Register 4004 always returns zero even when the system is active. What is wrong?

A zero reading most often means the Modbus TCP read interface is not active on the FEMS box. Open the FEMS online monitoring, go to Settings and the system profile, open the Modbus/TCP component, and confirm the read-access app is installed and enabled. Also verify the FEMS IP address and TCP port 502 are reachable from the Gateway.

The Gateway connects but the active power value looks wrong or inverted. Why?

The register is a signed int16 value in watts. Positive means grid import; negative means grid export. If the sign appears inverted relative to your metering convention, check the physical current-transformer orientation on the Fenecon Mini installation. Do not negate values in the portal; resolve polarity at the hardware level.

No Modbus TCP connection can be established to the FEMS box at all.

The device accessing the FEMS must be on the same physical local network as the FEMS box; TCP port 502 is not routed through the internet. Confirm the Gateway and FEMS are on the same LAN segment, check for firewall or VLAN rules blocking port 502, and verify the IP address entered in the portal matches the current FEMS address.

The FEMS firmware was recently updated and Modbus readings stopped. What changed?

FEMS firmware updates have occasionally changed register accessibility or required the Modbus/TCP app to be re-activated. After an update, log into the FEMS online monitoring, open the App Center, and confirm the Modbus/TCP read-access app is still installed and listed as active. Reconnect the Gateway once the app is confirmed running.

Free Modbus template: Fenecon Mini GridMeter

Name: Fenecon Mini GridMeter
Brand: Fenecon
SKU: 8de951c7-41ca-4106-8e4c-7d604b72dd50

The Fenecon Mini GridMeter is the integrated grid-metering component of the Fenecon Mini home energy storage system, managed by the FEMS (FENECON Energy Management System). It runs over Modbus TCP and exposes a single register: active power at the grid connection point (address 4004, int16, watts). Positive values indicate power drawn from the grid; negative values indicate export to the grid. Connecting the ModbusCloud Gateway to the same local network as the FEMS box lets your portal continuously record grid import and export flows, giving installers a reliable, time-stamped power baseline for auditing self-consumption performance.