What is Volvo EC120D Fault Code 128 PID 1570?

Fault Code 128 PID 1570 on the Volvo EC120D excavator indicates a problem with the Engine Coolant Temperature Sensor 1 Circuit – specifically, the ECM has detected a voltage reading that is above the normal operating range. This diagnostic trouble code (DTC) is generated when the Electronic Control Module (ECM) receives an abnormally high signal from the coolant temperature sensor, typically above 4.5 volts, suggesting an open circuit or sensor malfunction.

The coolant temperature sensor plays a critical role in the Volvo D4D engine's performance management system. It continuously monitors engine temperature and sends voltage signals to the ECM, which then adjusts fuel injection timing, idle speed, and fan operation accordingly. When this sensor circuit fails, the ECM cannot accurately regulate engine operations, potentially leading to overheating, poor fuel economy, or reduced hydraulic performance on your excavator.

Common Symptoms

When fault code 128 PID 1570 is active on your EC120D, you may experience:

• Check Engine Light or warning indicator illuminated on the instrument cluster
• Engine operating in limp mode or derate, with reduced power output and hydraulic flow
• Hard starting conditions, especially when the engine is cold
• Engine cooling fan running continuously at high speed, regardless of actual temperature
• Abnormally high or low temperature readings displayed on the gauge (often showing maximum reading)

Potential Causes

The most common technical reasons for this fault code on used Volvo EC120D excavators include:

• Open circuit in the wiring harness between the coolant temperature sensor and the ECM (often caused by wire chafing near the engine block mounting points)
• Failed coolant temperature sensor with internal resistance breakdown (common after 5,000+ operating hours)
• Corroded or loose connector terminals at the sensor or ECM connection points
• Damaged sensor harness due to heat exposure from exhaust manifold proximity
• Rare: ECM internal circuit failure affecting the 5-volt reference voltage supply

How to Troubleshoot and Fix Code 128 PID 1570

Step 1: Visual Inspection Begin with a thorough visual examination of the coolant temperature sensor (located on the cylinder head near the thermostat housing) and its wiring harness. Check for obvious damage, melted insulation, or connector corrosion. On used excavators, pay special attention to harness routing near moving engine components where vibration can cause wire chafing.

Step 2: Sensor and Circuit Testing Disconnect the coolant temperature sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At room temperature (68°F/20°C), resistance should read approximately 2,400-2,700 ohms. If readings are infinite (open circuit) or zero ohms (short circuit), replace the sensor. Next, check the harness wiring for continuity between the sensor connector and ECM pins using a wiring diagram – resistance should be less than 5 ohms.

Step 3: Voltage Reference Check With the ignition on and sensor disconnected, measure voltage at the harness connector. You should see approximately 5 volts on the signal wire (reference voltage from ECM) and continuity to ground on the ground wire. If the 5-volt reference is missing or incorrect, suspect ECM issues or harness damage. Before replacing expensive ECU components on used machines, thoroughly clean all connector terminals with electrical contact cleaner and apply dielectric grease.

Step 4: Clear Code and Verify Repair After repairs, use Volvo VOCOM diagnostic software or a compatible scan tool to clear the fault code and perform a functional test. Monitor live data to ensure coolant temperature readings correspond with actual engine temperature during warm-up.

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Disclaimer: This guide provides general diagnostic information. Always consult your Volvo service manual and consider professional assistance for complex electrical diagnostics. Improper repairs may void warranties or cause additional damage.