Ruud U802VA050317MSA Error Code A031_F: Open Fuse
What Does Code A031_F Mean?
Fault code A031_F on the Ruud U802VA indicates that the low-voltage fuse on the IFC board has blown open. The U802VA's 24VAC control circuit is protected by this fuse, which opens when a short circuit allows excess current to flow. With the fuse blown, the IFC, thermostat, and all 24VAC accessories lose power and the furnace cannot operate.
A short circuit in the thermostat wiring is the most common cause — for example, where a thermostat wire has been pinched under a staple or nail, causing the hot (R) wire to contact the common (C) wire. A defective thermostat, zone controller, humidifier, or other 24VAC accessory connected to the furnace can also create a short that blows the fuse.
On the Ruud U802VA, lockout conditions can normally be cleared by cycling power, but a blown fuse cannot be cleared this way — the fuse must be physically replaced and the underlying short must be corrected. Replacing the fuse without finding the short will result in the new fuse blowing immediately. A technician must systematically isolate the 24VAC circuit to locate the fault.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Short circuit in thermostat wiring | Most common | ✗ Call a pro → |
| Faulty thermostat or accessory | Common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- Any time this code appears — a technician must locate and repair the short circuit before replacing the fuse
- Simply replacing the fuse without fixing the short will cause immediate re-failure; a systematic diagnosis is required
- A technician should check all 24VAC wiring including thermostat runs and accessory connections to find the fault location
✓ Verified against manufacturer service manual — March 2026