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Problem diagnosis · Fronius
Fronius State Code Alarm — Inverter Error Codes Explained
Your Fronius inverter is displaying a numbered state code — a status message telling you something abnormal has been detected. Not all codes are faults. Some clear on their own, some need a manual reset, and some need an engineer. The first digit of the code tells you which.
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State code keeps coming back?
A recurring state code means the underlying condition has not been resolved. We review your Solar.web event history remotely to identify the pattern, classify the fault, and recommend the right fix — before booking an unnecessary site visit.
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Quick reference — state code classes
1xx — Temporary grid issue · auto-reconnects
3xx — Operational message · does not stop generation
4xx — Hardware / software fault · may need engineer
5xx — Non-critical notification · acknowledge to clear
Diagnostics
6-step state code diagnosis
Work through these steps to identify what your Fronius state code means, whether it needs action, and what that action should be.
1
Find the state code on your inverter or Solar.web
Fronius state codes appear in two places. On the inverter itself, the LCD display shows the current state code number and a short description. In Solar.web — Fronius's monitoring portal — the event log records every state code with a timestamp, so you can see exactly when each event occurred and how long it lasted.
Open Solar.web, navigate to your system, and check the event log. Write down the exact code number and when it first appeared. If the inverter display is showing a code but the system is still producing power, that is a Class 3 informational code. If production has stopped entirely, the code is more likely Class 1 (grid) or Class 4 (hardware).
2
Identify the code class from the first digit
The first digit of the state code tells you the severity and whether you need to act:
• 1xx — Grid issues — the inverter has disconnected because grid voltage or frequency is outside safe limits. It will reconnect automatically when the grid stabilises. State 102 (voltage too high) is the most common in this class.
• 3xx — Operational — informational messages during feed-in. State 306 (power low) and 307 (voltage low) appear at dawn and dusk when there is not enough sunlight to generate — this is completely normal.
• 4xx — Hardware fault — the inverter has detected a component failure. State 407 (temperature sensor), 401 (communication), and 457 are the most common. These usually need an engineer.
• 5xx — Notifications — non-critical messages that display until you acknowledge them. State 509 (no feed-in for 24 hours) is the most common and may indicate snow cover, heavy soiling, or a genuine fault.
3
Check whether the code is temporary or persistent
Open the Solar.web event log and look at the pattern over time. A code that appeared once during a grid disturbance and cleared on its own is temporary — no action needed. A code that appears every day at the same time, such as state 102 every sunny afternoon, points to a recurring grid overvoltage issue that may need investigation.
State 306 and 307 appearing only at dawn and dusk are completely normal — they indicate insufficient light for feed-in. A Class 4xx code that persists after a restart is a hardware fault. The key question is whether the code is reducing your energy production during the hours when the system should be generating.
4
Try a manual reset for non-hardware codes
For Class 1xx and 5xx codes, pressing the button on the inverter display clears the alarm and lets the inverter attempt reconnection. For a full power cycle:
1. Switch off the AC isolator at the consumer unit
2. Switch off the DC isolator below the inverter
3. Wait three minutes for the capacitors to discharge
4. Switch the DC isolator back on, then the AC isolator
5. Allow five minutes for the inverter self-test to complete
If the code returns immediately after restart, do not keep cycling the power. The fault is persistent and needs professional diagnosis. Class 4xx codes that return after a reset confirm a genuine hardware problem.
5
Review the Solar.web event history for patterns
The most useful diagnostic information is the pattern over time. Check the event log covering at least the last 30 days and look for correlations:
• State 102 every sunny afternoon — grid overvoltage from high solar export in your area. Multiple homes feeding in simultaneously pushes the local voltage above the safe limit.
• State 509 after several cloudy days — check panels for snow, heavy soiling, or bird mess before assuming a fault.
• A 4xx code with a matching drop in daily yield — confirms the fault is actively reducing your production and needs attention.
This event history is exactly what a diagnostic engineer needs to identify the root cause remotely — without an unnecessary site visit.
6
Book a diagnostic if the code keeps returning
If a state code is recurring and affecting your energy production, it needs professional diagnosis. STS reviews your Solar.web event log and production data remotely to identify whether the issue is grid-related, weather-related, or a hardware fault — before anyone visits the property.
For grid overvoltage issues (state 102), the solution may involve contacting your Distribution Network Operator. For hardware faults (4xx), the engineer can identify the likely component before attending site — reducing diagnosis time and cost.
