Yanmar Excavator Fault Code P0668-4: Complete Diagnostic & Repair Guide

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What is Yanmar Fault Code P0668-4?

Yanmar fault code P0668-4 indicates a low voltage condition on the Engine Control Module (ECM) internal temperature sensor circuit — specifically, the signal voltage has dropped below the acceptable threshold defined by the manufacturer.

This fault targets the ECM internal temperature sensor, which monitors the operating temperature of the control unit itself. The ECM uses this data to protect internal components from thermal damage and to ensure fuel delivery and timing calculations remain accurate under varying ambient conditions.

On Yanmar-powered equipment, this code is critical because an ECM operating outside its thermal parameters can produce erratic engine behavior, incorrect fueling, and in severe cases, permanent damage to the control module. Ignoring this fault on a used excavator can escalate a minor electrical issue into a costly ECM replacement.

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Common Symptoms

When fault code P0668-4 is active on a Yanmar excavator, operators typically experience:

• Amber or red warning light illuminated on the instrument cluster with no obvious mechanical cause
• Engine derate mode — reduced power output as a protective response from the ECM
• Erratic idle or unstable RPM, particularly during warm-up cycles
• Difficulty starting in cold ambient conditions due to compromised sensor feedback
• Fault code stored repeatedly, even after a key cycle reset, indicating a persistent electrical fault

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Potential Causes

The following are the most likely technical root causes for this fault, particularly on used or high-hour machines:

• Low supply voltage to the ECM caused by a failing battery, corroded ground strap, or weak alternator output
• Damaged or chafed wiring harness — on used excavators, harness sections near the firewall, boom pivot, or hydraulic lines are common rub points
• Corroded or moisture-contaminated ECM connector pins, frequently found on machines stored outdoors or in high-humidity environments
• Internal ECM failure — the temperature sensor circuit is embedded within the module and may fail due to age or heat cycling
• Poor chassis ground connections creating a floating reference voltage on the sensor circuit

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How to Troubleshoot and Fix Code P0668-4

Step 1 — Verify Battery and Charging System Voltage Using a digital multimeter, measure battery voltage at rest (should read 12.4–12.7V) and under load during cranking. Check alternator output at operating RPM (target 13.8–14.4V). Low system voltage is the first and most overlooked cause on used equipment.

Step 2 — Inspect the ECM Wiring Harness and Connectors Physically trace the ECM harness from the module back toward the battery and chassis ground points. On used excavators, look specifically for harness abrasion, heat shrink repairs, or corrosion on connector pins. Use electrical contact cleaner and a pin inspection tool to clean and verify each terminal in the ECM connector.

Step 3 — Check ECM Ground Integrity Using a multimeter set to resistance mode, verify ground circuit resistance between the ECM ground pin and the chassis. Resistance should read less than 1 ohm. Any reading above this indicates a compromised ground path requiring repair.

Step 4 — Connect Diagnostic Software Use Yanmar-compatible diagnostic software (such as Yanmar Dr. 3 or a compatible J1939 scanner) to read live ECM sensor data. Monitor the ECM internal temperature sensor voltage in real time. A healthy signal should fall within the manufacturer's specified voltage window (typically 0.5–4.5V). A reading at or near 0V confirms the low-voltage fault condition.

Step 5 — ECM Replacement (Last Resort) If all external wiring, connectors, and power supply circuits test within specification, the fault is likely an internal ECM failure. ECM replacement on Yanmar engines requires programming — consult a certified Yanmar dealer to ensure the replacement unit is properly configured for your machine's serial number and injection system.

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Disclaimer: This guide is intended as a general technical reference for qualified mechanics and experienced equipment operators. Fault diagnosis on heavy construction equipment involves high-voltage electrical systems and complex engine management software. Always consult a certified Yanmar technician or authorized dealer before performing ECM-level repairs, particularly on used or modified machines. Improper diagnosis or repair may void warranties and create additional damage.