Heating, Air Conditioning, Fridge, HVAC/gas fired forced air furnace


QUESTION: The only trouble code I am seeing is the green light flashing with the red and yellow lights off. If the blower is shutting down and the burner is still fired up. Wouldn’t Flame sensor Item 27 (not shown)Trip? It’s located on the burner box with Item 26 and 23 on the diagram attached. Mine is the blower deck item 11. Would the sensor reset itself if the temperature went back to normal? Because even though I power down the entire unit at the breaker and try to rest everything ,it still won't fire up. I physically have to remove the huge blower and disconnect the wires at the sensor to have it work. I buy the bulk roll of filter at the hardware place and cut my own to size. Yes it's green and on the door panel. Sorry for the picture the website won't let me upload a pdf file. I did all that you said to do. A new sensor is in the mail. I figured I would ask the expert to see if there was something I was missing.
Thank you


ANSWER: Hi Jim, the problem you are having is very uncommon, can't say I have ever run across this. I can't figure out by disconnecting the wires on the limit would get it running again.

I know your tired of messing with this, but could you do one more test. Trace the two blue wires from the deck switch back to where they are connected to the other limit switches. Remove those wires. Set your meter to OMH's and test the two wires. This will tell you if the deck switch is open or closed.


---------- FOLLOW-UP ----------

QUESTION: At the time of the failure and when the diagnostic Green LED was telling me the problem and I could not reset the furnace. I disconnected the 9 pin plug and tested the 2 blue wires with the VOhm and the circuit did in fact show open. I pulled out the blower so I could get access to the deck switch. When I tested the deck switch with no wires it was closed> I put the blower and wires all back together and tested the blue wires at the 9 pin plug And the circuit was closed. I am so confused why this happened. It last happened sunday morning same thing . It has been ok up till now

Jim, Now it's starting to make sense. When you test from the 9 pin your are checking all 4 limits. Here's what I think MIGHT be happening. Either the vent limit or the supply air limit is tripping. By the time you pull the blower to access the deck limit the other limit has cooled down and closed.

Here's a good way to find out which limit is tripping.

1. Find the vent limit, it will be on the inducer blower. Put your meter on the blue/white wire and the COM on the control board. You should have 24V.

2. Then put your meter on the other side of the limit, 24V if closed or 0V if opened. If you have 24V then do the same on the supply air limit.

The furnace will not reset as long as the limit is opened. The limit could take 30 min or longer to reset.

If it is the supply air limit make sure all the vents in the house are opened.  

Heating, Air Conditioning, Fridge, HVAC

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Jim S.


I can answer questions on gas or electric furnaces, ac units and heat pumps. I don't work on boilers,chillers,or home appliances.


I have been in the HVAC trade for over 35 years. For the past 15 years I have owned my own business. I am semi-retired so I can answer questions in a timely manner.

I attended trade school for 5 years on weekends and evenings. This was very hard to do, but well worth it!

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