What is Caterpillar Fault Code EID667?

Caterpillar Event Identifier (EID) 667 indicates "Engine Coolant Temperature Sensor Circuit - Voltage Above Normal or Shorted High." This diagnostic trouble code triggers when the Electronic Control Module (ECM) detects an abnormally high voltage signal from the engine coolant temperature (ECT) sensor circuit, typically above 4.5 volts.

The ECT sensor is a critical component in Caterpillar excavators, providing real-time coolant temperature data to the ECM for fuel injection timing, engine protection, and emission control strategies. When EID667 activates, the ECM cannot accurately monitor engine temperature, potentially leading to improper fuel delivery, increased emissions, and inadequate overheating protection. For used excavators operating in demanding conditions, this fault can compromise engine longevity and performance reliability.

Common Symptoms

Operators may experience the following when EID667 is active:

• Check Engine warning light illuminated on the instrument cluster
• Engine derate mode activated, limiting power output to prevent potential damage
• Hard starting conditions, especially in cold weather, due to incorrect fuel enrichment
• Cooling fan running continuously at high speed, as the ECM assumes maximum temperature
• Rough idle or poor throttle response from improper fuel mapping based on faulty temperature readings

Potential Causes

The most common technical causes for EID667 on used Caterpillar excavators include:

• Open circuit or damaged wiring in the ECT sensor harness, particularly at flex points near the engine block where vibration causes wire fatigue
• Corroded or loose connector pins at the sensor or ECM connection points, common in machines exposed to moisture or extreme environments
• Failed ECT sensor with internal open circuit, though less common than wiring issues
• Chafed wiring harness rubbing against engine components, frame rails, or hydraulic lines—a frequent issue in older machines
• ECM internal fault affecting the sensor supply voltage or signal processing circuitry

How to Troubleshoot and Fix Code EID667

Step 1: Visual Inspection Begin with a thorough visual examination of the ECT sensor wiring harness from the sensor to the ECM. Check for obvious damage, chafing, burns, or connector corrosion. On used excavators, pay special attention to harness routing near moving components and heat sources like exhaust manifolds.

Step 2: Connector and Resistance Testing Disconnect the ECT sensor connector and inspect pins for corrosion, bent terminals, or moisture intrusion. Using a digital multimeter, measure sensor resistance at various temperatures (refer to Caterpillar specifications—typically 2,000-3,000 ohms at 68°F). If resistance is infinite (open circuit), replace the sensor.

Step 3: Circuit Voltage Testing With the sensor disconnected and ignition on, measure voltage at the harness connector. You should see approximately 5 volts on the signal wire. If voltage exceeds 5 volts or shows battery voltage, inspect for shorted wiring to power sources. Check circuit continuity to the ECM and verify proper grounding.

Step 4: Repair and Verification Repair or replace damaged wiring, ensuring proper harness routing away from friction points. Apply dielectric grease to connectors on used machines to prevent future corrosion. Clear the fault code using Caterpillar Electronic Technician (Cat ET) software and perform a test cycle to verify repair.

---

Disclaimer: This guide provides general troubleshooting information. Always consult Caterpillar service manuals and qualified diesel technicians for machine-specific procedures. Improper diagnostics may cause further damage or safety hazards.