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Discussion Starter #1
Two Trane 90XL installed 2000 run off one thermostat. One runs great now. Second furnace blows cold air and burners have no flame.

First service tech said problem was drain tube loose and pressure set too high.
Adjusted pressure and cleaned drain tube and reservoir. Same thing happened again in 3 days.

Second tech says that the main board is shot because the 4 light pulses indicate the high limit switch is malfunctioning. He jumped the high limit and the furnace started normally then quit.

He says the board is shot and needs replaced.

Can any one confirm his diagnosis?
 

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AKA HVACTECHFW
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No, board is not shot. You have an airflow issue. (blocked supply vents, blocked returns, plugged filter, plugged evap orator coil, plugged secondary heat exchanger coil, weak blower motor, inoperative blower motor, or weak blower motor capacitor) First thing to do is identify why the main limit is tripping, then if the limit is sticking closed you should replace the limit.
 

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Discussion Starter #3
I think I jumped the high limit

Thanks for your response.

I took off the cover to expose burners but did not pull them and to access the high limit switch in the ex changer chamber.

The wire were in a connector. I put a small wire into the connector and turned the furnace back on. There was a zap noise and I had to reset the breaker.

After resetting the breaker the furnace came back on and the error code blinked 9 times. I checked the on the door of the furnace 9 blinks check ignition.

I checked several times and confirmed there are now 9 blinks instead of the 4 blinks.

The furnace still blows cold air. I checked and there are blocked supply or returns.

What do you think?
 

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Discussion Starter #5
Furnace now running

The furnace i now running I must have some how reset the high liit switch. I laso mad e sure there was nothing blocking supply and returns.

However, the flame is not bue like the first furnace it is amber yellow.

What is missing, thanks.
 

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Is the inducer running.
 

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Discussion Starter #7
Inducer Running

The inducer was running previously .When I returned to the property I really only felt the heat and looked in the heat exchanger through the see through window to see the amber fame. I initially thought the board was blown so seeing it working was a relief.

Things seem to be normal but there was not a strong air low at the affected vents.

Is there a serious problem if the inducer is not running?

Will the furnace be destroyed?

Thanks
 

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Shouldn't even be able to light the gas without the inducer running. but you shorted something out. So no telling what may happen. It need attention.
 

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I'd shut it off. You could be sooting up he secondary heat exchange. No telling if the safeties work now.
 
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Discussion Starter #11
Okay I shut it down. I'll get serviceman tomorrow or Monday. Other furnace running well.

Thanks for the info. safety First Always.
 

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Good decision.
 
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