FMI 3 — Voltage above normal or shorted to high source

The measured voltage on the circuit is higher than the sensor could legitimately produce — an electrical fault, not a process condition.

Start free J1939 (CAN) → canonical

Diagnosis

When you hit this

A signal wire chafed against a supply line.
An open circuit on a sensor whose input is pulled high when disconnected. Unplugging the sensor produces FMI 3 on many designs.
Water in a connector bridging signal to supply.

Resolution

How to fix it

Do not trust the reading
Unlike FMI 0, the value backing an FMI 3 is meaningless. Exclude it from any calculation rather than acting on it.
Unplug to differentiate
If disconnecting the sensor clears FMI 3 and produces FMI 5, the fault is in the harness rather than the sensor.
Inspect connectors first
Corrosion and water ingress cause far more FMI 3 and 4 codes than failed sensors do.

Normalization

What Forge does about it

This code is reported by the device itself, so it survives normalization as data. Forge maps the vendor's fault register into the canonical fields alarm_code and alarm_description (and alarm_severity where the vendor supplies one), so an agent reads the same three fields whether the fault came from a Haas, a SINUMERIK, or a Cat engine. Forge does not clear the fault — that is the controller's job.