When it comes to furnaces and combustion, it is important that we monitor the presence of flame, even if the furnace is NOT firing. This is the duty of the control board, flame sensor and the wire between the two. Having fuel present without an ignition source can be extremely dangerous in these cases. 

        So you might be thinking " looking for flame when the furnace is NOT running!?!?". YES! Obviously we do not want flame at the burners if the thermostat is not calling for heat. The main control board indeed monitors that, so long as power is applied to the furnace. 

        Now you may be wondering " how in the  (*$#@!)   does all of this voodoo and witchcraft work?"  Well, its actually pretty simple.

       The control board sends  A/C volts (low amperage so risk of shock is minimal) to the flame sensing rod through the wire. The flame rod is located in the ionization part of the flame. When flame is present and makes contact with the rod, the signal is then "rectified" if you will, and produces a micro-amp D/C signal that the control board must see. The flame must also be in contact with ground (usually the burners) for this process to work properly. This is also why a complete and dedicated ground to the furnace cabinet is a MUST! 

        Scenario time. Lets say we show up to a service call and we see that the furnace is locked out on "flame failure fault" or "excessive retries". Whats the procedure here? The first thing we should do is measure the A/C signal on the flame sensor wire (main power on), and we will place one meter lead on the wire and the other on ground. We will typically see 80-100 volts. If present we can safely assume the board is working properly. If not present, check wiring and test the signal at the board, if still no voltage, check for complete ground circuit before replacing the control board.

       If voltage is present, we should now try to give the furnace a call for heat and test the micro-amp signal. Your meter must have the ability to do this. The symbol is usually displayed with a lower case u and upper case A (uA). Again, D/C selection this time. Now what we do is place our meter leads in series with the flame rod and wire (black lead on wire, red lead on flame rod connection). When the furnace fires up and flame is present, we should see a reading displayed on our meter. This reading will vary from brand to brand but is usually anywhere from 1.7 to 6.0 micro amps D/C. Anything lower than 1.0 is a concern and the control board will most likely fault out. 

     If a low reading or fault occurs at this point, it may be necessary to replace or clean the flame rod. If you chose to clean it, use something that is very non-abrasive, steel wool, sos pad, jeans ect. Do not use anything that is coarse grit, this will ruin the rod and cause nuisance faults. 

    On applications where the rod needs to be clean or replaced on regular basis (more than once every two years or so) we must now look into the combustion process and find out why. Advise the homeowner to keep chemicals away from or out of the mechanical room (paint, cleaners, cat liter box, laundry soap ect.) All of these can contaminate the combustion process and cause the flame sensor to become dirty thus, not produce a proper signal. 

     Good luck out there! :)