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thate 11-14-2007 09:12 AM

Lennox stuck in Watchguard Mode
Hello all,

I am having a furnace issue with my Lennox G40UH-36B-090-03 furnace.

The system tries to fire up about 5 times then it gives me an alternate blinking code, which is referred to as watchguard. It could mean a couple of different things according to the case:

A: Burners failed to ignite
B: Limit open more than 3 minutes
C: Lost flame sense 5 times in one heating cycle
D: Pressure switch opened 5 times in one heating cycle

Just by doing some research I have done the following.

Emptied the condensation pipe and cleaned the buildup
Took out the flame sensor and sanded it, it did not have the white residue buildup others complained of
Unplugged and reseated the igniter plug
Checked the screen, it is brand new looking still
The igniter IS getting hot, verified by touch.

Can anyone help me out with some additional troubleshooting steps?



thate 11-14-2007 11:54 AM

A friend just suggested that carbon may be on the ignitor and to either attempt to clean it or buy a new one.

Does this sound feasible?

metal-maniac 11-14-2007 03:12 PM

If this unit has a hot surface igniter you would see it glowing and not be able to touch it while on. If in fact it is and it has power to it then it will need to be replaced. Never touch the black part of the hot surface ignitors the oil residue from your hands will cause premature failure because of the intense heat.
If it has spark ignitor then cleaning it and the flame sensor is all you should do.If you have a vent issue you definitely need to have a tech look at it.

Hope this helps.

thate 11-14-2007 03:23 PM

thanks metal-maniac

yes it is a hot surface ignitor and I just ordered a new one today. It never gets to the point of glowing, but after it shut off I did touch it last night just to see what was going on. It was infact hot, just maybe not getting hot enough?

undone 11-14-2007 05:06 PM

check the venting
Given the probable failures and/or malfunctions you so listed (A thru D), you should make an attempt to verify ALL these possibilities to be the cause. The last thing you want to do is go and start replacing parts unnecessarily. The fact that you can feel the HSI getting hot could be due to it trying to. When the draft system (or venting) is restricted, the pressure switch will open and close intermittantly allowing the ignitor to get hot but not hot enough. Also, some systems have the pressure switch and the gas valve "tubed" together so to verify (to both components) system is venting properly.

#CARRIERMAN 11-14-2007 06:17 PM

Hi thate

Now you know why I am a Carrier man. However that does not mean I can't or won't help you. There are several things that will cause this on a Lennox furnace. If you have done some rescent rewiring and have the polarity wrong or do not have a good ground on the furnace. This would be the most likely cause. The watch guard portion of the furnce is looking for a rectified flame signal. If the burners have not been pulled and cleaned in a while, this is where I would start. To create a rectified flame siginal, the control board sends out about 90 volts a/c to the flame sensor. The ionized air that is created when the air is superheated causes a ground path to the burner. Hence the burner must have a clean surface to ground too. The other problem Lennox has had is formicary corosion around cabinet screws causing excessive resistance or poor ground path. The last possibility is that the flame rectification circuit has gone bad in the board. This would be the last resort as this board is going to cost you in excess of $100.00. You can test this by putting a ground wire from the burner in front of the flame sensor to the ground wire located on the control board. You can make a temporary wire to test this, but I would definately start here.

Good luck

thate 11-14-2007 07:30 PM

thanks you guys for the suggestions!

undone, what is the best way to get into the venting to check for clogs?

#CARRIERMAN, nothing has been cleaned in 5 years that I am aware of. There is some minor discoloring on the burners where it looks like maybe water dripped out of the condensation pipe into the furnace. Not much but just enough to possibly do damage. What do you suggest cleaning them with?


#CARRIERMAN 11-14-2007 08:13 PM

Hi thate

The best way to clean the burners are to remove them and to use a wire brush on the burner end. Use an air compressor or you can wash them out with water. If you use water make sure to set them on end and let them air dry completely.

Good luck

undone 11-14-2007 08:23 PM

I admit I'm not familiar with this particular product, but venting applications don't vary too much from one manufaturer to another. On an 80% afue furnace you should have "B" vent connected to it. If it terminates at the roof and you can access it safely, check there. During the off-season, spiders will spin webs at the cap causing the problems you described.
If you have a condensing furnace (afue is around the mid 90's), the venting should be PVC (plastic), same applies if it vents out the roof. If it terminates laterally (sideways), a sag may have developed allowing water to "pool" (or trap), causing restriction.
If you had your house reroofed or painted lately, you might be surprised what you'll find.

thate 11-15-2007 10:37 AM

Rusty thanks I will be performing this tonight when I get home.

Undone it's 80% so I'll have to climb out to the roof and check. There is also a pvc pipe that runs from top that I took off and checked. There was water in the pipe with minor blockage, but clearing it didn't fix anything. I haven't done any painting or roof work ever on this house. Trees are also a good distance from the house but spiders and whatnot could easily have access.

Thanks for the great suggestions!


VAheatcool 11-16-2007 09:15 AM

You really need to call a professional. Sanding the flame sensor is NOT recommended by Lennox. You can actually damage it. Touching the ignitor? HSI's get over 1000 degrees F. Do you have gas present? Low voltage on your valve?....please call someone who does this for a living.

thate 11-16-2007 09:25 AM


Thanks for your concern, it's much appreciated. However, this is the do it yourself chatroom (hence DIY). I wanted to give it a shot first, I have no problem calling a professional if I don't get this working by Saturday. The ignitor obviously didn't reach 1000 degrees F as it wasn't turning red, in fact just putting my finger near it didn't emit heat. I had to touch it before I could tell it was hot, and no my finger did not melt off. Lennox I'm sure would suggest you replace the flame sensor rather than get the old one working again. Common sense tells me they wouldn't recommend that, but I've found countless threads stating that the sanding method does in fact work (if carbon build up is the problem).

I'll make sure to keep you posted.



thate 11-17-2007 04:04 PM

Ok, I come home yesterday to find that my new HSI ignitor had arrived. I first removed the burners and the plate above it, then took a wire brush to clean them as carrierman had suggested. They had a rusty appearance to them and were sparkling afterwards. The old HSI did in fact appear to have a carbon buildup on it (or some other white compound) and instead of attempting to clean it off I installed the new one.

I turned the unit on and it worked like a charm, the HSI instantly turned yet and when the burners fired up the flame kicked on. SWEET! The new HSI was under $100 and I saved myself quite a few bucks.

Thanks for all the help guys, specifically undone and carrierman. When I get time I still need to get on the roof and check out the vent situation.

Praise God!

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