What is Komatsu Fault Code E426?

Komatsu Fault Code E426 indicates a malfunction in the Engine Controller (ECM) communication circuit, specifically related to a CAN (Controller Area Network) communication error or timeout between the engine control module and other machine control systems. This diagnostic trouble code triggers when the ECM detects interrupted, corrupted, or absent data transmission across the machine's electronic network.

The CAN bus system serves as the nervous system of modern Komatsu excavators, allowing the engine controller, hydraulic controller, monitor panel, and other electronic modules to share critical operational data. When communication fails, the ECM cannot properly coordinate engine performance with hydraulic demands, potentially causing the machine to enter reduced power mode or display multiple simultaneous fault codes. For used excavators, this code is particularly significant because communication failures often stem from age-related wiring degradation rather than component failure.

Common Symptoms

• Warning lights illuminated on the monitor panel, including check engine light and communication error indicators
• Machine enters derate mode, limiting engine RPM to 1500-1800 range or reducing hydraulic power output
• Intermittent loss of dashboard display functions, including gauge flickering or complete monitor blackout
• Multiple unrelated fault codes appearing simultaneously due to communication breakdown between modules
• Hard starting or rough idle when ECM cannot receive proper sensor data from other controllers

Potential Causes

The most common technical causes for Code E426 in used Komatsu excavators include:

• Damaged or corroded CAN bus wiring harness, particularly at flex points near the engine mount, cab base, or counterweight where vibration causes wire insulation breakdown
• Loose or corroded connector pins at the ECM, hydraulic controller, or junction boxes—especially the 38-pin and 58-pin ECM connectors
• Failed termination resistors on the CAN bus network (typically 120-ohm resistors located at network endpoints)
• ECM internal circuit failure or corrupted software, more common in machines with 8,000+ operating hours
• Voltage supply issues to controllers caused by failing alternator, weak batteries (below 12.2V), or damaged main power harness
• Aftermarket component interference from improperly installed monitoring systems or non-OEM controllers creating network conflicts

How to Troubleshoot and Fix Code E426

Step 1: Perform Visual Inspection and Basic Electrical Checks

Begin by inspecting all wiring harnesses along the CAN bus network path. Focus on known wear points: behind the cab where harnesses route through grommets, near the engine mount where vibration is highest, and along the chassis rail. Look for abraded insulation, exposed copper, or evidence of rodent damage. Check battery voltage with engine off (should read 12.4-12.8V) and while running (13.8-14.4V). Low voltage commonly causes intermittent communication errors in used machines.

Step 2: Inspect and Test Connectors

Disconnect and inspect the ECM connector and all related controller connectors. Apply electrical contact cleaner and check for green corrosion, bent pins, or moisture intrusion. Using a multimeter, verify continuity on CAN-High and CAN-Low wires (typically yellow and green wires) throughout the harness. Resistance should be approximately 60 ohms between CAN-High and CAN-Low with all controllers connected (indicating proper 120-ohm termination at each end).

Step 3: Test CAN Bus Termination and Signal Quality

Locate the termination resistors at each end of the CAN bus network (consult service manual for exact locations—often at ECM and main monitor). Measure resistance across CAN-High to CAN-Low at each terminator; each should read 120 ohms. With ignition on and a digital oscilloscope (if available), check for proper CAN signal voltage: should show 2.5V baseline with 1V differential pulses. Erratic signals indicate harness issues; no signal suggests ECM or controller failure.

Step 4: Update Software and Replace Components as Needed

Connect Komatsu diagnostic software (KOMTRAX or Komatsu Diagnosis System) to check for ECM software updates and perform communication tests between modules. For used excavators, outdated firmware often causes compatibility issues. If all wiring and termination tests pass, the fault likely resides in the ECM itself or another controller. Replace components only after confirming wiring integrity, as harness issues account for approximately 70% of communication faults in older machines.

Disclaimer: This guide provides general troubleshooting information for experienced technicians. Always consult the official Komatsu service manual for your specific model and serial number. If you lack proper diagnostic tools or electrical expertise, contact a certified Komatsu technician to prevent further damage to electronic systems.