U0679

Code

Generic U — Network/User

Lost Communication With Transmission Fluid Pressure Sensor/Switch A

Brand: Generic

Type: U — Network/User

Views: UK: 34 EN: 43 RU: 32

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open or short in the sensor signal/wiring harness
Poor or missing sensor power or ground
Corroded or loose connector at the sensor or TCM
Failed transmission fluid pressure sensor/switch
Faulty Transmission Control Module (TCM) or ECU receiving the data
CAN or LIN bus fault (open, short, high resistance, missing termination)

Symptoms

Malfunction Indicator Lamp (MIL) or transmission warning lamp illuminated
Transmission enters limp/inverted shift strategy or limited gears
Harsh, delayed, or incorrect shifting
Incorrect or missing transmission fluid pressure reading in scan tool
Stored U0679 (and possibly other communication U-codes)
Intermittent or no communication with the sensor on a scan tool

What to check

Read all stored codes and freeze frame data with a capable scan tool; note related U- and P-codes
Verify battery voltage is stable and above ~12V during testing
Visually inspect sensor connector and wiring for corrosion, bent pins, breaks, or water intrusion
Backprobe sensor connector to check for proper power and ground
Check CAN/LIN bus voltages at a nearby module (CAN_H and CAN_L ~2.5 V idle each; verify termination ≈60 Ω across bus)
Wiggle wiring while observing live data to look for intermittent faults

Signal parameters

Sensor output: typically analog 0–5 V or a switch closure; some systems send pressure data as a CAN/LIN message
Expected idle/normal CAN voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (recessive state); differential ~0 V
Typical bus termination: ≈60 Ω between CAN_H and CAN_L when network intact
Message rate: sensor CAN/LIN messages often 1–20 Hz (varies by manufacturer)
Typical pressure reading ranges: 0–5 V analog corresponds to 0–(manufacturer value) kPa; check OEM spec for exact range

Diagnostic algorithm

Retrieve all DTCs and freeze-frame/live-data with a scan tool. Note timestamped events and any other communication codes.
Identify whether the transmission fluid pressure sensor communicates via analog/switch circuit or over CAN/LIN (consult wiring diagram).
Inspect connectors and harness along the route for physical damage, corrosion, or signs of heat/wear. Repair/clean as necessary.
With ignition ON (engine off), backprobe the sensor connector: verify battery/power feed, reference voltage (if present), ground continuity, and signal presence. Compare voltages to spec.
If the sensor is on a data bus (CAN/LIN), measure CAN_H and CAN_L voltages at the sensor connector and at the TCM. Check for correct idle voltages and for shorts to battery/ground.
Check termination resistance across CAN_H/CAN_L (~60 Ω). If termination is incorrect, trace for missing/failed modules or wiring faults.
Perform continuity/resistance checks on signal/power/ground wires between sensor and TCM. Repair any open/short circuits found.
If wiring and power/ground are good, substitute a known-good sensor (if available) or bench-test the sensor per manufacturer procedure.
If sensor replacement doesn’t clear the fault, inspect/diagnose the TCM: verify it receives other transmission sensors, check for internal faults, and consider reflashing software if a bulletin exists.
After repairs, clear codes and perform a functional test/road test while monitoring live data to ensure the sensor communication is restored and the DTC does not return.
Note: Always consult the vehicle manufacturer service manual/wiring diagrams for connector pinouts, expected voltages, and special test procedures.

Likely causes

Damaged or disconnected sensor connector/wiring
Failed sensor element (no response or invalid data)
Power/ground circuit fault to sensor
CAN/LIN node issue or bus wiring short to battery/ground
Faulty TCM software/firmware or internal module failure

Fault status

⚠️ Status

Lost communication with Transmission Fluid Pressure Sensor/Switch A — no valid sensor message or signal received by the transmission control module (CAN/LIN or direct circuit).

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours