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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 21936
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I have Lennox Diplomat Furnace, Model 80MGF2-45A-1. The Furnace

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I have Lennox Diplomat Furnace, Model 80MGF2-45A-1. The Furnace will run, just no heat. Light on board blinked 3 times rapid, then I replaced thermostat and light blinks normally, but still no heat from unit. No flame from burners. I followed instruction manual for relighting burners, no success,... need assistance.

ComfortZone :

Does this furnace have a standing pilot that you must manually light?

JACUSTOMER-82x2vjvj- : No
JACUSTOMER-82x2vjvj- : Auto-light not manual
ComfortZone :

Does the ignitor glow bright orange?

JACUSTOMER-82x2vjvj- : I'm confused. Do you not have the service manual for this unit. The ignitor isnot
JACUSTOMER-82x2vjvj- : The ignitor is not visible
I do not so I will opt out.
Customer: replied 5 years ago.
OK, great. Since that is the case, I take it that this matter was forward to another service expert?
Customer: replied 5 years ago.
Relist: Answer quality.
...need another "expert" to reply to original questions....ASAP





This is a pressure switch issue.


First here is what I assume is your model. Please verify



Customer: replied 5 years ago.
Thanks for the fast response. I looked at the repair manual that you attached and this appears to be the exact furnace that I have. I just have 3 burners though.


Thank you,


Are you familiar with the pressure switch that must prove air flow?



Customer: replied 5 years ago.
I am familiar with the location of the pressure swith location on the furnace. As a matter of fact, I check the hose leading from this switch to the fan, blockage detected.


Do you mind a hands on test?



Customer: replied 5 years ago.
Certainly. How shall I perform this test?


Turn furnace off

Locate and remove the 2 wires connected to pressure switch and hold apart.

Turn power back on with thermostat turned to heat and up.

AFTER the exhaust fan starts, touch the 2 wires together so they make metal to metal contact. Some find a paper clip with the ends stuck into the bottom of the wire connectors helpful. If you do this do not let metal touch the cabinet. It is only 24v but could blow a fuse.


I need to know if the unit progresses after the pressure switch wires are touched together.




Customer: replied 5 years ago.
I follow the steps and placed the 2 wires together for >60 secs, no response, burner did not ignite.


Thanks for your help....


Now there are a couple different circuit boards used in this unit. How many diagnostic lights are on your board?



Customer: replied 5 years ago.
1 diagnostic light, RED, labeled "LED1"




and are you positive the code is 3? When the furnace was shut down and then restarted...did the diagnostic start solid or flashing....and then lock out with 3?



Customer: replied 5 years ago.

As I recall, the LED blinked 3 times rapidly, then I replaced the thermostat and now the LED blinks normally but slow. The light on the other furnace blinks normally, but fast. It runs normally, also.

Customer: replied 5 years ago.
When the furnace was shut down and restarted, the diagnotics LED started in flashing mode; no solid LED noted. I can recheck.
Customer: replied 5 years ago.
Just verified. Turned the furnace off for 60 sec, then back on. There is a steady, slow LED with no interruptions. The two wire for the pressure switch remain disconnected, ....did not reconnect.


Thanks...was the upper exhaust fan running during this time?



Customer: replied 5 years ago.
The upper exhaust fan (blower) was not running. The blower deeper inside the cabinet seems to work. The upper blower does not respond at all.


Take doors off furnace. Tape safety door switch down at top of bottom door. Locate where the thermostat wires attach to the circuit board. You should see R, Y, W, G, C.


Tell me if you have identified them and we will do another quick test. We are close to an answer now...



Customer: replied 5 years ago.

I noticed that this unit does not have a safety door switch. I looked for this on today and never located this door switch. Does every unit absolutely have a safety door switch? I may have overlooked it.


I see the R, Y, W, G, C on the circuit.


Before testing, should I reconnect the wires leading to the pressure switch?

Customer: replied 5 years ago.
There is a single door for the top and bottom, not two doors



page 7 figure 1 shows where a normally placed door switch would be.


Now..yes put wires back on pressure switch.


Turn power at this point we have the circuit board exposed...power is on.


