Solar · Error codes · Fox ESS
bat-buck-faultFox ESS · EVO all-in-one
bat-buck-fault on a Fox ESS EVO all-in-one: The battery buck circuit mosfet is fail
Short answer
On a Fox ESS battery, bat-buck-fault means: The battery buck circuit mosfet is fail. The battery stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 24 September 2026 · 1 source
What it means
The manual says: The battery buck circuit mosfet is fail.
Is it safe?
| Does the battery stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The battery stops until the fault clears or is reset.
Resetting itself: no.
Manual reset: power cycle of PV, grid and battery (installer).
What you can check
- Read and note the exact code or light pattern and the time (photo or app screenshot). The EVO shows the fault name on its LCD touchscreen (red LED = fault mode).
- Check the battery/inverter AC isolator and the consumer-unit breaker are ON. Do not keep resetting a breaker that trips again straight away.
- Check the monitoring app shows the system online and the home Wi-Fi/router is working.
- The manual's fix ('disconnect PV, grid and battery, then reconnect') includes DC switching inside an all-in-one battery - ask your installer to do it or talk you through the safe sequence.
- Never open the battery, inverter or gateway casing and never touch DC cables, connectors or battery terminals.
- If the code stays or keeps coming back, contact your installer (quote the code) - do not keep power-cycling.
What the engineer will check
- Follow the EVO Alarm List solution; check event history on the LCD/Fox Cloud; escalate to Fox ESS support with serial number.
Who to ask for: battery storage installer.
Certification: Fox ESS approved installer (Fox ESS warranty excludes repairs by unauthorised persons); MCS battery certification.
What you might notice
Sources
- EVO 5 / EVO 10 Series User Manual V1.0.0 (10-500-10451-00) ↗ · V1.0.0 · page 53 · Reviewed 24 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 24 September 2026