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DTC codes / U06xx · Network communication / U0679

U0679Lost Communication With Transmission Fluid Pressure Sensor/Switch A

U0679 is a network communication code indicating that a control module, usually the transmission control module or the engine control module, has lost the data signal from the transmission fluid pressure sensor or switch A. The computer detected that the expected periodic message from this sensor is missing on the vehicle's communication bus, not that the sensor itself has necessarily failed electrically.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaNetwork communication

Symptoms you may notice

  • Illumination of the check engine light or transmission warning light, often with no other noticeable symptoms
  • Harsh or delayed transmission shifts as the control module defaults to a limp-home strategy
  • The transmission may stick in a single gear or refuse to downshift during acceleration
  • A message on the driver information display indicating a transmission fault or reduced power
  • In some cases, the vehicle may enter limp mode and limit engine speed
  • No drivability complaints at all if the fault is intermittent and the module retains a default pressure value

Common causes, most likely first

  1. A faulty transmission fluid pressure sensor or switch A that has stopped transmitting data on the bus
  2. Damaged, chafed, or corroded wiring between the sensor and the transmission control module
  3. A loose or contaminated electrical connector at the sensor or at the transmission case pass-through
  4. Low or degraded transmission fluid causing the pressure sensor to read out of range and drop its message
  5. A software or calibration issue in the transmission control module that misinterprets the sensor signal
  6. A failing transmission control module or a fault in the communication bus itself that disrupts the sensor message
  7. Internal transmission mechanical wear that prevents the sensor from seeing expected pressure, leading to a communication timeout

How to diagnose and fix it

  1. Connect a scan tool and record all stored and pending codes in every module, then verify the live data for transmission fluid pressure sensor A to see if the value is missing or frozen
  2. Inspect the transmission fluid level and condition according to the manufacturer's procedure, looking for low level, burnt smell, or contamination
  3. Visually inspect the sensor connector and the wiring harness from the sensor to the transmission control module for chafing, corrosion, or loose pins
  4. Perform a voltage and resistance check on the sensor circuit at the connector, comparing readings to the manufacturer's specification
  5. Check for technical service bulletins or software updates for the transmission control module that address communication faults with the pressure sensor
  6. If the circuit and fluid check out, substitute a known-good sensor or perform a wiggle test on the harness while watching live data to isolate an intermittent fault
  7. Clear the codes after repair and perform a relearn or adaptation drive cycle as required, then verify that U0679 does not return

What the pressure sensor message really means

  • U0679 is a communication code, so replacing the sensor immediately is a common misdiagnosis; the fault often lies in the wiring or connector rather than the sensor itself.
  • A low transmission fluid level can cause the pressure sensor to report an out-of-range value, which the module may interpret as a lost message and set U0679.
  • Intermittent U0679 codes that appear only on cold mornings often trace to a connector that loses contact as the transmission case expands with heat.
  • Some vehicles use a pressure switch rather than a variable pressure sensor, and a stuck switch can produce the same communication timeout without any electrical failure.
  • A failing transmission control module can drop the sensor's message from the bus, so always check for other communication codes in the same module before condemning the sensor.

Can I keep driving with U0679?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy acceleration or towing because the transmission may be operating on default pressure values. If the vehicle enters limp mode or shifts harshly, stop driving and arrange a tow to prevent internal transmission damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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