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
SPN 102 FMI 4Boost pressure sensor circuit voltage below normal · CATSPN 102 FMI 4Boost pressure sensor circuit voltage below normal · DetroitSPN 102 FMI 4Boost pressure sensor circuit voltage below normal · InternationalSPN 102 FMI 4Boost pressure sensor circuit voltage below normal · PACCARSPN 102 FMI 4Boost pressure sensor circuit voltage below normal · MERCEDESSPN 102 FMI 4Boost pressure sensor circuit voltage below normal · VolvoSPN 102 FMI 4Boost pressure sensor circuit voltage below normal · MackSPN 102 FMI 4Boost pressure sensor circuit voltage below normal · CumminsSPN 171 FMI 4Ambient air temperature sensor low · MackSPN 1172 FMI 4Turbo inlet temperature sensor low · InternationalSPN 1172 FMI 4Turbocharger compressor inlet temperature sensor voltage low · CATSPN 1172 FMI 4Turbocharger compressor inlet temperature sensor voltage low · DetroitSPN 1172 FMI 4Turbocharger compressor inlet temperature sensor voltage low · PACCARSPN 1172 FMI 4Turbocharger compressor inlet temperature sensor voltage low · MERCEDESSPN 1172 FMI 4Turbocharger compressor inlet temperature sensor voltage low · VolvoSPN 1172 FMI 4Turbocharger compressor inlet temperature sensor voltage low · MackSPN 1172 FMI 4Turbocharger compressor inlet temperature sensor voltage low · Cummins
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