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The Cat Man 11-14-2010 10:27 PM

Lennox G51MP Elite Series
Good Evening:
I have a Lennox G51MP furnace that has quit working tonight.
The LED diagnostic code states primary or secondary limit switch open. Limit must close within 3 minutes or unit goes into 1 hour Watchguard
I don't know what that means.
I have taken the covers off and cleaned things up a little.
When I put the cover back on, the fan starts but only lasts for about 3 minutes. No heat is produced.
I have also turned the Thermostat off and back on again after a few minutes.
When the thermostat is off, the sure lights state normal operation.
turn the thermostat back on and I get the error again.

yuri 11-15-2010 06:16 AM

It means that the limit control above the burners has failed open from the furnace continuosly overheating from a lack of airflow.

The Cat Man 11-15-2010 06:39 AM

Thanks Yuri
any idea how to fix it

yuri 11-15-2010 04:36 PM

Do you own a electrical meter/multitester and know how to check voltages and continuity?

The Cat Man 11-15-2010 04:45 PM

I just reset the primary limit switch by pulling off the ground wire and putting it back on.
that's all it needed I guess.
It was caused by the lack of airflow like you said and when the new filter didnt help......I was a little concerned
All is well now and we once again have heat in Regina.

Marty S. 11-15-2010 05:25 PM

Pulling the wire probably jiggled it enough to work for now. Sometimes a screwdriver tap will do the same. I never trust them after that and always order a new one to be safe.

yuri 11-15-2010 05:30 PM

Yep. Not sure if you can order them online in Canada but in the US has them and so does your local Lennox dealer. MAKE ABSOLUTLELY sure you do not stick it in upside down or it will fail. Note how the old one was installed. Check the temp rise thru the furnace. Subtract the return air/house temp from the supply air temp measured a foot downstream from the plenum with a cooking thermometer stuck in the duct. Then check the specs on the rating plate inside the furnace ie: 50-70F. If it is overheating then the fan needs speeding up or the ductwork fixed. Too bad our Bombers bombed out. Go Banjo pickers:laughing:

The Cat Man 11-15-2010 05:43 PM

Thanks for your help guys.
Go Riders.
Did you watch that game last night!!
Do you get TSN in Winnipeg??:wink:

yuri 11-15-2010 05:49 PM

1 Attachment(s)
We get TSN but I don't watch too much football with our miserable bums. Now they are building a $200 million stadium for the sad sacks. Was watching Battle of the Blades on CBC. Hockey players on figure skates, go figure, must be Canada eh!

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