Cummins Fault Code
Free · no signup122 (SPN 102 FMI 3)
Significant power derate; Black smoke; Engine may feel "choked" or "lazy."
System
Not specified
Symptom
Significant power derate; Black smoke; Engine may feel "choked" or "lazy."
Diagnosis & fix steps
Intake Manifold Pressure (Boost) Sensor - Voltage High.
LOGIC: Signal > 4.5V (Open Circuit). The ECM loses the ability to measure boost and assumes a default failed state.
ACTION: 1. Unplug sensor and Jumper Signal to Return; if code becomes 123 (FMI 4), harness is good.
2. Verify 5V Ref on Pin A.
FIX: Replace Boost Sensor or repair broken wire in the harness loom near the intake.
Red Seal field insights
These sensors are often caked in soot. Before replacing, ensure the small orifice in the manifold isn't plugged with carbon, which can sometimes cause "implausible" high voltage readings.
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