Inverter isolation fault error: causes and fix

Your inverter shows 'ISO fault', 'low impedance', 'riso low' or similar. It's the protection detecting a loss of isolation between panels and ground. Without fixing it, it won't produce. Here are the real causes.

What it means exactly

The inverter constantly measures resistance between DC conductors (positive and negative) and ground. Must be >1 MΩ. If it drops to kΩ, there's a stripped cable, water in a connection, or a damaged panel touching the rack.

Cause 1: moisture in MC4 connector

Most common. A bad-crimped or unsealed MC4 lets water in. Morning (dew) triggers the error; afternoon it dries and goes away. Inspect all MC4s with flashlight: green oxidation or dirt = replace pair.

Cause 2: flooded panel junction box

Rear junction box with defective seal. Water inside shorts a cell to the frame ground. Identify affected panel: isolating one series at a time. Replace the panel.

Cause 3: degraded PV1-F cable

10+ years of UV degrade the insulation. Cable stripped and touching frame metal: ground leak. Full visual inspection of the run. Replace affected length.

Cause 4: leak in buried cable

Cable in buried conduit at foot of post or rack. Rodents chew insulation. Only visible by digging. Seasonal (rain) issue points here.

How to locate the panel/cable

Power down inverter. Disconnect all strings. Connect one alone: if it starts without error, that string is fine. Continue one by one. Within the bad string: isolate panel by panel.

When to call a pro

If visual inspection finds nothing: you need a 1000 V megger ($350-600) measuring panel-by-panel insulation. Pro: $250-450 to locate. Don't ignore: isolation fault can deliver lethal shock.

To avoid the issue, read correct grounding.