What is Komatsu PC1250-11R Fault Code CA691?

Fault Code CA691 indicates a communication error between the Engine Control Module (ECM) and the Machine Controller (MC) on the Komatsu PC1250-11R excavator. This diagnostic trouble code specifically signals that the CAN (Controller Area Network) communication link has experienced an interruption, timeout, or data transmission failure between these two critical control systems.

The PC1250-11R relies on continuous, real-time data exchange between the ECM and MC to coordinate engine performance with hydraulic demand, implement power modes, and manage fuel efficiency. When this communication pathway fails, the machine cannot properly synchronize engine output with operational requirements, potentially leading to performance limitations or complete machine shutdown. This code is particularly critical because it affects the integrated control strategy that makes modern excavators efficient and responsive.

Common Symptoms

• Warning lights illuminate on the monitor panel, typically showing engine and system error indicators simultaneously
• Machine enters derate mode or limited power output, restricting hydraulic flow and reducing digging force
• Intermittent loss of power modes – inability to switch between work modes (Power, Economy, Breaker mode)
• Erratic engine response to throttle inputs or hydraulic lever commands
• Complete machine shutdown in severe cases, with the engine refusing to start or immediately shutting down after ignition

Potential Causes

The CA691 code on used PC1250-11R excavators typically stems from several age-related issues:

• Corroded or damaged CAN bus connectors between the ECM and MC, especially at harness connection points near the engine bay
• Harness chafing or wire breakage at known rub points along the main chassis rail where the communication harness routes
• Failed termination resistors within the CAN network causing signal reflection and data corruption
• ECM or MC internal failure, though less common than harness issues in used machines
• Voltage supply problems to either controller due to failing relays, corroded grounds, or battery connection issues
• Water intrusion into controller housings from damaged seals, common in machines operating in wet conditions

How to Troubleshoot and Fix Code CA691

Step 1: Visual Inspection and Connection Verification Begin by inspecting all CAN bus harness connectors between the ECM (located on the engine) and the MC (typically in the cab area). Look for corrosion, bent pins, or moisture in connectors. On used excavators, pay particular attention to the harness routing along the right-side chassis rail where vibration and heat cycling cause insulation breakdown. Clean all connections with electrical contact cleaner and apply dielectric grease before reconnecting.

Step 2: CAN Bus Electrical Testing Using a digital multimeter, measure CAN-High and CAN-Low voltage at both the ECM and MC connectors with the key in the ON position (engine off). You should read approximately 2.5V on CAN-High and 2.5V on CAN-Low at rest. Check for 120-ohm termination resistance between CAN-High and CAN-Low lines with all controllers disconnected. Incorrect resistance indicates failed termination resistors or shorted wiring.

Step 3: Advanced Diagnostics with Komatsu Software Connect Komatsu diagnostic software (KOMTRAX or dealer-level tools) to monitor real-time CAN bus communication status. Check for message timeout errors specific to ECM-MC communication. Test communication integrity by commanding power mode changes while monitoring data traffic. If communication is intermittent, perform a wiggle test on harnesses while monitoring to identify mechanical connection failures.

Step 4: Component Replacement Priorities For used machines, replace components in this order: First, address any identified harness damage or connector corrosion. Second, replace termination resistors if resistance values are incorrect. Third, if all wiring tests normal, consider ECM or MC replacement, starting with the controller showing the most environmental exposure or physical damage.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific serial number, and consider engaging a certified Komatsu technician for complex electrical diagnostics to avoid costly misdiagnosis on expensive control modules.