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Fox ESS SW Bus Vol Fault

Updated June 2026 · Reviewed for technical accuracy · ~2 min read

SW Bus Vol Fault means your Fox ESS measured the internal DC bus voltage outside its safe range and the software protection tripped. The "SW" stands for software detected, which is the milder of the two versions. Its hardware twin, HW Bus Vol Fault, is more serious. A common and harmless cause of the software version is conflicting battery charge schedules set in FoxCloud, so it is often cleared without any hardware work.

What usually causes it

  • Conflicting or overlapping charge schedules. Two FoxCloud time slots that fight each other (for example a charge and a force-discharge in the same window) can push the bus voltage out of range. This is the most common trigger for the software version.
  • A duplicate schedule left over from testing or a changed plan, telling the battery to do two things at once.
  • A brief PV or battery voltage swing at a transition between modes, which the software flags as a protection event.
  • A failing internal power stage, less common, especially if the hardware variant (HW Bus Vol Fault) also appears or the fault keeps returning.

How to handle it safely

  1. Open FoxCloud and review your schedules. Look at the work mode and timed charge settings for the day. Note any slots that overlap in time.
  2. Remove duplicate or conflicting slots. Delete any leftover or overlapping schedule so only one clear instruction applies in each time window, then save.
  3. Power-cycle the system fully. Disconnect PV, grid and battery, wait about 15 minutes for the bus capacitors to discharge, then reconnect in the order your manual specifies.
  4. Watch for the fault returning. If it stays clear after fixing the schedules, you are done. If it comes back, or if HW Bus Vol Fault appears, stop and treat it as a hardware issue for an installer.
When to call a professional. The 15 minute wait is only to let the DC bus capacitors bleed down. Do not open the inverter or touch any DC connector. If the fault returns after you have cleared conflicting schedules and power-cycled, or if the hardware version (HW Bus Vol Fault) shows up, the internal power stage may be failing and needs a qualified Fox ESS installer.

Related Fox ESS codes

Bat Volt Fault PV Volt Fault BMS Lost

FAQ

Is SW Bus Vol Fault as serious as HW Bus Vol Fault?

No. The software version (SW) is the inverter's first level of protection and often comes from conflicting FoxCloud schedules rather than a defect. The hardware version (HW) is triggered by the protection circuit itself and points to a real power stage problem. If you ever see HW Bus Vol Fault, stop using the system and contact your installer.

I cleared my schedules but the fault came back, what now?

If the fault returns after you have removed conflicting schedules and done a full power-cycle, it is no longer a settings issue. The internal DC bus stage may be failing. Do not keep restarting it or open the unit. Note when the fault appears and have a qualified Fox ESS installer inspect it.

Helpful guides

Error codes explained How to safely restart your inverter

Sources

  • Fox ESS hybrid inverter user manual and FoxCloud alarm list (Bus Voltage Fault = internal DC bus voltage out of range; SW software detected, HW hardware detected).
  • Fox ESS installer guidance on FoxCloud work modes and timed charge schedules, and the safe shutdown sequence before any service.
⚠️ Safety disclaimer. Solar inverters carry lethal DC and AC voltage even when "off". This guide covers reviewing FoxCloud schedules and a normal shutdown only. Any internal inspection or DC work must be done by a licensed installer or electrician.