What Does This Error Mean?

Fronius State 408 means the inverter cannot detect the AC grid — it has lost its connection to mains power. Check that the AC isolator switch near the inverter is in the ON position and that the solar circuit breaker in your main switchboard has not tripped.

Affected Models

Fronius Primo
Fronius Symo
Fronius Galvo
Fronius IG Plus
All Fronius grid-tied string inverters

Common Causes

AC isolator switch (solar AC disconnect) in the OFF position — must be ON for inverter to operate
Solar circuit breaker in the main switchboard tripped
Power outage — inverter shuts down and shows State 408 during any grid outage
Loose AC wiring connection at the inverter AC terminal
Fronius inverter installed on a circuit that lost phase (3-phase systems)

How to Fix It

Check the AC isolator switch.

The AC isolator (solar AC disconnect) is a red or grey switch mounted near the Fronius inverter. Confirm it is in the ON position — if it is in OFF or has a lock on it, switch it to ON. This is the most common cause of State 408 — the AC isolator is sometimes switched off by household members who do not realize what it does.
Check the solar circuit breaker in the main switchboard.

Open your home's main electrical switchboard and locate the breaker for the solar/PV system. If it has tripped (moved to the middle or OFF position), reset it by switching fully off then fully on. If it trips again immediately, there is a fault on the AC wiring — contact a licensed electrician.
Confirm there is no grid power outage.

Fronius grid-tied inverters cannot operate without the AC grid — State 408 is the normal shutdown code during any power outage. Check if other appliances in your home have power — if not, the grid is down and the inverter will resume automatically when power is restored. Note: Fronius IQ8 (IQ microinverters) and systems with Enphase IQ Batteries have different grid-independence behavior.
Inspect the AC wiring connections at the inverter.

A licensed electrician should open the inverter AC terminal cover and verify that the Line (L), Neutral (N), and Earth (E) conductors are firmly connected. A loose terminal — especially the live (L) conductor — will cause State 408 even when the AC supply is present. Do not open the inverter yourself — AC terminals carry lethal voltage.
Check for a phase loss on 3-phase systems.

On 3-phase Fronius Symo installations, the loss of any single phase will trigger State 408 on some firmware versions. Check all three phase breakers in the switchboard — if one has tripped while the others are on, the Fronius will see no full AC supply. Reset any tripped phase breaker and the Fronius should restart within 60 seconds.

When to Call a Professional

If the AC isolator is on and the circuit breaker is on but State 408 persists, there may be a wiring fault or the inverter's AC detection circuit has failed. Contact a licensed electrician.