HVACopediaCarrier · Furnace
25.4 Main program invalid, running on backup
What the control is telling you
The main program is invalid, so the control is using its backup program. Operation is still normal.
Applies to: Carrier 59SC6A
Likely causes
- Main program memory corrupted
Test and repair steps
- Furnace keeps running; schedule the repair.
- Reprogram the control with the rating-plate Model Program Number (app/NFC or super plug).
- Verify in the inF menu.
- Replace the control if the code returns after reprogramming.
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
Save it for the next call
Star this code in the HVACopedia app and keep the whole library on your phone for jobs with no signal.
Source: Carrier 59SC6A Installation, Start-up, Operating and Service Instructions, Fig. 60 Service Label Information, p.54; Control Board Replacement p.53. 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.