What is Yanmar Fault Code P0668?

Yanmar fault code P0668 indicates a low voltage condition on the ECM (Engine Control Module) internal temperature sensor circuit. Specifically, this code is triggered when the ECM detects that the signal voltage from its internal temperature sensor has dropped below the manufacturer's acceptable threshold — typically below 0.2V on most Yanmar diesel platforms.

This is a critical fault because the ECM's internal temperature sensor directly influences how the module regulates fuel injection timing, cooling fan operation, and overall engine protection strategies. If the ECM cannot accurately monitor its own operating temperature, it may fail to protect itself from thermal damage, leading to costly control module failure.

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Common Symptoms

When P0668 is active on a Yanmar-powered excavator, operators typically report the following:

• Check Engine or Fault Indicator light illuminated on the instrument cluster
• Engine derate mode — reduced power output as a built-in protection response
• Erratic or inconsistent engine performance, particularly during warm-up cycles
• Unexpected engine shutdowns or refusal to start in extreme ambient temperatures
• Diagnostic tool shows frozen or irrational ECM temperature readings (e.g., -40°C displayed in normal operating conditions)

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Potential Causes

The following are the most common root causes for P0668 on used Yanmar equipment:

• Failed or degraded internal ECM temperature sensor — common in high-hour machines exposed to repeated heat cycling
• Low supply voltage to the ECM — caused by a failing alternator, corroded battery terminals, or weak ground connections
• Damaged or chafed wiring harness — particularly in areas where the harness routes near the engine block or frame, common on older used excavators
• Corroded or loose ECM connector pins — moisture ingress is a frequent culprit on machines stored outdoors
• Faulty ECM ground circuit — a poor ground can pull the sensor reference voltage below acceptable limits

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How to Troubleshoot and Fix Code P0668

Step 1 — Inspect the ECM Connector and Harness Begin with a thorough visual inspection of the ECM wiring harness and connector. On used excavators, look specifically for chafed insulation, rodent damage, and corrosion on the connector pins. Use electrical contact cleaner and a pin inspection tool to clean and check for bent or backed-out terminals.

Step 2 — Check System Voltage and Ground Integrity Using a digital multimeter, verify battery voltage at the ECM power supply pins — it should read 11.5V–14.5V with the engine running. Next, perform a voltage drop test on all ECM ground circuits; anything exceeding 0.1V drop indicates a resistance problem requiring repair.

Step 3 — Verify Reference Voltage Using Yanmar diagnostic software (YEDIAS or equivalent), monitor the ECM internal temperature sensor live data. Cross-reference the displayed value against actual ambient temperature. A reading of -40°C or below 0.2V signal confirms sensor or circuit failure.

Step 4 — Replace or Repair as Needed If wiring and grounds test good, the fault likely resides within the ECM itself. In this case, consult a certified Yanmar dealer for ECM replacement or reprogramming, as internal sensor replacement is not field-serviceable on most units.

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Disclaimer: This guide is intended for informational purposes and general diagnostic reference only. Always consult a certified Yanmar technician or authorized dealer before performing repairs on safety-critical engine management systems. Improper repairs may void warranties and could result in equipment damage or personal injury.