Why state code 102 is increasingly common across the UK
State 102 — AC voltage too high — is the most common recurring Fronius state code in the UK. The inverter disconnects because it has measured grid voltage above the safe threshold, typically above 253 volts on the UK nominal 230V supply. It reconnects automatically when the voltage drops, but if this happens repeatedly during peak sun hours, the system loses significant generation.
The underlying cause is grid infrastructure that was not designed for widespread solar export. On rural and suburban networks with multiple solar installations, simultaneous export during midday pushes the local voltage upward. The houses furthest from the transformer see the highest voltage and are the first to trip. This is a network issue, not an inverter fault — reporting it to the Distribution Network Operator is the correct next step, as they may need to adjust the transformer tap or reinforce the local network.
Fronius inverters operating under G98/G99 grid codes in the UK have specific voltage and frequency trip settings. In some cases, an installer can adjust the voltage threshold within the allowed range to reduce nuisance tripping — but this must be done by a qualified person with access to the inverter service menu and only within the parameters permitted by the grid code.
Reference
Most common Fronius state codes in the UK
102
AC voltage too high
Grid voltage exceeds the safe limit. Common on networks with high solar penetration. Auto-reconnects when voltage drops. Report to DNO if it happens daily.
306 / 307
Power low / voltage low
Insufficient DC power or voltage to feed in. Normal at dawn and dusk. Only a concern if it appears during peak sun hours — check for panel shading, soiling, or a DC wiring issue.
407
Temperature sensor faulty
Heat sink temperature sensor has failed or disconnected. Hardware fault — try a power cycle first. If it returns after restart, the inverter needs professional repair.
509
No feed-in for 24 hours
The system has not generated for a full day. Check for snow, heavy soiling, or a tripped AC breaker before assuming a fault. If panels are clean and breakers are on, the inverter needs investigation.
475 / 502
Isolation fault / earth fault
DC insulation resistance has dropped below the safe threshold. Almost always caused by water ingress. Do not restart — see our dedicated isolation fault guide.
State code alarms — common questions
A state code is a numbered status message that Fronius inverters display when they detect an abnormal condition. Class 1xx codes are temporary grid issues, 3xx are operational messages during low light, 4xx are hardware or software faults, and 5xx are non-critical notifications. Not all state codes are actual faults — many represent normal operating conditions such as insufficient light at dawn and dusk. The code appears on the inverter LCD and is logged in Solar.web with a timestamp.
State 102 means the grid voltage at your property is too high. The inverter disconnects to protect itself and the network, then reconnects automatically when the voltage drops. In the UK, this commonly happens on rural or suburban networks with high solar penetration — when multiple systems export simultaneously, local voltage rises above the 253V limit. If it happens daily during peak sun, the grid in your area may need reinforcement. Contact your Distribution Network Operator to report the issue.
For Class 1xx and 5xx codes, yes — press the button on the inverter display to clear the notification, or do a full power cycle by switching off the AC isolator then the DC isolator, waiting three minutes, and switching them back on in reverse order. Class 3xx codes clear automatically when conditions improve. However, Class 4xx codes that return after a reset indicate a hardware fault that needs professional diagnosis. Repeatedly power-cycling an inverter with a persistent 4xx code will not fix it.
State 407 means the temperature sensor on the inverter heat sink is faulty or disconnected. This is a Class 4 hardware fault — the inverter cannot safely monitor its own temperature, so it shuts down as a precaution. Try a full power cycle first. If state 407 returns after restart, the sensor has failed and the inverter needs professional repair or replacement. This fault is more common on older Primo and Symo units where years of thermal cycling degrade the sensor connection.
Log into Solar.web at solarweb.com, select your system, and navigate to the event log or notifications section. Each state code is recorded with the code number, a short description, and the date and time. You can filter by date range to see patterns over weeks or months. The Solar.web mobile app for iOS and Android also shows current codes and push notifications. If you cannot access Solar.web, the code is also shown on the inverter LCD — press the display button to cycle through current and recent codes.
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Recurring state code? We find the cause remotely.
We review your Solar.web event history and production data to identify whether the issue is grid-related, weather-related, or a hardware fault — before booking a site visit.
- Not affiliated with Fronius
- No fix, no fee
- Covers all Fronius state code classes
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