon the problem. Typically either one of these solutions will likely be a relatively cost effective repair. Cleaning and checking the flame sensor should really be a regular element of a yearly maintenance tune up. Call Alexander Heating and Air for a tune up or acquire an annual service contract and feel confident of how your furnace is in great shape.

Warning: most furnace repairs involve high voltage and flammable gas. It is strongly advised you don’t perform this yourself. Have a skilled HVAC technician check and clean your flame sensor.

Just what is a flame Sensor and are they all necessary in my furnace?

The flame sensor is one among the many safety features inside of a gas furnace. There are some being checked right before the gas is allowed to burn and some right after the gas is burning. Those that are checked right after the gas is burning are available to safeguard you and your home from dangerous conditions while your gas is burning. The flame sensor is just one of those safety measures. The flame sensor is constructed out of metals that will create an electrical current if in the presence of a flame. It is usually meant to shut the product down when not enough burning fuel is detected from the last burner in the gang of burners inside the furnace. A constant flame from the last burner would indicate a reliable burning environment and also that the whole set of burners are getting approximately the very same amount of fuel and air. If no flame is there, then any one of the burners inside the group could possibly be burning fuel in an unsafe way. Therefore the system checks for the flame and closes the fuel valve when no flame is detected. Any system can have a false reading, so the control board usually tries to light the burners two more times. If these three times the system delivers the same message that the flame sensor isn’t detecting a flame, the whole furnace is shut down via the control board either for a couple of hours or completely until reset by a service technician. This is called a lockout condition.

What triggers flame sensors to malfunction?

You can find two possible challenges with flame sensors. A dirty flame sensor could be the most likely problem. At this point it is essential to understand that burning fuels do produce trace amounts of water when burning. And whenever water contacts most metals, the metals usually rust. This is also true with flame sensors. The metal end of a flame sensor can rust after a while. The rust accumulates and prevents the flame sensor from producing the current that tells the control board which it detects the flame. The other possible concern is that the flame sensor is now bad. This is usually a cracked insulator or perhaps short in the electrical components. Replacement is suggested for a bad flame sensor.

If you think you may have a problem with your furnace,

call The HVAC Guy at Alexander Heating and Air Conditioning.

For Quick,  Honest and Affordable  heating system services.

1-919-886-HVAC (4822)

40 thoughts on “Dirty Flame Sensor Problem”

  1. I have an commercial carrier HVAC roof top unit, since last year, it’s been acting weird, the heat will blow when you first turn it on and will stop when termpeture is meet, but for those cold day when customer keeping coming in the room termputure will drop, then the unit will keep working for hour or two then It will start to blow cold air only, and will only stop when I turn off the break, cut off the power, can’t turn if off from the thermostat, then after while it will work again when turn it back on. could it be the flaming sensor? or heat exchange?

    1. Being a commercial unit, there are many more control items than a simple residential unit. I cannot diagnose from the internet. Someone would need to be on site and test different components to eliminate what is working to narrow down possible root cause.

  2. 3 years ago we experienced an issue where the small fan on the furnace would stay on after heat and not shut off. The furnace, in turn, would not light the burners in a call to heat. My HVAC guy replaced the board (it was definitely bad). That fixed the issue. This year we experienced an issue (3 times now) where the house is heated, the flames turn off, but the blower fan (that feeds the vents in the house) stays on for an hour or so (preventing the burners from being lit)…it then turns off for 5 secs. Since only the fan was running for the hour, the internal house temp. dropped, and after the 5 secs. a call to heat is done and the burners light. The only way to get it out of that “on fan cycle” is to turn off the power to the furnace and then turn it back on. But I then have to monitor the furnace very hour or so to ensure it turns the heat back on.

    Any ideas? Dirty/defective flame sensor?

    1. Two possibilities. 1) Poor air flow such as dirty filters and the heat exchanger is over heating or 2) Cracked heat exchanger and the roll out switch is tripping. Check the status code on the circuit board to see which safety switch is preventing the unit from running again.

