FMI 4 — Voltage below normal or shorted to low source
The signal is pinned low — usually the signal wire touching ground, or the 5 V reference lost.
Common causes
- Signal wire shorted to ground or to the return wire
- Open 5 V reference feeding the sensor
- Internally shorted sensor
- Corroded or water-filled connector
What to test, in order
- Unplug the sensor and check for the 5 V reference at the connector
- With the sensor unplugged, check the signal pin for continuity to ground
- Jumper the signal to reference through a resistor and watch the value swing
- Inspect the shared reference — one shorted sensor can pull the whole 5 V circuit down
Codes we document with FMI 4
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