Question 1

What does error code State 509 / Grid Overvoltage u Fronius / GoodWe / Solax mean and how to fix it?

Accepted Answer

Grid overvoltage (above 253 V) – inverter disconnected from grid. Troubleshooting & fix: High grid voltage around midday; inspect terminal contact resistance in the distribution board and check the 10-minute average voltage setting.

Question 2

What does error code Isolation Fault / ISO Error u Fronius / GoodWe / Solax mean and how to fix it?

Accepted Answer

Low PV array insulation resistance to ground (moisture inside MC4 connector). Troubleshooting & fix: Water ingress inside a rooftop MC4 connector or solar DC cable chafed against roof tiles/mounting rails (most common during morning dew).

Question 3

What does error code BMS / Battery Low Voltage u Fronius / GoodWe / Solax mean and how to fix it?

Accepted Answer

Deep discharge of battery storage bank in winter. Troubleshooting & fix: Trigger a grid Force Charge or inspect the battery fuse disconnector.

Solar Inverter Faults: State 307, 509, Isolation Fault, Grid Overvoltage – Error Code Meaning, Troubleshooting & Repair

Error Code Reference Table for Fronius / GoodWe / Solax and step-by-step repair procedure:

Error Code	Meaning / Fault Symptom	Most Common Cause & Repair Procedure
State 509 / Grid Overvoltage	Grid overvoltage (above 253 V) – inverter disconnected from grid	High grid voltage around midday; inspect terminal contact resistance in the distribution board and check the 10-minute average voltage setting.
Isolation Fault / ISO Error	Low PV array insulation resistance to ground (moisture inside MC4 connector)	Water ingress inside a rooftop MC4 connector or solar DC cable chafed against roof tiles/mounting rails (most common during morning dew).
BMS / Battery Low Voltage	Deep discharge of battery storage bank in winter	Trigger a grid Force Charge or inspect the battery fuse disconnector.