      1. The roll-out switch has a manual reset. “Mark” didn’t say anything about pushing a manual reset, but he did suggest that the flame rod may be bad. So, the question is: is a symptom of a bad or weak flame rod that the blower fan stays on for an hour after the flame goes out?

      2. Yes it could be. The safety program on the circuit board does in some cases run the fan indefinitely until the problem is resolved. Typically the board will indicate some sort of LOCK OUT state as well.

  3. I appreciate all your info you put online. Maybe you may have an idea on this, problem occurs while either in heat or cool. Both blowers cycle on off randomly. I have tried putting the thermostat in a non-call temperature and placing the system switch on thermostat to off. Still happens. I took the thermostat off wall, still happens. Only way to shut it off is the switch on furnace. It is a heil throwing 5 blinks = flame sensed out of sequence. Even while I am using it in AC. Looking at schematic I feel it is the board. Any assistance is appreciated.

  4. My landlord has had problems with our furnace for 2 years finally got a new one and still the same problem. Im guessing its not the furance its self its maybe the vents or something. im a girl so what do i know. but anyway. i hear it go on, first the pilot then the burner then the blower. well the blower goes out as soon as the blower goes on. the the burner lights up again and goes off. i keep hearing aclicking sound too. landlord said its our cats hair. is the cat the problem?

  5. My furnace started acting up and the logic board had an alarm of 3 fast blinks continuously. From the code it meant the pressure limit switch was open and wouldn’t close. I recently changed the limit switch out and it worked fine for a day and went out again. I checked the vent, it was fine. The blower had insulation that fell on top of it and restricted air flow so I removed it and now it works fine. The next day it quits again. Could it be a bad flame sensor even though the furnace kicks on, but the ignitor never comes on?

    1. It doesn’t sound like a flame sensor. It sounds like your inducer is failing. either the blades have rusted out or the motor has gone bad. It probably will require replacing the assembly.

  6. I clean my flame sensor and runs fine for about 24 hours then stops, clean sensor again and starts right up, any help would be greatly appreciated!

    1. Sometimes you just have to replace the flame sensor. But it could be the burners are too dirty and are preventing the flame from transitioning all the way to the third burner . Another thought is there is a crack in the heat exchanger and the blower is blowing the burner backward causing it to miss the flame sensor.

  7. Can a flame sensor be cut to length, and if so how? I am being told the the OEM replacement part for my Furnace is the manufacturers listed replacement part, but it is much longer than the sensor in the furnace now.

    1. You may damage the sensor if you cut it. Several sensors can be universal mounting positions. However, I have never run into a OEM replacement part that doesn’t fit. You may have gotten the wrong part. Selecting the right part can sometimes be confusing with the variations of model numbers.

  8. My heating system works ok for the first time. Once it reaches the right temperature it shuts down, good. However, when the house temp drops it doesn’t fire back on. I tried turning it off then back on and the machine will start but shut off after 30 seconds. Could it be the sensor? I have a coleman furnace

    Also my electricity bill is higher then my gas bill for whatever reason but i guess my system does use electricity

    1. These symptoms are common for most every problem a furnace has. Can you be more specific? What code is the circuit board showing?

  9. My Trane furnace would cycle on and off three times like you were saying but gave me a blink code of a poor draft. I tested the vacuum switch, fan, and flue but had no way to check the heat exchanger. I figured it was cracked but a friend told me to check the flame sensor. I sandpapered it and now it’s fired up every time I’ve tested it. But the furnace also has a code for low or no flame sensed. Why would it give me a poor draft code instead? Thanks.

  10. I’d like to explain my problem a little bit, hopefully this is a simple fix.
    My furnace had been working fine until one morning I woke up with the sound of the furnace unit running, but the house was freezing. Two different repair people came over to look at it. One said that the sensor was the problem, the other said it was just an old thermostat. Both times, they switched off the furnace, then switched it back on and it worked fine.

    The first time it was switched off and on, it worked perfectly for two weeks before the problem started again. The next time it was switched off and on, it was less than a week before it stopped working again.

