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Home / DTC / U1309 — Fault: absence of communication with the automatic gearbox ECU: No signal / ECU absent

U1309 — Fault: absence of communication with the automatic gearbox ECU: No signal / ECU absent

Detailed page for trouble code U1309.

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U1309

PEUGEOT U — Network/User

Fault: absence of communication with the automatic gearbox ECU: No signal / ECU absent

Brand: PEUGEOT
Type: U — Network/User
Views: UK: 0 EN: 1 RU: 1
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Page language: EN

Causes

  • Blown fuse or loss of power supply to gearbox ECU
  • Poor or corroded ground to gearbox ECU
  • Loose, disconnected or damaged connector/wiring to the gearbox ECU
  • Open or shorted CAN/LIN bus wiring or broken termination
  • Faulty gearbox ECU (internal failure)
  • Intermittent bus communication due to wiring harness damage or chafing

Symptoms

  • Transmission warning lamp or gearshift indicator fault
  • Automatic gearbox stuck in limp mode or limited gears
  • No gear selection or vehicle does not move when in Drive
  • Relevant transmission-related functions unavailable (e.g., electronic parking, torque management)
  • Other modules reporting communication errors or related U-codes
  • Possible engine derate or check engine light if ECM loses TCU data

What to check

  • Read all stored and pending codes from all modules and note freeze frame data
  • Attempt to communicate with gearbox ECU using a dealer-level scan tool
  • Inspect fuses related to transmission ECU and vehicle data bus power rails
  • Visually inspect gearbox ECU connector for corrosion, bent pins, water ingress or damage
  • Check battery voltage (key on, engine off) and chassis ground condition
  • Perform wiggle test on harness while monitoring live communication/messages

Signal parameters

  • CAN_H idle voltage approx 2.5 V (recessive ~2.5 V; dominant ~3.5 V under activity)
  • CAN_L idle voltage approx 2.5 V (dominant ~1.5 V during activity)
  • Differential voltage during activity ≈ 1.5–2.5 V (dominant bits)
  • Termination resistance across CAN_H and CAN_L ≈ 60 ohms (two 120Ω in parallel) with ignition OFF or bus disconnected
  • Power supply to gearbox ECU (ignition-switched +12 V) present with key ON
  • Ground continuity from gearbox ECU to chassis (low ohms)

Diagnostic algorithm

  1. Record all DTCs from all control modules and note whether U1309 is current or intermittent.
  2. Try to connect with an OE-level scan tool to the gearbox ECU. If ECU does not respond while other modules communicate, proceed.
  3. Inspect fuses and relays for gearbox ECU and bus power; replace any blown fuses and retest communication.
  4. Visually inspect gearbox ECU connector and harness for corrosion, water ingress, damaged pins or loose clip. Repair/clean as required and retest.
  5. With ignition ON (engine off), measure supply voltage at gearbox ECU power pin(s) and check ground continuity. Repair any open or high-resistance circuits.
  6. Measure CAN_H and CAN_L at the gearbox ECU connector: verify idle voltages (~2.5 V) and correct differential during activity. If voltages are incorrect, trace wiring toward the nearest junction, ABS module or gateway module.
  7. Check termination resistor across CAN lines (~60Ω). If open or wrong value, inspect bus wiring and termination resistors or modules providing termination.
  8. Perform continuity and resistance checks on suspect wiring sections, and perform a wiggle test while monitoring bus messages to find intermittent faults.
  9. If wiring and power/ground are confirmed good, verify no other module is dominating or shorting the bus by disconnecting devices one at a time (start with easily accessible modules) while monitoring bus activity.
  10. If only the gearbox ECU is non-responsive and all wiring/power/ground are confirmed OK, consider replacing or exchanging the gearbox ECU. Ensure correct part number and perform any required programming/coding after replacement.
  11. After repair, clear codes and perform road test and re-scan to confirm fault is resolved and no new communication errors appear.

Likely causes

  • Connector disconnected at gearbox ECU or broken wires near gearbox
  • Failed fuse(s) feeding gearbox ECU or vehicle network power rail
  • Damaged CAN bus feed, short to battery/ground, or missing termination
  • Corroded ground point at gearbox/nearby chassis
  • Failed gearbox ECU (last-resort after wiring verified)

Fault status

⚠️ Status
No valid messages received from the automatic gearbox ECU on the vehicle data bus; ECU appears absent or not transmitting.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.5 - 4.0 hours

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