MAV_RESULT 4 (4) — FAILED — command accepted but execution failed

The vehicle accepted the command and tried to execute it, but execution did not succeed.

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Diagnosis

When you hit this

A mission upload that failed partway through.
A parameter write rejected by the parameter subsystem.
A calibration routine that could not complete.

Resolution

How to fix it

Read STATUSTEXT for the reason
As with DENIED, the explanation is in the text stream rather than the ack.
Verify state afterwards
FAILED can leave partial state, such as half an uploaded mission. Read back and confirm rather than assuming a clean rollback.

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.