Fault code index · Fronius inverter all models
Fronius Fault Code Index
Every Fronius fault code and state code — what it means, the most likely cause, and what to do. Find your code in the index below or use the category sections to browse by system area.
Fronius Primo, Symo, Gen24 and GEN24 Plus
Sourced from Fronius technical documentation
State codes and error codes explained
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How to find your fault code. Fronius codes appear on the LED status lights on the inverter front panel. For detailed information, log into Fronius Solar.web → My Systems → [Device] → Events. The event log shows code numbers (State 240, Error 301, etc.) with timestamps and descriptions. Note the exact code number including the prefix — this indicates the code type and severity.
State codes
State codes indicate normal operating conditions, temporary restrictions, or routine maintenance — not faults. The inverter continues to operate normally with these state codes active.
State 240
Calibration active
Inverter is performing automatic calibration — a routine part of normal startup. No power will be generated during this brief period. This is normal behaviour and clears automatically. Usually lasts only a few seconds.
State 307
Active power limit engaged
Fronius is limiting output power as configured — export limit (power limitation due to grid requirements) is active. The inverter is generating but power is being capped. Check your Power Limitation settings in Solar.web to confirm the intended limit value.
State 406
Waiting for grid voltage
Inverter has disconnected from the grid due to an undervoltage or frequency event and is now waiting to reconnect. Typical reconnection delay is 5 minutes. No power is being generated during this hold period. Grid voltage and frequency must return within normal limits before the inverter resumes operation.
Grid & AC faults
Fault codes generated in response to AC grid voltage or frequency being outside safe operating limits. These cause the inverter to immediately disconnect from the grid.
Error 301
⚠ Grid overvoltage
AC grid voltage has exceeded the upper safety limit (typically above 253V). Inverter has disconnected. This is usually a temporary grid disturbance. Monitor the event log — if the fault is persistent, contact your DNO about grid quality issues in your area.
Error 302
Grid undervoltage
AC grid voltage has dropped below the lower safety limit (typically below 216V). Inverter has disconnected. This often occurs during high-load periods on the local network. Wait for grid voltage to recover — the inverter will reconnect automatically.
Error 303
Grid overfrequency
Grid frequency has exceeded the upper safety limit (above 50.2Hz). Inverter disconnected. Frequency disturbances are usually caused by grid events affecting many users — check your local grid status. The inverter will reconnect when frequency normalizes.
Error 304
Grid underfrequency
Grid frequency has dropped below the lower safety limit (below 47.5Hz). Inverter disconnected. This indicates a significant grid disturbance — wait for frequency to recover. The inverter will reconnect automatically once frequency returns to normal range.
Error 306
⚠ No mains connection
Grid power has been lost completely — AC supply is missing. Check circuit breakers at the consumer unit on the solar output circuit. Check the AC isolator below the inverter is switched ON. Verify the AC terminal connections are tight and not corroded.
DC & Insulation faults
Fault codes related to DC string voltage, insulation resistance, and ground faults. These faults typically prevent the inverter from starting or cause an immediate trip if detected during operation.
Error 401
High string current
DC string current is exceeding the inverter's safe input limit. Typically caused by incorrect panel array configuration — panels may be wired with too many in parallel. Check panel string sizing against Fronius documentation for your inverter model.
Error 402
String overvoltage
DC string voltage is exceeding the maximum input voltage (Voc) for this inverter. Check panel array configuration — may have too many panels in series. Do not touch DC cabling. Contact an engineer to review the string design.
Error 403
No DC power
DC input voltage is below the minimum start-up threshold. Check the DC string isolator (the breaker on the positive DC cable) is switched ON. Verify all MC4 connectors between panels and inverter are fully seated and locked. Check for panel shading or cloud cover reducing panel output.
Error 404
⚠ Isolation fault — URGENT
Leakage current has been detected between the DC string and earth — a ground fault. This is a critical safety issue. Isolate the system immediately at the consumer unit. Do not touch any DC cabling. Check for water ingress at MC4 connectors, damaged insulation on string cables, or a faulty panel junction box. Contact a qualified engineer before any restart.
Error 405
⚠ Arc fault detected — URGENT
A DC arc has been detected in the PV string — this is a fire safety hazard. Isolate the system at the consumer unit immediately. Do not touch any DC cabling or attempt to restart the system. Contact a qualified engineer to inspect string cabling and connectors for damage or loose connections.
Internal & thermal faults
Fault codes originating from inside the inverter — processor communication failures, cooling fan issues, and temperature protection events.
Error 501
DSP communication fault
Internal processor communication fault — the two processing units inside the inverter have lost synchronization. Perform a full power cycle (AC isolator off, wait 60 seconds, back on). If the fault persists, the control board may require a firmware reflash or replacement — contact Fronius support or a qualified engineer.
Error 502
Fan failure
Cooling fan is not operating. The inverter will derate (reduce power output) or shut down to protect internal components from overheating. Check for any visible debris or blockage around the inverter cooling vents. Ensure 20cm minimum clearance on all sides. If fan does not resume operation after restart, the fan motor may have failed and requires replacement.
Error 503
Overtemperature
Internal heatsink temperature has reached the thermal shutdown threshold. Check ventilation — ensure minimum 20cm clearance on all sides, no direct sunlight exposure, and no heat sources nearby. Clear any dust from cooling vents using compressed air. If ambient temperature is normal and clearances are adequate, the internal temperature sensor may be faulty.
Frequently asked questions
How often should I expect to see state codes on my Fronius? ↓
State 240 (calibration) typically appears once monthly or after firmware updates — this is normal. State 307 (power limit) appears whenever your export limit is active. State 406 (waiting for grid) only appears after grid disturbances. None of these indicate a problem with your system.
Why does my Fronius show Error 301 even though my grid voltage seems normal? ↓
Grid voltage varies constantly — it may be at normal levels during off-peak times but spike above 253V during peak generation periods, especially with many solar systems exporting power. Monitor how often Error 301 appears in your event log. If it's frequent, contact your DNO to investigate grid quality in your area.
Can I read Fronius fault codes without logging into Solar.web? ↓
Yes — many faults show as LED codes on the inverter's front panel. However, for complete diagnostic information including timestamps and detailed descriptions, you'll need to log into Fronius Solar.web → My Systems → Events. This index will help you interpret both the LED codes and the event log entries.
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