Trane XL80 High limit code 4 even when high limit probe removed and jumpered
#1
10-28-15, 03:14 PM
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Trane XL80 High limit code 4 even when high limit probe removed and jumpered
Hello- Any help would be greatly appreciated. My trane xl80 furnace has been shutting off and showing code 4 "high limit". Furnace starts fine and blower pushes air strong but in 10 to 15 mins it shuts off.
Checked all the vents and there free and clear. Intake and filter are clean. Roll out limits have not been activated / popped tried reset and it doesn't go in. Replaced a two year old thermostat just incase to see if it was that and no go =(
Ive replaced the high limit with a new one and even tried jumping the wires to test with no resolve. Furnace still shuts off with code 4.
any suggestions would be great.... I'm leaning towards purchasing a new control board at this point.
Checked all the vents and there free and clear. Intake and filter are clean. Roll out limits have not been activated / popped tried reset and it doesn't go in. Replaced a two year old thermostat just incase to see if it was that and no go =(
Ive replaced the high limit with a new one and even tried jumping the wires to test with no resolve. Furnace still shuts off with code 4.
any suggestions would be great.... I'm leaning towards purchasing a new control board at this point.
#2
10-28-15, 04:34 PM
Banned. Rule And/Or Policy Violation
The purpose of the high temperature limit switch is to turn off the furnace if it overheats.
Normally, the fan that circulates air around the house strips the heat produced by burning gas from the heat exchanger and prevents the furnace from overheating.
However, if something impairs the air flow through the furnace, the heat can't be removed and the furnace will get hotter and hotter until the limit switch opens and shuts off the burners.
So very likely you need to find what is impairing the air flow through the furnace. The most common causes of this problem are a plugged air filter, too many warm air vents closed off or a return air vent covered over.
Normally, the fan that circulates air around the house strips the heat produced by burning gas from the heat exchanger and prevents the furnace from overheating.
However, if something impairs the air flow through the furnace, the heat can't be removed and the furnace will get hotter and hotter until the limit switch opens and shuts off the burners.
So very likely you need to find what is impairing the air flow through the furnace. The most common causes of this problem are a plugged air filter, too many warm air vents closed off or a return air vent covered over.
Last edited by SeattlePioneer; 10-28-15 at 05:06 PM.
#3
10-28-15, 04:59 PM
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Thank you seattlepioneer Ill check all the vents again to see if any blockages. All vents are open to my knowledge, one of the first things ive checked. Glad to see another seattleite helping others 
#4
10-28-15, 05:12 PM
Banned. Rule And/Or Policy Violation
>
I see you already did the initial checks I suggested.
Since they didn't identify the problem, inspect the fan assembly to see if the fan is loose on the motor shaft, has broken blades or is plugged with dust and debris.
If you have air conditioning (rare in Seattle), check to see if the AC coil in the furnace is being plugged by dust, dirt and debris.
Look for any signs that the sheet metal or duct insulation has come loose and might be plugging up the flow of air.
If you tape the furnace door switch closed, does the fan have a real powerful effect sucking the door closed? That might indicate something plugging up the air being supplied to the furnace from the return air register.
Very likely you have SOMETHING plugging up the air flow. You have to find that and correct it.
I see you already did the initial checks I suggested.
Since they didn't identify the problem, inspect the fan assembly to see if the fan is loose on the motor shaft, has broken blades or is plugged with dust and debris.
If you have air conditioning (rare in Seattle), check to see if the AC coil in the furnace is being plugged by dust, dirt and debris.
Look for any signs that the sheet metal or duct insulation has come loose and might be plugging up the flow of air.
If you tape the furnace door switch closed, does the fan have a real powerful effect sucking the door closed? That might indicate something plugging up the air being supplied to the furnace from the return air register.
Very likely you have SOMETHING plugging up the air flow. You have to find that and correct it.
#5
10-28-15, 08:47 PM
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Ive been watching the furnace for a while and it looks more like the reason its activating code 4 is because the fan turns off 10 mins in as its still calling for heat? So the fan stops for no reason as the fires still going which causes the high limit to trip.
#6
10-29-15, 12:53 PM
Banned. Rule And/Or Policy Violation
Unreliable operation of the fan motor can certainly cause the limit to trip.
Use an AC voltmeter to determine if the AC voltage to power the motor is being shut off, or whether the AC power stays on but the motor itself is bad.
A common reason for the motor to shut off is that the motor is overheating, and the hot motor opens a heat sensitive limit switch in the motor which shuts it off.
Try turning the fan to see if the motor is stiff and hard to turn. Lack of lubrication of the motor can cause the motor to be hard to turn, which can cause the motor to overheat and the thermal limit to open.
Use an AC voltmeter to determine if the AC voltage to power the motor is being shut off, or whether the AC power stays on but the motor itself is bad.
A common reason for the motor to shut off is that the motor is overheating, and the hot motor opens a heat sensitive limit switch in the motor which shuts it off.
Try turning the fan to see if the motor is stiff and hard to turn. Lack of lubrication of the motor can cause the motor to be hard to turn, which can cause the motor to overheat and the thermal limit to open.