    Essentially, what I have observed is that when the heat is first switched on, the first thing that happens is that the unit starts running making a humming sound. About 15 seconds after the humming starts, you can hear the sound of the fan kick in in and start blowing the air through. This is normal. What’s not normal is that at some point the fan stops blowing the heat in, but the humming sound of the unit continues. When the unit is working, the sound of the fan and the humming of the power to the unit both stop at the same moment when the temperature that I set on the thermostat is reached.

    However, since this malfunction has started, the fan will randomly stop… sometimes it stops only 5 minutes after the unit is turned on, and other times it stops at the correct time (when it reaches the right temp), but in all cases, when the warm air stops blowing through, the power to the unit is still on and humming. I’ve gotten to the point where I have to actively listen for the unit to stop and go switch it off by hand. It’s a real problem at night when we fall asleep with the furnace on, only to wake up with the temp being, say, 50 degrees in the house when the thermostat is set to 72, and I can hear that the power to the unit is on but the fan is not on and no heat is coming into the house.

    The month before I figured out how to shut off the furnace by hand this problem ran my electric/gas bill up to $500, when it’s normally only $100 in the winter.

    Does this sound like the same kind of problem that you described in your article? I need to be able to describe the problem to a local technician in words they can understand. Often then come here and turn on the furnace, and it seems to work fine while they’re here, only to shut off later in the day, way after they’ve gone. Neither time that they came before, could they say they were sure about what the problem was, and this was after seeing the unit with their own eyes.

  11. I have an 8 year old Lennox G43UF-60D-135-06 doing a very similar dirty flame sensor every other month. Brand new furnace. Was fine for first 7 years. The diagnostic code on the control circuit is always the same – Slow RED flashing LED mated with Fast GREEN Flashing LED (low flame sense detected). It started about a year ago, cleaned flame sensor – was fine for 3 months. Then became worse. (about once a month the error code would return). As of 3/30/15, the flame sensor has been cleaned three and replaced 4 times. Also replaced: Circuit board ($400) and gas control module ($600). 3 different repair business have looked at my furnace – yet the problem remains unresolved. About every other month, the flame sensor must be cleaned. (which isn’t an easy task on this model, given its mounting location within the furnace) – flames within the chamber appear normal, blue. air intake has strong suction. no blocked exhaust. filter is replaced on normal intervals. Michigan house.
    Thoughts, comments – much appreciated. thanks!

    1. Since the most reasonable parts have been replaced, I’m thinking now something a little less common may be going on. Dirty fuel? Are you using Propane? Maybe the byproducts of combustion are excessive. Another possible is the combustion air is insufficient. This can cause excessive and dangerous byproducts of combustion. The biggest concern is carbon monoxide. But there can also be excessive carbon on the flame sensor due to poor combustion. There should be 50 cubic feet of room air for the combustion per 1000 btu. So a 135,000 btu furnace such as yours needs to be located with either proper outside air ducts or 6750 cubic feet of room air. That is about the equivalent of a 30×30 room with 8ft ceilings if vented from the installed room. If it is vented through an outdoor 4in PVC pipe.
      Here is a link to the installation documentation. LINK
      That’s where I would start my investigation next.

      1. Thanks much for the information. It is GAS, not Propane. New sub (8 years ago). All homes have same gas, similar heating systems. None of my close four neighbors have experienced this issue. Don’t know about the others. House size I believe is 2800 sqft. Open basement, with 9.5′ ceiling. not finished. Furnace is in center of basement. Vented at the furnace. No outside pipe. (The last tech out said they would have installed outside venting) I would guess the basement is at least 30×30. I did remove the vent pipe – assuming it is the air intake into the burner chamber? it isn’t blocked. looked clean. although much rust is on the burners themselves. The last tech (Lennox cert tech) dismantled and removed the burner assembly, then reassembled. there is ample space around the furnace – no boxes, shelving or any items within 10-15 feet of the furnace. I do have a carbon monoxide detector – has never tripped. Again, was fine for 7 years – never even considered cleaning the flame sensor – now It’s almost monthly. Thanks again for your time.

      2. It would be difficult to diagnose without my teat equipment given those events. Try finding a reputable company with Nate certified technicians to help.

