HVACopediaCarrier · Furnace
12.1 W (heat call) on at power up
What the control is telling you
The control saw a heat call (W) when power came on, typically after a power interruption during a heat call. About 2 s after power returns it runs the blower alone for 90 s (LED flashes 12, display 12.1), then resumes normal operation.
Applies to: Carrier 59SC6A
Likely causes
- Power outage or breaker trip during a heat call
- Blower door opened (interlock switch ILK) during a heat call
- Loose line-voltage connection causing a momentary power loss
Test and repair steps
- Wait out the 90-second blower-only period; after it the LED should go to steady ON if no faults are present and the furnace should answer the thermostat normally.
- If 12.1 keeps appearing, check FLt history for repeated power-related codes (12.1, 46.1).
- Warning: Turn off and lock out power before checking line-voltage connections.
- Check the junction box and disconnect/breaker connections for looseness.
- Make sure the blower door is installed securely so the door interlock switch stays closed.
- Confirm the thermostat W wire is not intermittently shorting to R, which can look like a heat call at power-up.
Settings for these units
Filter sizes by casing width and filter cabinet · 59SC6A
| Location | Size | Side return filter | Bottom return filter | Type |
|---|---|---|---|---|
| Furnace casing | 14-3/16 in. wide | 16 x 25 x 3/4 in. | 14 x 25 x 3/4 in. | Washable |
| Furnace casing | 17-1/2 in. wide | (1) 16 x 25 x 3/4 in. | (1) 16 x 25 x 3/4 in. | Washable |
| Furnace casing | 21 in. wide | (1) 16 x 25 x 3/4 in. | (1) 20 x 25 x 3/4 in. | Washable |
| Furnace casing | 24-1/2 in. wide | (1) 16 x 25 x 3/4 in. | (1) 24 x 25 x 3/4 in. | Washable |
| Filter cabinet | 16 in. high | (1) 16 x 25 x 3/4 in. or (1) 16 x 25 x 4-5/16 in. | - | Washable or media |
| Filter cabinet | 20 in. high | (1) 20 x 25 x 3/4 in. or (1) 20 x 25 x 4-5/16 in. | - | Washable or media |
| Filter cabinet | 24 in. high | (1) 24 x 25 x 3/4 in. or (1) 24 x 25 x 4-5/16 in. | - | Washable or media |
Side-return filters can differ in size - measure the existing filter.
Every return-air inlet needs a filter.
Minimum clearances to combustibles · 59SC6A
| Position | Minimum clearance |
|---|---|
| Rear | 0 |
| Front (combustion air openings in furnace and in structure) | 1 in. (25 mm) |
| Required for service | 24 in. (610 mm) (see note) |
| All sides of supply plenum | 1 in. (25 mm) (see note) |
| Sides | 0 |
| Vent | 0 |
| Top of furnace | 1 in. (25 mm) |
Service and supply-plenum values are marked 'consult local building codes'.
Keep 1 in. (25 mm) from combustibles to supply ductwork for 36 in. (914 mm) horizontally from the furnace.
Models that show this code
11 models in 59SC6A
Other codes on the same units
- 10.1 L1 polarity fault
- 13.1 Limit lockout - main limit open over 3 minutes
- 14.1 Ignition lockout after 4 tries
- 14.2 Lockout - flame lost 3 times after 70 s of heating
- 14.3 Lockout - 7 flame-loss events in one heat call
- 21.1 24 VAC on gas valve when it should be off
- 22.1 False flame
- 23.1 Main pressure switch stuck closed
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Source: Carrier 59SC6A Installation, Start-up, Operating and Service Instructions, Fig. 60 Service Label p.54; Sequence of Operation note, p.68. Summarized in our own words; check the original manual for your model.
Homeowner looking at this code? See what it means for you and when to call a technician.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.