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 HVAC Guru Your Own Question
HVAC Guru
HVAC Guru, HVAC TECH
Category: HVAC
Satisfied Customers: 14023
Experience:  15yrs in the HVAC industry with certificates and degrees.
21107370
Type Your HVAC Question Here...
HVAC Guru is online now
A new question is answered every 9 seconds

My furnace control board light is not coming on. Yes, the

Customer Question

My furnace control board light is not coming on.
JA: Did you check the furnace's filters?
Customer: Yes, the furnace filter is new.
JA: Do you plan on doing the work yourself?
Customer: Yes.
JA: Anything else we should know to help you best?
Customer: No, that's all.
Submitted: 1 month ago.
Category: HVAC
Customer: replied 1 month ago.
This is a propane furnace. The furnace fan works. I have an electrical tester and tools to troubleshoot the problem.
Expert:  HVAC Guru replied 1 month ago.

Hello, one minute while I get some information together.

Customer: replied 1 month ago.
ok
Expert:  HVAC Guru replied 1 month ago.

Hello and thanks for using this service! If you are a JA Member I would like to personally thank you and welcome you back! I am HVAC Guru an extremely experienced tech on the site and I try to get to as many customers as I can. NOTE it takes time for me to type research and add links so please wait for me to respond back I can only do this so fast. Secondly, I do not know your skill level so please try and let me know if I am going beyond your knowledge or comfort zone thanks! I strive for a 5 star rating from everyone especially our JA members.

Answer:

What likely happened is the low voltage fuse in the furnace or airhandler blew. There typically is a SLOT STYLE 3-5AMP FUSE on the board itself inside the furnace/airhandler like this.

CLICK HERE

If that is burned out all power to the thermostat will be lost. 95% of all units use some sort of SLOT FUSE or inline BUS STYLE FUSE to protect the low voltage circuit. You need to find that and change it out. If you just bump two wires together you could have just popped it without even knowing it. Now if for some reason your unit does not use a fuse often the board will be damaged due to the short. You would need to confirm you have 24 volts to the board inside the furnace/airhandler with a volt meter to make sure though.

Note if you have a condensate pump check the float and make sure the pump is not full of water...as that could cut low voltage power to the unit as well if the pump is not working and full of water.

All I ask is please use the Star Rating Link at the top of your page before leaving, A 5 STAR RATING ALLOWS UNLIMITED FOLLOWUP QUESTIONS AND A DIRECT LINK BACK TO ME.....tips are greatly appreciated so thank you very much for using this service. If you have any other questions just post back to me here anytime!

Customer: replied 1 month ago.
OK, that sounds reasonable. I did change out the blower motor, and may have tripped the fuse somehow. It's a Honeywell S861OU control board. How do I check the fuse? Do I have to open up the control board housing?
Expert:  HVAC Guru replied 1 month ago.

It should be on the main board you should see it. That S86 control is just the spark ignition control it should be on the other board.

Customer: replied 1 month ago.
Looks like this: http://i.ebayimg.com/00/s/NTAwWDUwMA==/z/ZA0AAOxyVaBSuKV9/$_3.JPG?set_id=2 . Is the fuse there?
Expert:  HVAC Guru replied 1 month ago.

Yes but that is just the ignition controller. You should see another board besides for that one.

Customer: replied 1 month ago.
I don't. It's an old furnace, from around 1988. Am I missing something?
Expert:  HVAC Guru replied 1 month ago.

Can you get to the transformer to check it for power? Also do you see the wiring diagram on it? If so take a picture of it and upload it here please.

Customer: replied 1 month ago.
Wiring diagram: http://i.imgur.com/OGHQt5Z.jpgI see a transformer. How exactly do I check it for power?
Expert:  HVAC Guru replied 1 month ago.

The fan works right?

Customer: replied 1 month ago.
Correct. The blower fan works.
Customer: replied 1 month ago.
OK, so the transformer must be good, I guess.
Expert:  HVAC Guru replied 1 month ago.

OK so you either have a limit switch tripped or the vent switch is tripped.

Customer: replied 1 month ago.
OK, how do I check those?
Expert:  HVAC Guru replied 1 month ago.

The limit switch should be around the burner and have two wires on it. Check it for continuity.

Customer: replied 1 month ago.
OK, I think I see both. The vent switch is small, about the size of a dime, with two wires coming out of it, and a small red button in the middle? And the limit switch is after it, but has a rotating dial with pointers, and says things like "fan limit" and "load limit" on it? Is that right?
Customer: replied 1 month ago.
Still there?
Expert:  HVAC Guru replied 1 month ago.

Yes that is it press the red button in the middle one may be tripped.

Customer: replied 1 month ago.
Alright, I can't press the red button in the middle, so I assume it's already pressed down. That switch has continuity.
Expert:  HVAC Guru replied 1 month ago.