  12. My rheem furnace malfunctions about every 2 weeks . it will all of the sudden blow only cold air . I will manually switch off the furnace and switch it back on and this is what it does. It does everything normal up to the igniter working no flame so it pauses a couple of seconds and igniter turns on but again no flame but this time the blower turns on and throws cold air , I’ve noticed that after turning off the power and turning it on after an hour or two then it works normally , do you have any advice on how yo fix the problem

  13. Hate to ask but extremely frustrated, I’m a renter and my landlords thankfully have a protection plan on my furnace however; at LEAST once a year parts need replaced which I’ve been told are now obsolete, today alone I’ve had tech here 2 and now will need to wait until am due to furnace keeps trying to turn on but sometimes either just instantly shuts off or is running but no air (warm or cold ) is happening, sometimes it literally clicks every 1/2 second. Heard them say could be getting dirty gas? I don’t know what that exactly means but with a child I don’t like the sound of anything dirty trying to push through our furnace, they cleaned sensor but within 6 hours I’m back to same clicking issue and no heat, what does dirty gas mean and/or is there something I can/should do? Please help

    1. Hate to say but just not enough information to give you a decent answer. It could be any one of several things as the symptoms are all the same. But dirty gas? That sounds very far fetched. There are traps and filters that are put in the gas pipes to prevent that. As far as safety concerns about dirty gas, nothing to worry. It will only affect the valves and cause them to malfunction. Not a safety issue. More than likely it is an internal issue and I say this because you said the furnace is obsolete and parts unavailable. That would only tell me that the furnace is over 20 years old. In which case there are numerous items in your furnace that can cause the same symptoms. replacement is the only real solution.

  14. My flame sensor has been cleaned 4 times & I want the problem resolved.
    My repairman is saying that it might need a new circuit board (about $500).
    Is this likely or would just replacing the sensor fix the problem?
    Any help will be appreciated.

    1. Has he checked the micro amps DC on the flame sensor? If it is pulling 4-5 mA then the flame sensor is not the problem. It could be something else. The circuit board is expensive. But I would need to know more, like what is the error code the board is detecting?

      1. I am having a heating problem myself, I have done some research trying to trouble shoot this myself. I pulled my flame sensor out to find the ceramic was broken and the rod would spin freely in the ceramic collar. I replaced it and still the same result. I did test the Micro amps and would read 1.8-3 micro amps for a second or 2 then it jumps to 5.0-5.5 and shuts off. It then repeats 3 more times and then the lock out occurs. I have also check continuity of many wires ONE at a time and putting them back where I found them ONE at a time. Everything is checking out. I am concerned that it is the circuit board since everything else seems to be working.
        In case you want to know the exhaust fan for the gas spins up every time it prepairs to light and continues till the furnace does its lock out. The cold air works, the main squirrel fan for the furnace vibrates as if it is out of balance (I feel I need to pull it out and clean it). I have a clean filter that I replace on the first of every month.

        I have a Carrier furnace 58pav070-12 it is about 16 years old, the circuit board is 1012-940-j.
        When I did apartment maintenance this would be the time I call a service tech out, I do not have it in my budget yet to call a service tech and it is getting colder. If you could point what to look at next I am all ears.

      2. The circuit board should be blinking a specific sequence of blinks which will identify the issue the board is reading. Count the blinks then check it against the chart on the inside door to the furnace. This will give you direction on where the issue may be. If the sensor is correct, then the board or wiring may need to be replaced.

      3. This is happening with my Rheem 90 furnace. It’s reading code 11. replaced flame sensor and still reading code 11. the furnace tec. said the only other thing it could be is the circuit board. My question is, why wouldn’t read code 68 (which is the code for a bad circuit board). When he checked the amps of the new flame it only read 2.2 for just a second and then went to o.

      4. Actually it could be several things. The flame sensor should read over 3.0 Mill-Amps to be properly operating. It will fluctuate with the flame. it could also be dirty or rsty burners not allowing the flame to properly migrate to the sensor. The circuit board code will not always be indicating a bad board for All Board failures. An absent code isnt proof that the board is good, but an obvious code is definite proof the board is bad.

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