What is Caterpillar Fault Code 135?

Caterpillar Fault Code 135 indicates an Engine Coolant Temperature (ECT) Sensor Circuit High Voltage condition. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects a voltage signal from the coolant temperature sensor that exceeds the manufacturer's specified range, typically above 4.5-4.8 volts.

The ECT sensor is a critical component in Caterpillar excavators, responsible for monitoring engine coolant temperature and sending real-time data to the ECM. This information directly affects fuel injection timing, engine warm-up cycles, and protective shutdown protocols. When Code 135 appears, the ECM cannot accurately determine engine temperature, potentially leading to improper fuel delivery, increased emissions, and risk of engine damage from overheating or overcooling conditions.

Common Symptoms

When Fault Code 135 is active on your Caterpillar excavator, operators typically experience:

• Check Engine Light or diagnostic warning lamp illuminated on the instrument cluster
• Hard starting or extended cranking time, especially during cold starts
• Rough idle or inconsistent engine performance during warm-up phase
• Engine derate mode activation, limiting machine power output to prevent potential damage
• Inaccurate temperature gauge readings, often showing extremely low or no temperature reading
• Increased fuel consumption due to the ECM defaulting to a fixed temperature value

Potential Causes

The most common technical reasons for Code 135 on used Caterpillar excavators include:

• Open circuit in the sensor wiring harness due to wire breakage, particularly at flex points near the engine block
• Corroded or damaged connector pins at the ECT sensor or ECM connections—extremely common in machines exposed to harsh environments
• Failed Engine Coolant Temperature sensor with internal circuit failure causing high resistance
• Chafed or abraded wiring where the harness rubs against engine components or frame members (known wear point on older models)
• Moisture intrusion into connector seals causing intermittent high resistance readings
• Rarely, ECM internal circuit failure, though sensor circuit issues are far more prevalent

How to Troubleshoot and Fix Code 135

Step 1: Visual Inspection Begin with a thorough inspection of the ECT sensor connector and wiring harness from sensor to ECM. On used excavators, pay special attention to harness routing near the engine block where heat and vibration cause insulation degradation. Check for corrosion, bent pins, or moisture in connectors. Clean all connections with electrical contact cleaner before proceeding.

Step 2: Sensor and Circuit Testing Disconnect the ECT sensor connector and measure resistance across the sensor terminals using a digital multimeter. At room temperature (68°F/20°C), resistance should typically read 2,000-3,000 ohms (consult your specific service manual). Then check for open circuits by measuring continuity from the sensor connector back to the ECM pins—you should see less than 5 ohms resistance on good wiring.

Step 3: Voltage Testing With the key on and engine off, backprobe the sensor connector to measure supply voltage from the ECM, which should read approximately 5 volts. If voltage is present and wiring tests good, the sensor itself has likely failed and requires replacement with a genuine Caterpillar ECT sensor.

Step 4: Verify Repair After replacing faulty components, clear the fault code using Caterpillar Electronic Technician (Cat ET) diagnostic software or an appropriate scan tool. Run the engine through a complete warm-up cycle and monitor live data to confirm the sensor is reading accurately across the temperature range.

Disclaimer: This guide provides general troubleshooting information for Caterpillar Fault Code 135. Always consult your machine's specific service manual and consider professional diagnostic assistance for complex electrical issues. Improper diagnosis can lead to unnecessary parts replacement and continued downtime.