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bausin 01-18-2013 11:27 PM

Furnace blowing cold air

I'm trying to help a neighbor with his furnace, Carrier 48SDN030060311. It seems to run erratically. It will periodically blow hot air for a few minutes before it turns cold, but from what I observed there is no repeatable pattern.

I took off the front panel and noticed that the control board LED was off. The error code for that says "Hardware failure". Not sure what hardware that refers to; is it the control board itself or can it be caused by external factors? Obviously no power to the board would cause this.

I also noticed that even when the main blower started up, the inducer motor never turned. I figure the control board is probably bad and I can't troubleshoot the system until that's fixed, so I ordered a new one.

I appreciate any comments or suggestions.


Doc Holliday 01-18-2013 11:52 PM

When you opened the door you disengaged the door switch which in turn stops voltage to the board. Carrier is notorious for a 90 second "off" delay meaning that the board has to have power after initial applied voltage is returned to the board before it will operate. This means the doors switch needs to be taped or held in the entire time.

That's just a heads up. As for the problems you need to see what the code is when it's blowing cold air. It could be a high limit switch is tripping and resetting (hot, cold, hot,cold and if that is the case the inducer should come on with the blower when the flames go out by way of gas valve shutting off) or it could be a pressure switch or inducer problem. It very well could also be a bad board. It could be a faulty stat or wire. Is the stat reading the temp right? Is it sending a constant 24 volts through the W terminal? You can bypass the stat by jumping the R to the W terminal in the furnace on the control board to bring on heat. Doing this removes the stat completely from circuit so we now can only focus on the furnace. If heat stays on full time than we know it was the stat but that's only one possibility.

We need to know if the inducer is working. To do this you need a meter to test the output voltage to the inducer on the board. You should see 120 volts where the inducer is plugged into the board when the stat is calling for heat. If you don't have a meter then simply unwire the main furnace voltage wires and wire the inducer directly. If it runs then we can exclude the inducer as the issue but without the inducer running the pressure switch won't close and the furnace will not light.

Also, check the back of the control board for burn marks. Let us know.

bausin 01-19-2013 01:52 AM

Doc, Sir,

I forgot all about the door switch. Before I took the panel off, I looked through the little hole to see if there was an error code flashing and there was no light. So I think the board is bad. There were burn marks next to a couple of resistors on the board; the resistors look OK. There wasn't a fire but they must have been hot.

The thermostat seemed to be reading correctly. I'll remember to check the W terminal if there are still problems with the new board installed.

Thanks for the tips,

bausin 01-26-2013 09:30 PM

Got the new board and installed it today. It blinks 6 times which means induced draft motor fault. The inducer motor never turns and there was 120v going to it from the board. The wires to the motor connect to what looks like a motor start capacitor before going to the motor. Could it be a bad cap? Do you just replace the cap along with a bad motor?


how 01-27-2013 03:06 AM

First just replace the cap with one having an identical rating. If that doesn't work (it's cheap), then change the ID assy.

bausin 01-28-2013 04:31 PM

OK, replaced the capacitor for the inducer motor. No change; motor dead. I ordered a new motor.

I tried taking the old motor out but the fan is too big. No access from the back side of the furnace either. I guess I have to take the whole panel that the motor mounts to out through the front.

Anyone know how much stuff I have to remove to get it out?


bausin 02-02-2013 04:04 PM

Put in a new motor today. I still get the same error code; six flashes -> inducer motor error. The motor never turns. This motor has a hall effect sensor to detect motor operation. The stat is calling for heat.

There is 120 VAC on the yellow and brown wires. The purple wire from the motor, when disconnected, has 120 VAC. The CM connector on the control board (where the purple connects) shows about 1 VAC with no wire connected and the stat calling for heat.

Any ideas?


bausin 02-02-2013 08:47 PM

OK, more info. I was able to locate a wiring diagram for this furnace. It shows a CR relay (combustion) in series with the inducer motor. I assume this shuts down the inducer motor after the gas is lit. I jumpered across this relay and the inducer motor started up. However, after the stat was turned down and then back up, the inducer wouldn't come on and the LED flashed the inducer error code again even though it was still running.

Question 1: Anyone know what controls the CR relay?

Question 2: Any ideas about why the furnace won't run a second time? I was wondering if the software could be checking the hall effect sensor and detecting the motor turning before it was commanded to. But why does it work the first time after power up?

Thanks for your help.

beenthere 02-02-2013 09:49 PM

If you left the jumper on when there was no cll for heat. you'll get an inducer error.

bausin 02-02-2013 11:48 PM

That's good to know. So maybe the only problem is whatever controls the relay is keeping the relay open. If I don't get an answer, I'll trace out the board to find which input is the culprit.


bausin 02-03-2013 03:12 PM

The control for the CR relay, which is in series with the inducer motor, is driven by the controller chip. I'll check for voltage on the other side of the relay coil.

Any of this ringing a bell for someone?

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