Check the other one then they both have to have continuity

Customer: replied 1 month ago.
Alright I checked the other one, and it has continuity. It looks like this image I found in Google: http://f01.justanswer.com/JACUSTOMERvx5b099e/2014-03-03_025017_img_4606_-_copy.jpg . It has continuity between the topmost and bottommost screw.
Expert:  HVAC Guru replied 1 month ago.

Can you jumper the 24V terminal to the TH-W terminal and see if the unit fires up?

Customer: replied 1 month ago.
I can do that, but I'm not sure what you mean by 24V terminal and TH-W terminal. I'm looking at the wiring diagram now.
Expert:  HVAC Guru replied 1 month ago.
Customer: replied 1 month ago.
OK, I can do that. FYI there is currently a wire connected to the TH-W terminal, but not the 24V terminal. Should there be a wire connected to the 24V terminal already?
Expert:  HVAC Guru replied 1 month ago.

There should be

Customer: replied 1 month ago.
There isn't. Maybe that's it. BTW, jumping it did not work. Where should the other end of that 24V wire be?
Expert:  HVAC Guru replied 1 month ago.

Can you take a picture of the control with wiring and upload it here. There has to be a wire connected to the terminal marked 24V or 24 V ground is there on either?

Customer: replied 1 month ago.
http://i.imgur.com/lwusunB.jpgThere is a wire connected to the 24V(GND), but not to the 24V.
Expert:  HVAC Guru replied 1 month ago.

Put the TH-W wire on the 24 volt terminal also did anyone change any wiring on it?

Customer: replied 1 month ago.
There is no wire labeled TH-W. There are 6 wires, but 7 spade terminals. It's possible that I screwed it up when troubleshooting it previously.
Customer: replied 1 month ago.
Wires are: MV, MV-PV, PV, GND, 25VAC-GND, 25VAC
Customer: replied 1 month ago.
Should there be a TH-W wire? If so, where does the other end of it connect?
Expert:  HVAC Guru replied 1 month ago.

The wire to the right of the 24 Volt terminal should be on the 24 volt terminal it is not working since you have the Thermostat wire on the wrong terminal it has to be on the 24 volt terminal...did you take wires off at any point before all of this?

Customer: replied 1 month ago.
I took the wires off to troubleshoot it. Quite possible that I screwed it up. Hold on, let me fix it.
Customer: replied 1 month ago.
OK, now I believe it's correct:MV wire is on MV terminal.
MV-PV wire is on MV/PV terminal.
PV wire is on PV terminal.
GND wire is on GND terminal.
25VAC-GND wire is on 24V(GND) terminal.
25VAC wire is on 24V terminal.
TH/W terminal has no wire connected.Is that correct?
Expert:  HVAC Guru replied 1 month ago.

yes

Expert:  HVAC Guru replied 1 month ago.

Now try it

Customer: replied 1 month ago.
OK, I turned the heat on and turned the heat temp up, but the thermostat is reading "System: Wait". What do I do now?
Expert:  HVAC Guru replied 1 month ago.

Let it wait for 5-7 minutes and see if it starts

Expert:  HVAC Guru replied 1 month ago.

I am signing off for the night get back to me in the morning.

All I ask is please use the Star Rating Link at the top of your page before leaving, A 5 STAR RATING ALLOWS UNLIMITED FOLLOWUP QUESTIONS AND A DIRECT LINK BACK TO ME.....tips are greatly appreciated so thank you very much for using this service. If you have any other questions just post back to me here anytime!

Customer: replied 1 month ago.
Now the thermostat reads "System: Heat On", but the heat is still not on.Anyway, let's catch up in the morning. Thanks.
Expert:  HVAC Guru replied 1 month ago.

Did it come on?

Customer: replied 1 month ago.
No, and the board light is still not coming on. Maybe the control board is toast?
Expert:  HVAC Guru replied 1 month ago.

Yes sure sounds like it check for 24 volts from the 24 v connections at the board, if you have 24 volts across them the ignition control is bad.

All I ask is please use the Star Rating Link at the top of your page before leaving, A 5 STAR RATING ALLOWS UNLIMITED FOLLOWUP QUESTIONS AND A DIRECT LINK BACK TO ME.....tips are greatly appreciated so thank you very much for using this service. If you have any other questions just post back to me here anytime!

Customer: replied 1 month ago.
I turned on the call for heat, and checked for 24V AC across the 24VAC and 25VAC-GND terminals on the board. It doesn't read any voltage. I don't think 24V power is even reaching the board.
Expert:  HVAC Guru replied 1 month ago.

Then you either have a thermostat issue or a transformer issue.

Customer: replied 1 month ago.
The blower fan is working, so then it can't be a transformer issue, right? What do I test next?
Expert:  HVAC Guru replied 1 month ago.

The you have to check power from the thermostat back out to the unit, a wire may be broken or a limit is bad.

Customer: replied 28 days ago.
OK, I think the thermostat is working correctly, because I have continuity between the R and the W when the thermostat is calling for heat. What do I check next?

Related HVAC Questions