Furnace Trouble

We have one of those Carrier combination air conditioner and furnace units. This sort of makes sense since they share the same duct work. We had some trouble with the air conditioning side a few years ago and last year began having intermittent trouble with the furnace.

The symptom for the furnace trouble is that when the thermostat calls for heat, the inducer blower runs but there is no flame. At least no flame when you realize that you hear it running but there is no heat coming from the register. This has always occurred when there was precipitation and falling temperature. Around here that means fog forming or ice storm, usually at night. Not the best time to lose heat or work on the system. So far we have been lucky. If you smell gas, turn the gas off, turn the power off, and call a repairman. Otherwise Quick Fix: Turn the circuit breaker off. Wait 20 seconds. Turn circuit breaker on. The system seems to restart ok. This might be the best solution to get the heat back on. But it does not fix the problem.

This unit (Model number starts with 48SS. The next 3 digits are a capacity of the A/C unit) has an electronic circuit board that performs the starting and safety logic. When it detects an error, it displays an error code as a sequence of flashes of a red LED. It may display more than one code so count pulses and pauses until you get the same count twice. For example it could do flash-flash-pause-flash-flash-flash-pause-flash-flash-pause-flash-flash-flash-pause…. This would indicate errors 2 and 3 (if this were a possible condition). Understanding the error code is essential to pin-pointing the problem. But the errors are reset if you turn the power off. So if you can, leave the power on. If you smell gas, turn the gas off, turn the power off, and call a repairman.

In order to see the circuit board and the indicator LED you will need a 5/16″ socket to remove 2 screws at the bottom edge of the panel marked “Burner Control” on right hand side. If the label is gone, this is to the left of and around the corner from where the exhaust and gas line come out of the unit. After removing the screws, bring the bottom of panel toward you and allow it to slide down, out from under the lip at the top. The left side of the panel sits in a groove to provide a seal so there may be a bit of friction getting it loose.

The control board will be at the top left of the open compartment. Look for a red LED. If it is on solid, no error is indicated. If it is off, there is no power to the unit or the control board has failed. Call a repairman. If it is flashing, count the flashes and write them down. Replace the cover by inserting the left edge in the groove and the upper edge under the lip. Press the panel against the unit and start the screws. Tighten the screws. Reset the unit by turning the circuit breaker off. Wait 20 seconds. Turn circuit breaker on. Verify that the thermostat is calling for heat. If the unit does not start, you will need to call a repairman.

Longer term fix: The error code we saw was an 8. This indicates an “internal controller fault”. A web search for related problems suggest several things:

  1. Check the circuit board above the 2 white squares. There are two resistors there. Charing may indicate that the circuit board has overheated.
  2. Check for corrosion. This may affect the ground connection of the circuits. If there is evidence corrosion, you may need to clean it off to get a good connection. Examine the inside of the firebox (heat exchanger) with flashlight. If excessively corroded, you will need replacement. Call a repairman.
  3. Remove debris from the control box. The burner(s) are located in the bottom of the box. While the circuit breaker is off, vacuum cob webs, wasp nests, leaves, etc. out of the control box.
  4. Some units may have a “sail switch” to detect air flow. If one exists, check for free operation.
  5. Other units have a Hall Effect sensor to detect rotation on the inducer fan motor. Google for or check this thread (scroll down to djken post) or themanual for instructions on how to test the Hall Effect sensor.
  6. When you turn the circuit breaker back on and the unit restarts, check to make sure that the flame from the burner is induced completely into the hole in the heat exchanger before you replace the cover and tighten the screws.  Oversize flame or insufficient draft require adjustment. Flame outside of the combustion chamber is known as “flame roll-out”. Treat this condition as dangerous and get it repaired promptly. When roll-out is detected on this unit, the inducer blower and the house blower continue to run to cool things off.

Error codes for Carrier 48SS series

Error LED Indications
Normal Operation On
Hardware Failure Off
Fan On/Off Delay Modified 1 Flash
Limit Switch Fault 2 Flashes
Flame Sense Fault 3 Flashes
Four Consecutive Limit Switch Faults 4 Flashes
Ignition Lockout Fault 5 Flashes
Induced-Draft Motor Fault 6 Flashes
Rollout Switch Fault 7 Flashes
Internal Control Fault 8 Flashes


  1. There is a 3-second pause between error code displays.
  2. If more than one error code exists, all applicable error codes will be displayed in numerical sequence.
  3. This chart is on the wiring diagram located inside the burner access panel.

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