Fronius Primo / Symo State 307, State 475 & State 522: DC Low & Insulation Error Fix
Fronius Primo or Symo inverter displays "State 475 (Solar module ground, insulation fault)" or remains stuck on "State 307 (DC Low)" / "State 522 (DC1 Input Low)" during sunny weather.
Safe LCD Diagnostic Check
🔬 Root Cause Electrical Physics
State 475 means the resistance between the ungrounded PV array and earth ground dropped below the 100 kOhm threshold (usually wet rooftop MC4 plugs or water in an attic transition box). State 307/522 during full sun indicates an open string fuse, a failed rooftop Tigo/APsmart rapid-shutdown transmitter, or a melted DC disconnect switch inside the Fronius SnapINverter base.
⚡ Step-by-Step Multimeter & Diagnostic Test Procedure
On the Fronius LCD display, press the capacitive buttons to view live DC1 and DC2 string voltages.
If DC1 shows 0V at noon, open the lower SnapINverter wiring compartment (after switching DC disconnect OFF) and test continuity across the internal string fuses and check the integrated rotary DC switch contacts for thermal pitting.
🛠️ Safe DIY Reset & Remediation Steps
- 1.Remember that State 307 and State 522 are 100% normal at sunrise, sunset, and under heavy snow.
- 2.If State 475 appears after heavy rain, inspect the rooftop junction box (SolaDeck) and external conduit fittings for water intrusion.
👷 When to Escalate to a Licensed Solar Electrician
Early Fronius Primo SnapINverter lower wiring bases had a known service bulletin for high-resistance DC rotary disconnect switches—an electrician can replace just the lower connection bracket without replacing the upper inverter power stage.
🔋 Direct Replacement & Upgrade Hardware
Upgrading an aging 10-year-old Fronius string inverter to an EG4 18kPV (/go/eg4) lets you plug your existing high-voltage strings right in while adding 240V LFP battery backup.
Frequently Asked Questions: Fronius State 307 & State 475 Fix
What does Fronius State 475 mean?
State 475 is the Fronius code for an Isolation / Ground Fault (moisture or damaged wire insulation allowing DC current to leak from the solar array to the grounded mounting rails).