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airheatman
airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 7708
Experience:  Furnace, A/C and heat pump specialist.
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1998 Lennox, model G20Q3/4E-100-1, pilot comes on, blower

Customer Question

Hi, 1998 Lennox, model G20Q3/4E-100-1, pilot comes on, blower runs, but burner doesn't light, so no heat. Where to start?I have the same furnace with the same problem. Per your response to this question, I cleaned the probe. It made no difference. The probe does glow red hot. Does it matter what part of the flame the probe is in? I acquired the furnace used and installed it myself to replace an old gas converted oil burner. I have the Lennox User’s Information, Unit Information, and Installation Instructions manuals.I measured the flame signal going to the Robertshaw Ignition Controller. It measured in the 1 – 5 microamp range but the signal rapidly jumps around. Measured as voltage it jumped from zero to 230 microvolts. I checked the connections. Could it be a cold solder joint in the controller?The controller never brings up the MV signal. Jumping the valves MV line to the controller’s pilot signal will bring the main burner on and the furnace completes its heat on cycle.Any suggestions?Herbert ***************
Submitted: 22 days ago.
Category: HVAC
Expert:  HVAC Guru replied 22 days ago.

Hello, one minute while I get some information together.

Expert:  HVAC Guru replied 22 days 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:

Replace the controller it is bad.

All I ask is please use the Star Rating Link at the top of your page before leaving, that is the only way I am credited through the site, tips are appreciated.

Customer: replied 21 days ago.
At the moment I rate the service as poor. I was pretty sure it was the controller but to me the jumping current from the coulr come from a bad connection. If the response said that is normal for the probe's output to jump around I might be more accepting to drop $200 to $400 for a new controller. I stated that funds were tight and reassuring me that one of the $100 generic units were fine. Since you are not selling anything a suggestion of the best place to buy would help up my rating.
I said that I was capable of component level repair. A wiring diagram and reassurance that some have done it would also up the rating.
Questions to answer before a rate you.
1. Does the current from the probe jump around?
I have line on a SP735P for $40. I it a direct replacement for my SP735L?
3. do you know of someone who has fixed one by touching up the cold soldering joints?
4. If I have to go for a new one what and where is the best buy and should I get a kit with a new probe?
Lastly since I signed up for a 30 day trial, do you know anything about a Honeywell F300 Electronic Air Cleaner
Customer: replied 17 days ago.
It is not clear to me that the remarks below reached you. so I am entering it in this box and resending it.11 November 2016 03:43
At the moment I rate the service as poor. I was pretty sure it was the controller but to me the jumping current from the coulr come from a bad connection. If the response said that is normal for the probe's output to jump around I might be more accepting to drop $200 to $400 for a new controller. I stated that funds were tight and reassuring me that one of the $100 generic units were fine. Since you are not selling anything a suggestion of the best place to buy would help up my rating.
I said that I was capable of component level repair. A wiring diagram and reassurance that some have done it would also up the rating.
Questions to answer before a rate you.
1. Does the current from the probe jump around?
I have line on a SP735P for $40. I it a direct replacement for my SP735L?
3. do you know of someone who has fixed one by touching up the cold soldering joints?
4. If I have to go for a new one what and where is the best buy and should I get a kit with a new probe?
Lastly since I signed up for a 30 day trial, do you know anything about a Honeywell F300 Electronic Air Cleaner
Expert:  airheatman replied 17 days ago.

I am a different expert.Lets' back up and start fresh. Do the burners ever actually light?

Customer: replied 17 days ago.
If I jump the MV to PV after pilot lights main burner lights and furnace works
Expert:  airheatman replied 17 days ago.

OK , let me ask this .When thermostat calls for heat, inducer comes on , then ignitor.While igniter is on check and see if you get 24v to valve.

Customer: replied 17 days ago.
The SP715l never supplies 24v to main valve. The probe supplies the 1-5 uamp to control but current jumps up and down rapidly. Is this or normal?
Expert:  airheatman replied 17 days ago.

What probe.Are you referring to the lame sensor on pilot?

Customer: replied 17 days ago.
Expert:  airheatman replied 17 days ago.

I see so pilot is lighting, just not the burners?

Customer: replied 17 days ago.
Expert:  airheatman replied 17 days ago.

The 1-5 is normal as it will fluxuate.Looks like it will be the board as everything else in circuit has done as it should, board is just not getting signal.It is common for these boards to develop stress cracks.Not very many people attempt trying to trace and repair the solder joints.Best bet is to get number off of board and lets Google it to find best price on a replacement.

Customer: replied 17 days ago.
I have. I have a line on one for $40. Thanks.
What I didnt get from first tech was that probe output was normal.
Expert:  airheatman replied 17 days ago.

I understand. I never like to just throw out an answer telling someone to replace a part, especially an expensive part without doing some troubleshooting and verifying a few things.

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