U0689: Lost Communication With Turbocharger/Supercharger B Temperature Sensor

What it means

The vehicle bus reports loss of communication with smart turbocharger B temperature sensor. Turbo overheat protection switches to a model-based estimate.

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 turbocharger/supercharger B temperature sensor is a smart sensor that sends its readings to other vehicle modules over the communication bus. This code sets when the bus stops receiving messages from that sensor, so turbo overheat protection must rely on an estimated temperature instead of a live reading.

Common causes

Most likely first.

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

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Save freeze-frame data and scan all modules for communication codes to see when the turbo temperature signal dropped out and whether other devices on the same bus were affected.
2. Check the turbocharger/supercharger B temperature sensor connector and nearby harness for loose pins, corrosion, heat damage, or chafing.
3. Check the sensor module's power and ground under the conditions specified by the manufacturer, since a supply or ground loss can make it go offline.
4. Inspect the bus wiring to the B temperature sensor for opens, shorts, or damaged splices, and test continuity and shorts using the manufacturer's procedure.
5. If wiring, power, and ground are sound, use a capable scan tool to check whether the sensor communicates; replace the sensor/module only after confirming the bus and its supply circuits are good.

Common fixes

• Reconnect or repair a loose, corroded, or heat-damaged connector at the B temperature sensor.
• Repair an open or short in the bus wiring or a damaged splice serving the sensor.
• Restore the sensor module's power or ground connection.
• Replace the turbocharger/supercharger B temperature sensor module if it remains offline with sound wiring and power.

Can I keep driving?

A short, gentle trip may be possible if the engine runs normally and no overheating or boost warning appears, but avoid heavy acceleration, towing, and sustained boost because overheat protection lacks the live B sensor reading. Stop if the engine overheats, power drops sharply, or additional warnings appear; continued operation could expose the turbocharger to excessive heat.