Now use a short wire, jumper etc...and jump R to W. One end of wire on R while other touching W...and hold in place....tell me if exhaust fan starts...and if it does if the unit tries to run..




Customer: replied 5 years ago.

Definitely no door switch present.


I jumped R and W with paperclip. The blower that lies encased behind the circuit board powers on, and the LED light flashes a steady, fast, "normal", RED. The blower at the top, above the pressure switch, does not come on at all. It has not ran during any of these procedures.


Note: In the past, it seemed that the when I turned on furnace from the thermostat, the blower behind the circuit board would come on but the LED would blink slow and steady, not fast. I may be incorrect.


Ok here is the issue at hand.


The reason you got the initial code 3 is because the exhaust fan did not start. Since it did not start the air switch could not be closed.


Now we are down to 2 choices....


1) the exhaust motor gets power but doesnt run

2) the exhaust fan does not get power which means circuit board is bad.


Now the exhaust motor could have power but not run due to obstruction in exhaust housing.


Try the R to W again...and this time spin the black fan wheel on the rear of the exhaust motor. This is a cooling fan for the exhaust motor. See if when you spin it if it tries to start seized...if it hums... any sign of life. After this we would need a volt meter to see which is not working.



Customer: replied 5 years ago.

Just so that I am on the same page, you are referring to the exhaust motor that is displayed on page 19, figure 21, correct?

Good could also be page 7 figure 2....It appears the unit could have either.




I am very sorry but I have to go offline. I will respond first thing tomorrow morning.


I hate to go offline this close to an answer for you, but I am out of time.




Any update?



Customer: replied 5 years ago.

No heat. I think that the next step was to test something with a Multi-Meter, which I have. I am not at home presently. How long are you available today? I plan to return home in the next 3 hrs.

Send a response to this thread...I will be notified on my phone and I will respond as soon as I can.



Customer: replied 5 years ago.
I am online and at home. I just await your reply to this message.


Very sorry, had an emergency service call.


Which motor did you have ...the one with the cooling fan on back of motor or not?



Customer: replied 5 years ago.

I have one with the fan on the back of the motor. The fan moved freely when I jumped R and W.


So with R and W jumped...the fan did not run.....but spin by hand freely?



Customer: replied 5 years ago.

Yes, that is correct. No resistance from fan with R and W jumped.


There is a plug to the starts as wires from the circuit board and then there is a plastic you know what I refer to?



Customer: replied 5 years ago.

Yes, I am familiar with this connector. There is numbering on the connector for each connection.


Ok....we are at the point we need something to see if power is at that plug. A meter, a light tester, anything that can tell me if power is present...



Customer: replied 5 years ago.
OK. I have a Multi-Meter. I need to review the how exactly it works. The is a red and black wire, and a dial in front.

set dial to ac volts ~

and set for range less than 600.


Test meter by sticking leads into a outlet you know works...look for 115 - 125 volts.


This will tell us if meter is set and working

Customer: replied 5 years ago.
OK. I tested the meter and it works. unplug the motor and start the furnace....see if the plug that comes from the circuit board energizes with power or not.



Customer: replied 5 years ago.
OK. Leading from the circuit board is the female connector. There is a large black cable and a white one which interconnect with the opposing black cables on the male end. I plugged the meter "rods" into the holes on the female end of the connector and the meter indicates that there is power.


Thank you....was it over 115 volts?



Customer: replied 5 years ago.
I have the ACV dial set at 300 (max). The dial goes mid-way when I insert the rods into the female connectors.


Ok, that is sufficient.


You have a bad exhaust fan motor. It may spin by hand but apparently has bad windings.


Looks like you will need to replace the inducer/exhaust motor.



Customer: replied 5 years ago.

OK. So I will need to simply replace the entire part seen on page 7, Figure 2 (Combustion Air Blower), correct? or will I need to open the item and remove the motor and replace that?


Nope the whole assembly. Which is really not a bad job. I believe you unhook the wiring and vent pipe...then 3 - 4 screws. Pretty quick job.


Thanks for all your help.



BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 21936
Experience: Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
BillyHvac and 2 other HVAC Specialists are ready to help you
Customer: replied 5 years ago.
Thanks for all of your assistance.

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