SPN 2659 FMI 0: Turbocharger boost pressure control error – overboost or underboost

What this fault means

SPN 2659 FMI 0 indicates an air or turbocharger-control condition where the monitored value is above the expected range. Depending on the engine calibration, it may appear with overboost, incorrect turbo control or another air-management problem.

Variable-geometry turbo systems depend on correct actuator movement, clean exhaust flow, accurate pressure sensing and proper charge-air plumbing. A problem in any of these areas can cause the ECM to lose control of boost.

What you may notice

Possible symptoms include surging, reduced power, unusual turbo response, changes in turbo sound, smoke or additional boost and air-system faults.

What to check first

Inspect the charge-air system for loose clamps, damaged boots, cracked piping and other leaks. Check the turbo actuator and linkage for smooth movement and verify that commanded and actual turbo position agree when the diagnostic tool supports those values.

Inspect related pressure and temperature sensors because incorrect sensor data can make a mechanically good turbo appear faulty.

Soot and sticking concerns

On engines using a variable-geometry turbocharger, soot accumulation can restrict vane movement. Frequent idling, low-load operation and other combustion problems may contribute to deposits, but the turbo should not be condemned without testing actuator operation and the rest of the air system.

Before replacing the turbo

Check companion EGR, boost-pressure and exhaust-pressure codes. Confirm that the problem is not caused by wiring, a sensor, a charge-air leak or actuator control before replacing an expensive turbocharger assembly.

Use the OEM diagnostic procedure because SPN mappings and acceptable boost values differ between engine families.