U0657: Lost Communication With Turbocharger/Supercharger Bypass Valve A Position Sensor

What it means

A control module has stopped receiving expected network messages from the turbocharger/supercharger bypass valve A position sensor. The effect on boost control depends on the vehicle's system design.

At a glance

• Usually lights the warning light of the system that lost communication
• Affects emissions; the car can fail an emissions test

How it works

The bypass valve A position sensor reports the turbocharger or supercharger bypass valve's position to vehicle control modules over the communications network. This code sets when a module stops receiving the expected messages from that sensor, so it can no longer confirm the valve's position for boost control.

Common causes

Most likely first.

1. Bus wiring open or shorted
2. Module without power or ground
3. Module failed

Symptoms

• Warning light or message related to the affected system
• Engine performance may be affected

How to diagnose it

1. Use a scan tool to record freeze-frame data and check which modules report U0657 or other communication faults, since multiple faults may point to a shared network or power problem.
2. With the ignition off, inspect the bypass valve A sensor connector and nearby harness for loose pins, corrosion, chafing, or damage from heat and moving parts.
3. Check the sensor's specified fuses, power, and ground, because a sensor module without either supply cannot send network messages.
4. Inspect the relevant network wiring for opens, shorts, or poor connections, and test it against the manufacturer's specification.
5. If wiring and power are sound, use a capable scan tool to see whether the bypass valve A position sensor appears on the network and follow the vehicle's diagnostic procedure before condemning a module.

Common fixes

• Reconnect or secure a loose bypass valve A sensor connector.
• Repair damaged network wiring or corroded connector terminals at the bypass valve A sensor.
• Restore a failed fuse, power feed, or ground after finding the underlying cause.
• Replace the bypass valve A position sensor or its module only if testing confirms it has failed.

Can I keep driving?

If the engine runs normally and no other warnings are present, a short, gentle trip may be possible, but avoid hard acceleration or towing because boost control may be affected. Stop if power drops sharply or the engine runs poorly, and arrange diagnosis promptly.