U0679: Lost Communication With Transmission Fluid Pressure Sensor/Switch A

What it means

The vehicle bus reports loss of communication with smart transmission fluid pressure sensor / switch A. Hydraulic pressure feedback for shift control is unavailable; TCM may switch to default shift logic.

At a glance

• Usually lights the warning light of the system that lost communication
• The engine or gearbox may go into limp mode

How it works

Smart transmission fluid pressure sensor or switch A reports hydraulic pressure information over the vehicle network for shift control. U0679 sets when the receiving module loses those messages, so the transmission control module may use default shift logic without sensor A's pressure feedback.

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. Scan the engine, transmission, and other modules for communication and transmission codes, and save freeze-frame data before clearing faults.
2. Check live data for pressure sensor or switch A and note whether its reading is missing, along with any abnormal shifts or limp mode.
3. Inspect sensor A's connector and network harness for corrosion, loose connections, fluid intrusion, or physical damage.
4. Check the sensor or module's power and ground against the manufacturer's specification.
5. Test the network wiring to sensor A for opens or shorts using the manufacturer's diagnostic procedure.
6. If the wiring and supplies are sound, follow the manufacturer's procedure to determine whether pressure sensor or switch A or its communication module has failed.

Common fixes

• Reconnect or clean sensor A's connector and repair damaged wiring.
• Repair a faulty power or ground connection to the sensor or its module.
• Repair an open or short in the network wiring to pressure sensor or switch A.
• Replace the confirmed failed sensor A or its communication module.

Can I keep driving?

Avoid towing, hard acceleration, and long trips if shifting is harsh, delayed, or the transmission is in limp mode; lost pressure feedback can interfere with shift control. Stop driving if the transmission slips, will not engage, or overheats, since continued operation can worsen damage.