SPN 1127 FMI 16 — Turbo Boost Pressure — High / Overboost
Measured boost is higher than commanded (overboost), or the boost reading is high. The ECM derates to protect the engine. On a variable-geometry turbo (VGT) this is frequently a sticking vane mechanism rather than a blown turbo.
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How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Sticking VGT vanes (carbon/soot) holding too much boost
- VGT actuator fault or calibration issue
- Boost (MAP) sensor reading high or a wiring fault
- Wastegate/control fault on a wastegated turbo
First moves (roadside)
- Compare commanded vs actual boost — a large overshoot points to VGT/wastegate control.
- Check the boost sensor and wiring before assuming a mechanical turbo fault.
- Look for companion VGT actuator/position codes.
- Avoid heavy load until repaired; repeated overboost can damage the engine.
Common misdiagnoses
- Replacing the whole turbo when sticking VGT vanes just needed service/calibration.
- Condemning the actuator when the boost sensor was reading high.
- Clearing the derate without fixing the boost-control cause.
FAQ
Measured boost is higher than commanded (overboost), or the boost reading is high. The ECM derates to protect the engine. On a variable-geometry turbo (VGT) this is frequently a sticking vane mechanism rather than a blown turbo.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Sticking VGT vanes (carbon/soot) holding too much boost. Other possibilities include VGT actuator fault or calibration issue; Boost (MAP) sensor reading high or a wiring fault; Wastegate/control fault on a wastegated turbo.
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.