What is Caterpillar Fault Code SPN135?

SPN135 indicates a problem with the Coolant Temperature Sensor circuit. This Suspect Parameter Number (SPN) specifically monitors the engine coolant temperature sensor voltage signal being sent to the Electronic Control Module (ECM). When triggered, it means the ECM has detected an abnormal voltage reading—either too high, too low, or erratic—from the coolant temperature sensor circuit.

For Caterpillar excavators, this code is critical because the ECM relies on accurate coolant temperature data to manage fuel injection timing, engine protection protocols, and fan operation. An inaccurate reading can cause the engine to run inefficiently, overheat without warning, or enter derate mode to prevent catastrophic damage. In used machines, this fault often stems from sensor degradation or harness wear rather than actual overheating issues.

Common Symptoms

When SPN135 is active on your Caterpillar excavator, you may experience:

• Check Engine Light or malfunction indicator illuminated on the dashboard
• Engine derate or reduced power output as a protective measure
• Cooling fan running continuously at high speed, even when engine is cold
• Hard starting or rough idle, particularly in cold weather conditions
• Inaccurate temperature gauge readings showing extremely high or low temperatures that don't match actual engine condition

Potential Causes

The most common technical reasons for SPN135 appearing on used Caterpillar excavators include:

• Coolant temperature sensor failure due to internal resistance drift or element corrosion
• Damaged wiring harness from rubbing against frame rails or engine components—especially common near the thermostat housing where vibration is constant
• Corroded or loose electrical connectors at the sensor or ECM due to moisture intrusion
• Broken or frayed wires in the sensor circuit, often found in older machines with 5,000+ operating hours
• ECM issues such as internal circuit failures or corrupted calibration files (less common)
• Poor ground connections affecting signal voltage reference

How to Troubleshoot and Fix Code SPN135

Step 1: Visual Inspection Begin by locating the coolant temperature sensor (typically mounted on the cylinder head or thermostat housing). Inspect the sensor connector for corrosion, moisture, or damage. On used excavators, check the entire wiring harness from sensor to ECM for rub points, particularly where it contacts the engine block or frame. Look for worn insulation exposing bare wires.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance. A typical Caterpillar coolant sensor shows approximately 2,000-3,000 ohms at room temperature. Compare your reading against specifications in the service manual. Next, check the supply voltage at the connector—you should see approximately 5 volts from the ECM with the key on, engine off.

Step 3: Circuit Verification Perform a continuity test on the wiring between the sensor connector and ECM pins. Check for shorts to ground or voltage. Wiggle the harness while monitoring resistance; fluctuating readings indicate internal wire damage common in high-hour machines.

Step 4: Component Replacement If the sensor tests outside specifications or the harness shows damage, replace the faulty component. Always apply dielectric grease to connectors on used equipment to prevent future corrosion. After replacement, clear codes using Caterpillar Electronic Technician (Cat ET) software and monitor during operation.

For used excavators specifically: Before replacing expensive ECM components, thoroughly inspect all connectors and harness routing. Many SPN135 codes on older machines resolve with simple connector cleaning and harness repositioning away from heat and friction points.

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Disclaimer: This guide provides general troubleshooting information. Always consult your Caterpillar service manual and consider professional diagnostic services for complex electrical issues or if you're uncertain about any procedure.