Cummins Fault Code

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122 (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.

Same code keeps coming back?

That's a program problem, not a part problem. Book a 30-minute uptime review with Grant, a Red Seal Heavy Duty Technician.

Book an uptime review

Read before you touch anything

  • This is general reference information, not a repair manual. It is for trained, qualified technicians.
  • Fault codes, meanings and procedures vary by model, serial number and software version. Confirm the code and the procedure against the manufacturer's service manual before acting.
  • Follow the OEM service manual, torque specs and your site's safety procedures: lockout/tagout, block raised equipment, relieve hydraulic, fuel and air pressure, and let hot systems cool.
  • You use this information at your own risk. Adventure Mechanical and AM Fleet Integrity accept no liability for damage, injury, downtime or loss arising from its use.
  • Not affiliated with or endorsed by any manufacturer. Brand names are used only to identify the equipment a code applies to.

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