What is Volvo EC120D Fault Code 128 PID 1720?

Fault Code 128 PID 1720 on the Volvo EC120D excavator indicates a high voltage condition detected in the Engine Coolant Level sensor circuit. This Diagnostic Trouble Code (DTC) is triggered when the Electronic Control Module (ECM) reads a voltage signal above the normal operating range (typically above 4.5 volts) from the coolant level sensor located in the expansion tank.

This fault is critical for the EC120D's D6E engine because the ECM relies on accurate coolant level monitoring to prevent overheating and catastrophic engine damage. When this code activates, the system cannot reliably determine if coolant levels are adequate, potentially allowing the machine to operate in dangerous low-coolant conditions. The Volvo VECU (Volvo Engine Control Unit) uses PID 1720 specifically to monitor coolant reservoir status, making this sensor integral to the engine protection system.

Common Symptoms

• Coolant level warning light illuminated on the instrument cluster, even when coolant is at proper level
• Audible alarm or intermittent beeping indicating coolant system fault
• Possible engine derate mode engagement on some software versions, limiting machine performance
• Malfunction indicator lamp (MIL) activation on the dashboard
• Inability to clear the fault code without addressing the underlying electrical issue

Potential Causes

The most common causes of Code 128 PID 1720 on used EC120D excavators include:

• Open circuit in the sensor wiring harness — particularly common where the wire bundle passes near the engine mount on the left side, causing wire breakage from vibration
• Corroded or damaged connector at the coolant level sensor (prone to corrosion from coolant weepage on older machines)
• Failed coolant level sensor with internal circuit breakdown (the float mechanism can fail after 5,000+ operating hours)
• Chafed wiring rubbing against the engine block or hydraulic lines in the tight engine compartment
• ECM internal fault (rare, but possible on high-hour machines with moisture intrusion)

How to Troubleshoot and Fix Code 128 PID 1720

Step 1: Visual Inspection Begin with the expansion tank located on the right side of the engine compartment. Inspect the coolant level sensor connector for corrosion, bent pins, or coolant residue. On used excavators, this connector frequently shows green corrosion from years of heat cycling. Check the wiring harness routing from the sensor to the main engine harness — look specifically for wire chafing near metal edges or zip-tie compression points.

Step 2: Electrical Testing Disconnect the coolant level sensor connector. Using a digital multimeter (DMM), measure the voltage at the harness side (machine side) connector with the ignition key ON. You should read approximately 5 volts on the signal wire (typically the center pin). If you read battery voltage (12-14V) or significantly higher than 5V, you've confirmed the high voltage condition. Check for continuity to ground on the signal wire — any continuity indicates a short circuit in the harness.

Step 3: Sensor and Harness Testing Measure the resistance of the coolant level sensor itself (sensor side, disconnected from harness). Specifications vary with coolant level, but an open circuit (infinite resistance) indicates sensor failure. For used machines, remove the sensor and physically inspect the float mechanism — debris buildup or a stuck float is common. Before replacing components, thoroughly clean all electrical connectors with electrical contact cleaner and apply dielectric grease to prevent future corrosion.

Step 4: Harness Repair If testing reveals damaged wiring, repair by cutting out the affected section and splicing in new 18-gauge automotive-grade wire. Use heat-shrink solder connectors rather than crimp connectors for lasting repairs on vibrating equipment. Re-route the harness away from sharp edges and secure with proper vibration-resistant clamps.

Step 5: Verification Clear the fault code using Volvo VCADS (Volvo Computer Aided Diagnostic System) or compatible diagnostic software. Run the engine through temperature cycles and monitor for code recurrence. On used excavators with 8,000+ hours, consider preventive replacement of the sensor even if testing shows marginal values, as sensor degradation accelerates after this threshold.

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Disclaimer: This guide provides general troubleshooting information for experienced technicians. Always consult the official Volvo service manual for your specific machine serial number and software version. If you're uncomfortable performing electrical diagnostics, contact a certified Volvo dealer or qualified heavy equipment technician to prevent further damage to your excavator's electrical system.