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Fox ESS M1 Close Fault / M2 Close Fault

Updated June 2026 · Reviewed for technical accuracy · ~2 min read
⚠️ Safety-critical fault. A welded or stuck grid relay means the inverter may be unable to disconnect itself from the grid during a fault. You can try a single controlled power cycle. If the fault returns, stop using the inverter and call a qualified installer. Do not keep resetting it and do not open any DC connectors.

M1 Close Fault and M2 Close Fault (you may also see S1/S2 Close Fault or Main Relay Open) report a problem with one of the grid relays inside your Fox ESS. These relays physically connect and disconnect the inverter from the mains. The fault is raised when the inverter checks a relay and finds it stuck, welded or not behaving as expected, so it refuses to run. Because a relay that cannot open is a safety risk, treat this as serious rather than a routine glitch.

What usually causes it

  • A welded relay contact. The contacts inside M1 or M2 have fused together and can no longer open, so the inverter cannot isolate itself from the grid.
  • A stuck or mechanically failed relay. The relay does not switch cleanly, often after long service or repeated heavy switching.
  • A one-off internal glitch. Occasionally a single failed self-check trips the fault and a clean power cycle clears it for good.
  • An internal control or driver fault on the relay circuit, which only a technician can confirm.

How to handle it safely

  1. Do one controlled power cycle. Shut the inverter down in order: turn off the AC isolator and grid breaker, switch the DC/PV isolator to off, then turn off the battery. Do not pull any DC connectors.
  2. Wait about 15 minutes. Let the unit fully de-energise and the internal capacitors discharge before you power it back up.
  3. Reconnect in reverse order. Turn the battery on, then the DC/PV isolator, then the grid and AC isolator, and let the inverter restart and run its self-checks.
  4. If the fault clears and stays clear, fine. If M1/M2 Close Fault returns, the relay has mechanically failed. Stop operating the inverter and call your installer. Do not keep cycling power to force it on.
When to call a professional. If the fault comes back after one power cycle, the relay is failed and the inverter must be repaired or replaced by a qualified technician. A relay that cannot disconnect from the grid is a genuine safety hazard, so do not run the system in this state. There are no owner-serviceable parts inside, and the unit holds lethal DC and AC voltage. Contact your installer or Fox ESS service.

Related Fox ESS codes

Res Cur Fault DCI Fault Grid Lost Fault

FAQ

Is it safe to keep using my inverter after an M1 or M2 Close Fault?

No. A stuck or welded relay means the inverter may not be able to cut itself off from the grid if something else goes wrong. You can try one controlled power cycle, but if the fault returns you should stop using the system and call your installer rather than running it.

Can I just reset it a few times until it goes away?

No. One power cycle is fine to rule out a one-off glitch. If M1/M2 Close Fault keeps coming back, the relay has failed mechanically and repeated resets will not fix it. The inverter needs a qualified technician to repair or replace it.

Helpful guides

Error codes explained How to safely restart your inverter Inverter not turning on

Sources

  • Fox ESS H1/AC1 inverter user manual and FoxCloud alarm list (M1/M2 Close Fault = grid relay stuck or welded, inverter cannot disconnect; service required).
  • Fox ESS installer and service guidance on relay faults and safe shutdown sequence before any inspection.
⚠️ Safety disclaimer. Solar inverters carry lethal DC and AC voltage even when "off". This guide covers a single controlled power cycle using the external isolators only. A failed relay, any AC wiring or internal work must be done by a licensed installer or electrician.