What is Yanmar General Fault Code P1605?

Fault Code P1605 indicates a malfunction in the Engine Control Module (ECM) Internal Circuit, specifically related to the processor's internal monitoring system or watchdog circuit failure. This code signifies that the ECM has detected an abnormality in its own internal operations, which can compromise the entire engine management system's reliability.

In Yanmar diesel engines, the ECM serves as the central computer controlling fuel injection timing, air-fuel ratios, and emission controls. When P1605 triggers, it means the module's self-diagnostic system has identified a potential corruption in data processing, memory errors, or voltage irregularities within the controller itself. This is critical because an unreliable ECM can cause erratic engine behavior, unpredictable power delivery, or complete shutdown to prevent engine damage. For used excavators, this code often appears after years of exposure to vibration, moisture ingress, or voltage spikes from aging electrical systems.

Common Symptoms

• Check Engine Light (CEL) illuminated continuously on the instrument panel
• Engine derating or power reduction mode, limiting machine performance to protect components
• Intermittent engine stalling or rough idling, particularly during startup or under load
• Erratic throttle response with delayed acceleration or sudden RPM fluctuations
• Inability to start the engine in severe cases, with the starter cranking but no ignition occurring

Potential Causes

The most common technical causes for P1605 in used Yanmar excavators include:

• ECM internal component failure due to age-related capacitor degradation or circuit board corrosion
• Voltage supply issues from failing alternators, corroded battery terminals, or damaged main power harnesses
• Moisture or water intrusion into the ECM housing through compromised seals or gaskets
• Vibration damage from worn engine mounts causing micro-fractures in solder joints on the ECM circuit board
• Software corruption from interrupted flash programming attempts or voltage spikes during operation
• Corroded ECM connectors particularly on machines with high hours in wet or corrosive environments

How to Troubleshoot and Fix Code P1605

Step 1: Verify Power Supply and Grounding Use a multimeter to check battery voltage (should be 12.5-13V with engine off, 13.5-14.5V running). Inspect all ground straps from the ECM to the engine block and chassis for corrosion or looseness. Check the main power harness connector at the ECM for bent pins, corrosion, or moisture.

Step 2: Inspect ECM Connectors and Wiring On used excavators, carefully examine the ECM connector for green corrosion, especially on pins related to power and ground circuits. Check harness routing for friction points where insulation may be worn through from vibration against the frame. Look for evidence of previous repairs with electrical tape, which often indicates recurring issues.

Step 3: Test ECM Voltage Inputs With ignition on (engine off), measure voltage at the ECM power supply pin (consult Yanmar wiring diagrams for specific pin locations). Voltage should be stable at battery voltage without fluctuation. Any reading below 11V or erratic voltage suggests upstream electrical problems that must be resolved before ECM replacement.

Step 4: Perform ECM Reset and Software Check Disconnect the battery for 15 minutes to allow the ECM to fully reset. Reconnect and attempt to start. If the code returns immediately, connect Yanmar diagnostic software to check for ECM software version and any available updates. Outdated firmware can sometimes cause internal monitoring errors.

Step 5: ECM Replacement Considerations If all electrical inputs test correctly and the code persists, the ECM likely has internal failure. For used machines, source a replacement ECM with matching part numbers and software versions. Always address the root cause (vibration, moisture, voltage spikes) before installing a new module to prevent repeat failure.

Disclaimer: This guide provides general troubleshooting information. Always consult qualified diesel mechanics or Yanmar-certified technicians for definitive diagnosis and repair, especially when dealing with complex electronic control systems.