Please help, The system has been working intermittent and just stopped. Found the 4 blinking red light error on the board which states a limit switch problem. I tested all the limit switches for continuity and all were fine.

Symptoms:

Both AC and gas heat don't work.
Blower runs continuously till I switch the furnace off.
Blower fan is out of sequence, when the thermostat calls for heat or AC both vent motor and Blower come on at the same time but no heat or ac (compressor and fan outside will not come on, neither will the gas heater start). But indoor blower will run till switched off.

Again I checked all switches for continuity they are fine, I went ahead and replaced Aux switch no changes, cleaned evaporator coil nothing, replaced air filter nothing, tested the 10uf run capacitor and got 10.25uf. Replaced thermostat with exact model nothing, thermostat is on correct settings and auto air.

I have to keep the furnace switch off to keep the blower from running all the time, when I switch it back on I sometime hear one click and other times I hear couple clicks, guessing coming from the switch. Sometimes when the furnace has been turned off for a while it will wok for maybe an hour and come on and off normal and then a limit switch gets stuck in open and shuts down the heater with the blower running non stop.

Not sure what else to look for, could it be the board itself? I dismounted it and inspected it visually front and back and don't see anything fried! Any suggestions? Thanks

 

Welcome to the forum!! To better assist, please post model number of furnace.

 

Thanks for the reply, the furnace Model is B14933-175
The model on the control board is B18099-13
I am including a schematics photo

 

It is difficult to read the wiring diagram but it appears you have a primary limit and an auxiliary limit switch. The primary is normally located right behind the gas valve. Have you checked the primary limit?

 

Yes I did. They are both fine. If the furnace control board is bad would it also stop the compressor and outside fan from coming on but keep the indoor blower constantly running?

 

Intermittent problems can be very difficult to diagnose. In your case the exercise will be to determine if the primary or auxiliary switch is doing it's job or is the control board/wiring faulty. The primary/auxiliary limit switches are self re-setting. As soon as they cool off to the temperature shown on the label they will close and show continuity. So to properly test them for continuity you would need to test them as soon as the 4-flash fault code presents. You can also test to see if they are closed during operation by testing for 24V to ground on each terminal. If you read 24V to ground on one side only it would indicate the switch is open. If you show 24V across the two terminals during furnace operation the switch is closed.

I suggest you remove the 9-wire molex plug going to the control board and inspect it to make sure none of the pins are sticking out further or in further than the other pins. Also, check for signs of corrosion on the pins and re-insert the plug making sure it is fully seated.

 

Thanks for the input. I got hold of a control board and installed it yesterday and has been working fine now. Since this issues was intermittent I am going to hold off few days before calling it a total success. But it does look like a board issue, I don't know if it's a bad relay on the board or what causing the 4 blinks and tripping the Aux limit switch.

One thing to note since this problem caused both heat and AC to stop working I also tested the AC and it worked fine, compressor and fan outside kicked on fine. One question I did notice when going back to the attic a hissing bubbling sound in the ac pressure line while running that I don't recall hearing before, is that normal or is it an indication of low antifreeze? thanks

 

Hopefully the control board will solve your intermittent problem. As far as the hissing sound, can you pinpoint where it is coming from? The hissing could possibly indicate a refrigerant leak and the "bubbling" sound could be due to a low refrigerant level. However, there are other reasons these sounds are present. Do you see any oily residue anywhere on the evaporator coil or refrigerant lines? What was the temperature outside when you were running the air conditioner? Most manufacturers state not to operate the a/c when outside temperatures are less than 55 degrees. My cutoff temperature is 60 degrees.

 

I don't see any oil, it sounds more like a pressure sound hiss/bubbling effect in the line. Temperature outside was in the 60's. I only ran AC to make sure it solved that problem too as initially the intermittent problems started last month while we were still using AC and transitioned into heat when we switched to heat. And just noticed the 4 blinking lights on the board recently. I did read an older post with someone having the same warning/symptoms and was only solved after replacing the board. I thought the pressure noise in the attic is associated with low antifreeze but the ac cools fine !!!