Caterpillar Fault Code
Free · no signupSPN 108 FMI 4
Engine is "lazy"; Poor fuel economy; Struggles at high altitude.
System
Not specified
Symptom
Engine is "lazy"; Poor fuel economy; Struggles at high altitude.
Diagnosis & fix steps
Barometric Pressure Voltage Low.
LOGIC: Signal < 0.5V (Short to Ground). The ECM thinks the machine is at 15,000+ ft and cuts fuel to prevent smoke.
ACTION: 1. Unplug sensor; check if code changes to FMI 3.
2. Check for 5V supply and ground.
3. Ensure the sensor atmospheric vent isn't plugged with mud.
FIX: Replace Baro Sensor or repair signal wire shorted to frame.
Red Seal field insights
These sensors have a small "breather" hole. If you pressure wash the engine and force water into that hole, it will short the sensor internally.
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