What is Doosan Excavator Fault Code E000678-04?

Fault Code E000678-04 indicates a malfunction in the engine coolant temperature sensor circuit, specifically detecting an intermittent or erratic signal from the ECT (Engine Coolant Temperature) sensor to the ECM (Engine Control Module). This code is triggered when the ECM receives voltage readings outside the expected range or detects inconsistent signal patterns that don't match normal engine operating conditions.

The engine coolant temperature sensor plays a critical role in Doosan excavators by monitoring engine temperature and providing real-time data to the ECM. This information directly affects fuel injection timing, idle speed control, and engine protection systems. When this sensor circuit malfunctions, the excavator's performance and longevity are compromised, potentially leading to overheating, poor fuel economy, or unnecessary engine shutdowns.

Common Symptoms

• Check Engine Warning Light illuminated on the instrument cluster with reduced engine power
• Engine running rough or hunting at idle, particularly during cold starts
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Difficulty starting the engine when cold, or excessive cranking time
• Inaccurate temperature gauge readings that fluctuate erratically or remain stuck at one position

Potential Causes

The most common causes for Code E000678-04 in used Doosan excavators include:

• Corroded or loose electrical connectors at the coolant temperature sensor, especially on machines with high operating hours
• Damaged or frayed wiring harness near the engine block where vibration and heat cause insulation breakdown
• Failed ECT sensor due to age-related degradation or coolant contamination
• Intermittent ground connection issues caused by rust or poor contact points
• ECM software glitches or corrupted calibration files (less common but possible in older units)
• Coolant leaks causing moisture intrusion into the sensor connector

How to Troubleshoot and Fix Code E000678-04

Step 1: Visual Inspection Begin by inspecting the ECT sensor located on the engine block near the thermostat housing. Check the electrical connector for corrosion, moisture, or physical damage. On used excavators, pay special attention to connector pins for green corrosion or bent terminals. Clean connections with electrical contact cleaner.

Step 2: Test Sensor Resistance Disconnect the coolant temperature sensor and use a digital multimeter to measure resistance across the sensor terminals. Compare readings against Doosan specifications (typically 2,000-3,000 ohms at room temperature). Test resistance at different temperatures if possible—resistance should decrease as temperature increases.

Step 3: Check Wiring Harness Continuity With the sensor disconnected, test wiring harness continuity from the sensor connector back to the ECM. Look for resistance readings above 5 ohms, which indicate damaged wiring. Inspect harness routing for rub points against the engine block or frame rails—common failure areas on used machines.

Step 4: Verify ECM Signal Voltage With ignition on and sensor connected, backprobe the sensor connector to measure reference voltage from the ECM (should be approximately 5 volts). If voltage is absent or incorrect, the issue may be ECM-related.

Step 5: Replace Components as Needed If testing confirms sensor failure, replace with genuine Doosan parts. For harness damage, repair using heat-shrink solder connections rather than crimp connectors for durability.

Disclaimer: This guide provides general troubleshooting information. Always consult your Doosan service manual and consider professional diagnostic assistance for complex electrical issues, especially when dealing with ECM programming or warranty concerns.