U0656 code meaning
Lost Communication With Turbocharger/Supercharger Boost Control A Position Sensor
Network system Generic code U06xx codes Check engine light
What U0656 means
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.
U0656 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyHard
- Typical labor0.5 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0656
- MIL on
- Limp mode with clearly reduced power
- Boost control codes stored at the same time
Common causes of U0656
- Most likelySENT, supply or ground line to the VGT actuator damaged
- PossibleConnector at the actuator exposed to heat, oil or water
- PossiblePosition sensor or electronics in the VGT actuator failed
- Less likelyEngine controller input or software fault
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Boost control a position sensorVehicle bus wiring
What each part of U0656 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 56 | The exact fault in that area. |
What to do next
- Note U0656 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0656
What does U0656 mean?
U0656 means "Lost Communication With Turbocharger/Supercharger Boost Control A Position Sensor". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with U0656?
Drive gently and only to a workshop. With U0656 the car can go into limp mode (reduced power) to protect itself.
Will U0656 fail an emissions test?
Yes, it can. U0656 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is U0656 to fix?
Difficulty: hard. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will U0656 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.