John Deere Error Code F00123 — What It Means and How to Fix It

Applies to: 6R Series, 7R Series, 8R Series, 9R Series

JD F00123John DeereManually verified

ECU — Loss of Communication with Engine Control Module

The main controller has lost communication with the engine ECU on the CAN bus. The tractor may derate or shut down as a safety measure.

Repair difficulty

Workshop recommended

Repair time

2h

Dealer cost

$500-3000

Likely causes

  1. 1

    Damaged CAN bus wiring— Most likely

    The CAN bus (usually a twisted pair, yellow and green wires on JD machines) is vulnerable to rodent damage and chafing. Inspect the full harness.

  2. 2

    Corroded or damaged ECU connector— Possible

    Moisture at the ECU connector degrades the CAN bus termination. Remove and inspect the connector — look for green corrosion on pins.

  3. 3

    Failed CAN bus terminating resistor— Possible

    The CAN bus needs a 120Ω terminating resistor at each end. Measure across the CAN-H and CAN-L pins with the key off — should read 60Ω (two resistors in parallel).

  4. 4

    Failed engine ECU— Less common

    ECU failures are uncommon but do occur, especially on machines that have had electrical issues. Rule out all wiring first.

Parts needed

Tools needed

  • multimeter
  • CAN bus wiring diagram
  • terminal pin removal tool
  • electrical contact cleaner

Want alerts when we add new codes? Drop your email.

Look up another code