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U0656 Lost Communication With Turbocharger/Supercharger Boost Control A Position Sensor

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

On a variable geometry turbocharger the boost control position sensor reports where the guide vanes stand, and the controller needs that feedback to regulate boost and to check, as CARB 1968.2 (f)(7.2.3) asks, that the vanes reach their target in time. GM's 2025 diesel table 25OBDG04B reads this VGT position by SENT and sets U0656 when 480 of 600 checks, run every 6.25 ms, find the protocol stuck low or high with stale data, a message error, or counters that no longer update; it is Type A with the MIL in the first trip. Low raw signal level on the same sensor is a different code there, P2564. Analog circuit faults are P2562 to P2566, charger B uses U02B0. The vanes usually go to a safe position and power drops.

Likely causes

SENT, supply or ground line to the VGT actuator damaged

Connector at the actuator exposed to heat, oil or water

Position sensor or electronics in the VGT actuator failed

Engine controller input or software fault

Symptoms

MIL on
Limp mode with clearly reduced power
Boost control codes stored at the same time

Repair estimate

DifficultyHardWorkshop job

Parts and labour€80–1,200

Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

boost control a position sensorvehicle bus wiring

References: gsi.ext.gm.com, en.wikipedia.org, law.cornell.edu.