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Fox ESS Iso Fault

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

Iso Fault (also shown as Isolation Fault, Ground Fault or Error 3) means your Fox ESS measured low insulation resistance between the DC circuit and earth. For safety it stops generating until the reading recovers. The most common trigger is moisture, so this fault often appears during or after rain and clears once everything dries out.

What usually causes it

  • Moisture in a panel junction box or connector. Water getting into a PV panel junction box or an MC4 cable connector is the number one cause, which is why it usually tracks wet weather.
  • Morning dew or high humidity. The fault can show first thing in the morning and clear by midday as the panels and cables dry.
  • A pinched, chafed or rodent-damaged DC cable where the insulation has worn through to a metal frame or roof structure.
  • A degraded panel or junction box seal that lets water in over time, which is a persistent fault rather than a passing one.

How to handle it safely

  1. Note the weather pattern. Write down whether the fault appears during or after rain, or in the morning, and whether it clears once things dry out. A clear wet-weather link points straight at moisture in the DC string side.
  2. Let it dry before judging. If it tracks rain or dew, give it a sunny spell. A moisture-driven Iso Fault commonly self-resolves once the panels and connectors dry.
  3. Isolate the PV at the inverter to test. You can turn the DC isolator under the inverter to OFF, then wait about 2 minutes for the internal capacitors to discharge before checking again. If the fault clears with the PV isolated, the issue is on the DC string side, not inside the inverter.
  4. Do not keep resetting a persistent fault. If the Iso Fault stays in dry conditions, stop power-cycling it and book an installer. Repeatedly clearing a real isolation fault leaves a safety issue unaddressed.

Quick decision flowchart

Iso Fault appears
↓ does it track wet weather?
✅ Wet or humid, clears when dry → likely moisture in a junction box or connector. Let it dry and keep noting the pattern.
❌ Persists in dry conditions → stop resetting. Book an installer to megohmmeter-test each string.
↓
Isolating the PV at the inverter clears it → confirms the problem is on the DC string side.
When to call a professional. A persistent isolation fault means current could be leaking to earth, which is a shock risk. If it does not clear in dry weather, do not keep resetting it and do not open DC connectors yourself. An installer will use a megohmmeter (insulation tester) to check each string and find the wet or damaged section safely.

Related Fox ESS codes

PV Volt Fault Res Cur Fault PV Connect Fault

FAQ

My Fox ESS shows Iso Fault every morning then runs fine, is that a problem?

If it appears with morning dew or after rain and clears by itself once the panels dry, it is almost always moisture in a junction box or connector. Keep a short log of when it happens. If the morning pattern gets worse, lasts longer, or starts showing on dry days, have an installer test the strings before it becomes a standing fault.

Can I just keep clearing the Iso Fault myself?

Only if it is clearly weather-driven and clears on its own. If the fault stays in dry conditions, do not keep resetting it. A persistent isolation fault is a real safety issue and needs an installer with a megohmmeter to locate it, not repeated power-cycling.

Helpful guides

Error codes explained How to safely restart your inverter

Sources

  • Fox ESS H1/AC1 inverter user manual and FoxCloud alarm list (Iso Fault / ISO Failure / Error 3 = insulation resistance to earth below the safe threshold).
  • Fox ESS installer guidance on PV string insulation testing and tracing moisture ingress in junction boxes and DC connectors.
⚠️ Safety disclaimer. Solar inverters carry lethal DC and AC voltage even when "off". This guide covers noting the weather pattern and using the inverter's own DC isolator only. Opening DC connectors, insulation testing and any wiring work must be done by a licensed installer or electrician.