What is Caterpillar Fault Code 91-13?

Caterpillar Fault Code 91-13 indicates an abnormal voltage condition detected in the throttle position sensor (TPS) circuit, specifically a "Throttle Position Sensor Voltage High" error. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) receives a voltage signal from the throttle position sensor that exceeds the manufacturer's specified threshold, typically above 4.5-5.0 volts.

The throttle position sensor is critical for engine performance as it communicates the operator's throttle input to the ECM. This data determines fuel delivery, engine speed control, and overall machine responsiveness. When Code 91-13 appears, the ECM cannot accurately interpret throttle commands, potentially forcing the machine into a protective derate mode to prevent engine damage. For used excavators, this code often points to sensor degradation or wiring issues common in machines with significant operating hours.

Common Symptoms

• Check Engine Light or warning indicator illuminated on the instrument panel
• Engine operating in reduced power mode or limited to low idle speed regardless of throttle input
• Erratic engine response with sudden RPM fluctuations or unresponsive throttle control
• Machine entering automatic derate, limiting hydraulic function speed and digging force
• Stored fault code in ECM memory even after apparent normal operation resumes

Potential Causes

The most common technical causes for Code 91-13 in used Caterpillar excavators include:

• Throttle position sensor failure due to internal component wear, contamination, or electrical breakdown
• Damaged wiring harness with exposed conductors causing short circuits to positive voltage supply
• Corroded or loose connector pins at the TPS connector, creating intermittent high resistance or open circuits
• ECM internal fault affecting voltage reference supply or signal processing circuits
• Chafed wiring at known rub points near the fuel injection pump, engine mounting brackets, or cab pivot areas common in older machines
• Moisture intrusion into connector terminals causing voltage irregularities

How to Troubleshoot and Fix Code 91-13

Step 1: Visual Inspection Begin with a thorough inspection of the throttle position sensor and its wiring harness. Check for physical damage, oil contamination on connectors, or visible wire chafing near engine vibration points. On used excavators, pay special attention to harness routing where movement and heat exposure cause insulation breakdown.

Step 2: Electrical Testing Using a digital multimeter, backprobe the TPS connector with the key on and engine off. Verify the 5-volt reference supply from the ECM and check the signal wire voltage, which should read between 0.5-4.5 volts depending on throttle position. Any reading consistently above 4.8 volts confirms the high voltage condition.

Step 3: Sensor and Harness Testing Disconnect the TPS and measure resistance across sensor terminals according to Caterpillar specifications (typically 1-5K ohms variable with throttle movement). Test harness continuity and check for short circuits to battery voltage using the ohmmeter function. Replace corroded connectors before condemning sensors.

Step 4: Component Replacement If testing confirms sensor failure, replace the throttle position sensor with genuine Caterpillar parts. For harness damage, repair using proper gauge wire and weatherproof connectors, ensuring proper routing away from heat and friction points.

Step 5: Verification Clear codes using Caterpillar Electronic Technician (Cat ET) software and perform a functional test through all throttle ranges. Monitor live data to confirm voltage stays within 0.5-4.5V range.

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Disclaimer: This guide provides general troubleshooting information. Always consult Caterpillar service manuals and qualified diesel technicians for machine-specific procedures. Improper diagnosis may lead to unnecessary parts replacement or equipment damage.