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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a Rheem Furnace that is not working properly. the

Customer Question

I have a Rheem Furnace that is not working properly. the code reads L 5.
Submitted: 1 year ago.
Category: HVAC
Customer: replied 1 year ago.
the serial number is: W271310792.
Expert:  Jason Whatley replied 1 year ago.

Hello my name is ***** ***** I'm here to help you out! I apologize for the delay!! One money while I look up documentation on your furnace.

Expert:  Jason Whatley replied 1 year ago.

The documentation for your furnace is not showing a L5 fault code. Can you remove the front panels on your furnace and see if you can find a reference to the L5 fault code? It should give you a very brief description as to what the L5 fault code is for.

Customer: replied 1 year ago.
I can not find anything on the furnace, the manual, or the internet that tells me what the L 5 code is or why it is flashing.
Expert:  Jason Whatley replied 1 year ago.

Okay, let's start at the beginning. If your turn the power off to the furnace for 5 seconds and turn it back on WITH the thermostat on heat and turned up, can you describe the process the furnace goes through?

Customer: replied 1 year ago.
it clicks "on" but does not light to produce heat.
Customer: replied 1 year ago.
fan starts to blow, but it does not light to produce heat so it shuts off and flashes "L" and then 5 and keeps alternating back and forth between L and 5. It periodically try's to come on again, but doesn't and still flashes the L and then 5.
Expert:  Jason Whatley replied 1 year ago.

This will be hard to troubleshoot not being there without knowing what the L5 is referring to. I will open up this conversation to other experts who may have insight on the L5 error.

Expert:  Phil replied 1 year ago.


The error codes are bogus about 40% of the time, we can diagnosis the old fashion way if you like.

Repeat this test 3 or 4 times until you get familiar with what the furnace is doing at each stage.


Next stand next to the furnace after the thermostat has been turned off for 5 minutes. Listen intently as someone else turns the thermostat to heat and sets it for 85F.

You should hear the small inducer fan start right away.. then a faint click after 1to 3 seconds, that is the pressure switch closing.

....then in15 to 45 seconds you should hear2 more faint clicks, that is the ignition relay and gas valve relay closing.

....then within 5seconds you should hear the main burner fire..

In about 30seconds the main blower should come on.


If the main burner goes off after 2 to 5 seconds or within a minute or so tell me about it and the number of seconds involved.

Repeat that a few times until you get familiar with the sounds, then tell me in detail how far the sequence goes before it stops.

It is also a good idea to replug all of the connections in the unit, that burnishes any oxidized terminal connections and will sometimes solve the problem,especially at the printed circuit card and in any sensor molex connectors. Do that with the power off to theunit of course.

We can go from there.

Customer: replied 1 year ago.
It lit this time. I didn't do anything. But I still don't know why it failed and has not come on for the last 5 hours. It is cold here in Alaska. Would like to know what was causing the L 5 code and why it was flashing the last 5 hours.
Customer: replied 1 year ago.
Actually, I did replace the furnace filters when my wife came home with them, but the old ones were not too terribly dirty.
Expert:  Phil replied 1 year ago.

Hello, no one here including myself has found an L or L5 error code... it is common with a defective printed circuit card to show a bogus error code... we can spend days, even weeks, sometimes years attempting to find a bogus error code in the manual... we have learned however that bogus error codes are common. And that reverting to other tests works quite well in most cases.

I can certainly opt out if you like, or you can do the listening test I prescribed in my previous post.

Let me know how you wish to proceed.