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flame sensor

drhvacdrhvac Member Posts: 189
I have a low micro-amp signal (.3) which is causing this 10 year old rheem 80% furnace to lockout. I cleaned and then replaced the sensor but still the same reading. I thought it might be a grounding problem so since the panel was so close I ran a new wire diectly to the panel and still same problem. This is a weird one. All the years in the business I still don't understand 100% how the sensor works. I didn't check the AC voltage to the sensor, I know the board is suppose to do this, and I remembered that on my way back to the shop. What should the voltage be? I suspecting the board may be the problem, but before I do that i'm going to get a clamp to run a ground wire to the cold water line, and maybe bypass the sensor and ground wires around the molex plug that goes from the burner cabinet to the blower compartment. Any other ideas, I haven't had a good one like this in quite awhile. Thank you


  • Tim McElwainTim McElwain Member Posts: 4,342
    What is the Make and number of the control board? If this is Direct Spark or HSI is the sensor 1" to 3/4" into the flame and about 1/4" to 1/2" above the burner? Is this direct spark, HSI or a spark ignited pilot?

    Do you have three wires running from the circuit breaker panel to the furnace Black Hot, White neutral and bare copper wire ground? If so the gas valve is a great ground for a low voltage system. Try running a wire to the gas valve to restablish a good ground.

    The fact that you have very low microamps could as you say be the board. Most boards create about 170 to 200 volts AC out of the board to pass through the flame for rectification.

  • drhvacdrhvac Member Posts: 189
    I did not get the make of the board, but no there's only 2 wires to the unit, and they are using the casing of the bx as the ground. There is a junction box somewhere buried because out of the panel is romex. That's why I ran a separate wire. There is a ground wire attached to the gas valve already. I should check voltage from the sensor and the other probe to ground, correct. Anything lower than 170 ac could be the board? At what point would you condemn the board. Ty
  • drhvacdrhvac Member Posts: 189
    This is spark ignition.
  • Tim McElwainTim McElwain Member Posts: 4,342
    Anything less than 170 volts AC is a problem. It would help if you had the board number, that is always key to getting all the correct info as I have knowledge of about 95% of the boards that exist and are used on gas systems.
  • drhvacdrhvac Member Posts: 189
    Ok, I could get the board info tomorrow. I did read 97 volts ac with no call for heat, and about 92 volts with the burner on. This was with one probe on the sensor where wire is attached, and the other probe to ground. I also ran a ground wire from had valve to cold water line, and it still didn't work.
  • drhvacdrhvac Member Posts: 189
    The board part number is 62-24140-02. Where do you find the specs for all the different furnace boards? Ty
  • unclejohnunclejohn Member Posts: 1,494
    Seems to me if you have a clean and or new flame sensor and still reading low, then there is nothing left but the board. If your reading the signal at the flame sensor it's self and not at the board. If reading at the board may you may have a wiring harness issue.
  • drhvacdrhvac Member Posts: 189
    I'm just trying to rule everything out because boards are expensive. I didn't check signal at board because it is difficult with the Molex plug, but I did check continuity from both the flame sensor wire and ground wire on both sides of the plug, and it was good.
  • Tim McElwainTim McElwain Member Posts: 4,342
    That board numberyou gave is a Rheem part number there is a manufacturers make and number which is what I need to help you.
  • SherlockOhmsSherlockOhms Member Posts: 13
    Flame rectification is the phenomenon of a flames ability to rectify the spark ignition's AC polarity as it passes through the flame. The reason cleaning the flame sensor helps quite often is that the DC signal that is "sensed" is very weak and as a result is easily impeded or hindered by a normal patina on any electrical connector or path to the Ground terminal on the ignition module which in the ignition modules case is also the SENSE or Flame sensor terminal, they are electrically common if you take them apart. You mentioned you ran a new ground, does this mean from the pilot assembly to the ignition module?
    I use 105 C rated wire to make this clear path home for the D signal in these cases, I burnish the mating surfaces for the pilot assembly to the mounting surfaces. Johnson controls makes a flame simulator that works on any flame rectification system to "prove" a flame exists by furnishing a steady DC signal to the ingnition module, the flame simulator has a spade connector and an alligator clip to jumper the ground to sense at the ignition module. I never worry about the signals strength just assuring it has a clean easy path home,and if it locks out this way then change out the bad stuff. Very often controls that were replaced test normal and it is the reseating of the connections that has restored function.
  • icesailoricesailor Member Posts: 7,265
    Is that why someone once told me that most of the returned boards that were supposedly bad, weren't?

    Maybe that thing about old computers is still true. If you can't find anything wrong, just pull out all the connections, re-connect them, and re-start it. If it starts working then, you had a bad connection.

    If you change the flame sensor and the board at the same time and it them works, which was bad?
  • Don_197Don_197 Member Posts: 184
    If that furnace ran all that time without a true ground, it probably took the triac out.................grounding is critical on newer furnaces................biggest symptom? Poor or no flame signal. Also, they are sensitive to polarity, and many even have a fault code for reversed polarity. We went on one last week where the homeowner had a generator installed this summer, and the first time they tried to run the furnace, no go. They had electrician come back and sure enough, neutral and hot were reversed....he put them back and badabing....furnaace is working great.
  • drhvacdrhvac Member Posts: 189
    I couldn't find a grounding rod in this house. In the panel all the grounds are screwed to the side of the panel. It looks like the electric is coming in from the ground with an emt type pipe, would this be considered ground? I would think that pipe would corrode of the years and maybe is what's causing this problem? Could I get a grounding rod just for the furnace? Is that crazy? I'm thinking if that works, then I would tell him to get a electrician to ground the panel properly. Going back today, wish me luck.
  • icesailoricesailor Member Posts: 7,265
    I'm not an electrician. But the houses I have seen where I worked, all had ground rods driven in front of the panel and connected to the panel and ground rod. Sparky and their masters can't seem to come to a consensus on the best way to do it. They used to depend on all our metal pipes in the ground to solve their problems. Because of CHEAP and all that plastic, non-conductive material in the ground, they now have a problem.
  • SWEISWEI Member Posts: 7,356
    Is this a stand-alone building with its own meter, or a subpanel off of the main building? Either way, it needs a grounding electrode (like a rod) but the wiring is slightly different for the two cases.
  • drhvacdrhvac Member Posts: 189
    This is a standard house.
  • drhvacdrhvac Member Posts: 189
    This is a standard house.
  • drhvacdrhvac Member Posts: 189
    The problem was the board. Boards are usually pretty straight forward in troubleshooting, power in, no power out, it's bad. But in this case, the only thing the board does is send an ac voltage to the sensor, which it was, about 95 volts. Nobody could tell me what the correct voltage should be, not even the Rheem technical rep., but apparently the voltage was correct because when I installed the new board, the voltage was right around 90 volts and the micri amp shot up to 3.5. So in this rare case, I guess it's just a matter of ruling everything else out, until there's nothing else left but the board. Thanks for your help.
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