!!
critical Honeywell
Error Code AC LED On, DC LED Off
No DC output — the power supply is not providing DC power to the system despite AC being present.
? What Causes This
Power supply failure, blown fuse, or internal power supply malfunction.
✓ How to Fix It
- Check all DC connections for proper polarity and continuity.
- Verify the power supply output voltage (should be approximately 13.8 VDC).
- If no DC output is present, the power supply may require replacement.
Source: page 2 of the official manual
From the manual
This error code appears in:
Honeywell Pro 5000 Thermostat