How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Phil Your Own Question
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
Type Your HVAC Question Here...
Phil is online now
A new question is answered every 9 seconds

My dependable ninety two gas furnace will not start just

Customer Question

My dependable ninety two gas furnace will not start just Changed board power indicator light is on but is not flashing any codes
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.

Hello, its very difficult to verify the safety switches with a volt meter on these AC electronic systems because of feed backs (that is not a problem on DC circuits like in a car but is with these systems)

Please attach a well focused, close up photo of the wiring diagram glued inside the service panel, you can use the paper clip icon at the right end of the toolbar at the top of this dialog box, Include a picture of the thermostat wire connections to the printed circuit board in the same area.

If there is no paper clip icon, look for the '+attach files' link at left of your send button.

I can mark it up with trouble shooting instructions. You will need a multi meter ($30 at a hardware store) and some experience working with appliances, let me know a lot about your skill sets in those regards.

Customer: replied 1 year ago.
I hope these pictures are helpful. Please know that this is the newly installed board, and since it was installed the furnace is doing entirely the same thing.
Expert:  Phil replied 1 year ago.

Try sending the pictures again, none arrived, use the paper ciip icon at the right end of the dialog box tool bar, or the '+attach files' button to the left of your send button do do that.

Or you can post them to and post the URL for the pictures on flickr here.

We can go from there.

Customer: replied 1 year ago.
It shows me that they were attached to the reply. Oh well... here they are again.
Expert:  Phil replied 1 year ago.

Our server has been known to be a bit buggy at times. I got 3 photo's this time. However they were all of the printed circuit board.

The wiring diagram is an 8 x 11 sheet of paper plus or minus a few inches, printed with the wiring diagram, and glued to the inside of the blower service door.

That photo must be very carefully taken with very good light, from straight on, not at an angle, and exceedingly well focused. That is the one I mark up with trouble shooting instructions...

I see the area is poor lit, you may want to take the panel with the diagram on it, into the house where the light is good and take a close up of it, there... again the photo must be taken from straight on, any angle will blur it out so that I cannot read the fine print on it.

Customer: replied 1 year ago.
boy, this diagram is pretty rough. I hope it is what you are looking for and it helps. Thank you!
Expert:  Phil replied 1 year ago.

Hello again, that type of igniter has on average a 2 to 5 year life span, it it has not been replaced in a long time there is a good chance its bad... during the ignition phase, check for voltage to the igniter, and if you have voltage there, usually 120 volts, but sometimes those are 24 volt igntiers. and the igniter is not glowing, it is a bad igniter... remove it, and if you see a black line across the face of the heating element it is bad for sure.

Beyond that, the way to text limit switches, is to bypass them with a jumper wire *briefly, for a test only... attempting to do it with a volt meter leaves you open to getting ghost reads.

Let me know how this goes.

Customer: replied 1 year ago.
We Ohm'd out the igniter. I am told that means it is good? (replaced last year)Tested all the limiters.
Expert:  Phil replied 1 year ago.

Thanks, I still need you to check voltage to the igniter... there is a mosaic of interactions here that I am considering... one test tells me a lot about a range of problems in the system.... ohm tests are also good, in my 51 years with the tools however, about 90% of my ohm reads were bogus, I had to take them repeatedly to insure the test probes were making good contact in order to get good reads.

Expert:  Phil replied 1 year ago.

Did you test the limits with a volt or ohm meter or by jumpering them... Jumpers are all that work 100% reliably, due to electronic feed back in the AC circuits.

Customer: replied 1 year ago.
it was ohm'd with the audio, not the numbers and everything was disconnectedI tested the limits with both voltage and Ohm. It had low voltage.when i disconnected one side of the limit switch, the blower motor came on, but NOT the ventor moter.If the ignitor isn't working, why wouldn't the venter motor come on? last time I changed the igniter, it came on first.venter motor is not burned up. Jumpered it... started right up.(Please bear with me, I am trying to talk to an older gentleman on the phone, decipher what he is saying and relate it back to you.)
Expert:  Phil replied 1 year ago.

The ventor motor is required to come on *before* the ignition will be powered... you have that sequence of operations backwards in your mind. That is the root of some of your confusions.

The other aspect is that you are relying on meters (sound assisted or not) to provide accurate readings, when in fact they very often provide false reading.

Operating on the basis is going to lead you off into the tall weeds. I don't know how better to say it.

I will opt out now so that other of our experts here can take a look and decide if they can help you on this range of issues and problem with your furnace. Good luck with all.

Expert:  Kcrand36 replied 1 year ago.

Good Afternoon, I am Kevin, one of the techs here.......

I read over what you and Phil did. Can I ask what symptoms you had before you changed the board?

Customer: replied 1 year ago.
blower motors will not come on, will not ignite... swapped out thermostat, no help. Swapped board out, no help, tests tests tests, motors are good. Running out of options.
Expert:  Kcrand36 replied 1 year ago.

So if I understand you correctly, you had exactly the same symptoms before you changed the board?

Customer: replied 1 year ago.
Expert:  Kcrand36 replied 1 year ago.

have you tried at the board, jumping R and W?