Hello all as stated I have the Lennox G51 natural gas Elite series. I believe it is about 11 years old since the house was built in '05.

I searched the forum and found another thread which seemed to have the same issue as I. However, the solution to that person's problem did not fix mine. So here goes: my furnace runs for about 3 minutes normally then I get the alternating slow blinkers on my control board. The things I did:

1) cleaned the air filter
2) cleaned the flame sensor using sandpaper (I know I should have used a brillo pad). This is also what the other thread I found said to do and it fixed their issue.

I checked the flame sensor after I cleaned it with a multimeter and got a reading of 5.4 uA when the burners turned on. However, I am not sure if that is normal. All the videos that I've seen show that the readings were around 2.4 uA. Is that a possibility that the flame sensor is bad because I cleaned it with sandpaper? I went ahead and ordered a new one from Amazon anyway.

I also researched how to check the limit switch. I think there are 2? One on the inducer and the other that goes into furnance? However, I am unclear as to how to really check if the limit switch is doing what it is supposed to do. I did check that it had continuity with my multimeter while the furnace was off, but I do not know how to check it while the furnance is running. There are two lead wires that go to it. Do I just put my multimeter on both leads while it runs? If that is the case, that is what I did and the reading stayed at zero the whole time the furnace was running. However, when I put one of the leads on the limit switch and the other to ground, I got a reading of about 26 Volts. I did that for the other lead on the limit switch as well and got the same results. Not sure if that means that it is open or not. I did read on the front panel that the watchguard mode would activate if the limit switch is open for more than 3 minutes.

When I was done messing with that, I also researched how to check the pressure switches. That was way beyond the scope of what I know how to do. Plus it requires having special tools, so I gave up on that. Is there anything else I should look for besides what was mentioned above? Thanks for any imput you all might have.

 

Welcome to the forums.

That is a good value. 2.4 would be low. Always better to be higher.... than lower.
A flame sensor problem will only allow flames to burn for a few seconds so that's not your problem.

Take your voltmeter set to read 24vac and connect it to the limit switch. In normal operation it will read 0v. It will read 24vac as soon as it opens.

There are at least two limit sensors.... one is limit for flame rollout and the other is a limit if the plenum overheats. The plenum one is most likely your problem one.

There should be a list of trouble codes on the back of the lower/blower door.

The G51 is a series. A full model number found on an ID tag near the burner would be more helpful.

 

Do I put my multimeter leads on both terminals (circled in pic below) of the plenum limit switch? If so, I have done that and it just reads 0.00 Volts the whole time it runs. When should it open up?



Is this what you were referring to? I didn't see an ID tag anywhere else.

 

If you connect the multimeter as you describe, one test lead to each limit switch terminal, you SHOULD see 24 VAC registered when the thermostat is calling for heat and zero VAC if the limit switch opens.


You might be doing something incorrectly to get the results you describe.

Another method I prefer and which you might try is to connect one test lead to the furnace chassis (ground). Select a range to measure 24 VAC or higher on your multimeter.

Use the test lead probe on first one connection to the limit switch and then the other.

When the thermostat calls for heat, one side of the limit switch should show 24 VAC all the time. The other side will show 24 VAC when the limit switch is closed, but show 0 VAC if the limit switch opens.

That is an alternative means of testing the limit switch you can try. Repairman like to have multiple ways of diagnosing the same problem so that you have multiple ways of confirming if a problem exists.

 

I have done this method as described in the original post - one lead to one side of the limit switch and the other lead to ground which gave me a reading of ~26 VAC. However, I did not check which side stays static on 24 VAC and which side changes from 0 to 24 VAC. I will have to check this when I get home later today.

So to reference the trouble-shooting chart of the alternating blinking lights, if the limit switch stays open for more than 3 minutes, that would mean that it is what is tripping the watchguard? Thus a bad limit switch? Does that sound about right?

 

Yes.

We don't know when it will open. Set your thermostat up and let the furnace run until it fails.