The DFC 90 uses 4 criteria to set the self-test failure bit.
1. Servo Fail - Compares expected current and voltage draws of the servos to the actual current and voltage draws of the servos. If the measured value is out of tolerance, the servo fail bit is asserted. (The measured servo readings must be within 500mA of the expected current draw, and within 15V of the expected voltage. The "expected" values are determined by the rate commands being sent to each servo.)
2. Temp Fail - Compares 3 onboard temp sensors to ensure they are within limits.
3. Voltage Fail - Compares the voltages at the internal power supplies against expected values. This test also includes a check on the clutch voltage when the servos are engaged to ensure the clutch is also engaged. (For 28V aircraft the clutch must read between 15-35V for "engaged" and 0-5V for "disengaged. On 14V aircraft it is 10.5-20V "Engaged" and 0-5V "disengaged".)
4. Current Fail - Compares current draw on the internal power supplies against expected values.
The #1 cause of spurious disconnects of the DFC is excessive current draw from a single servo.