What is Caterpillar Fault Code 970-31?

Caterpillar Fault Code 970-31 indicates a "Throttle Position Sensor Circuit - Voltage Below Normal" condition. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects an abnormally low voltage signal from the throttle position sensor (TPS), typically below the manufacturer's specified threshold of approximately 0.5 volts.

The throttle position sensor is a critical input device that monitors the position of the fuel control lever or accelerator pedal, communicating engine speed demands to the ECM. When the ECM receives insufficient voltage from this sensor, it cannot accurately determine operator throttle input, compromising fuel delivery, engine performance, and overall machine responsiveness. In Caterpillar excavators, this fault can severely impact productivity and may trigger protective engine derate modes to prevent potential damage.

Common Symptoms

• Check Engine Light or malfunction indicator lamp illuminated on the instrument cluster
• Reduced engine power or engine operating in a limited derate mode (often restricted to low idle)
• Erratic throttle response or complete loss of throttle control regardless of lever position
• Engine may only run at low idle and fail to respond to throttle input commands
• Intermittent stalling or difficulty maintaining consistent engine RPM under load

Potential Causes

The most common technical causes for Code 970-31 on used Caterpillar excavators include:

• Damaged or corroded TPS connector pins – particularly common in machines with high hours exposed to moisture and debris
• Broken or shorted wiring in the throttle position sensor harness, especially at known rub points near the fuel injection pump or along the chassis frame
• Failed throttle position sensor due to internal component degradation or mechanical wear
• Poor ground connection at the sensor or ECM, causing voltage drop in the signal circuit
• ECM internal fault (less common) affecting the reference voltage supply to the TPS
• Contamination or corrosion inside the sensor housing from fuel leaks or environmental exposure

How to Troubleshoot and Fix Code 970-31

Step 1: Visual Inspection Begin by thoroughly inspecting the throttle position sensor and its wiring harness for obvious damage. Check for corroded connector pins, damaged insulation, or evidence of wire chafing at common rub points. On used excavators, pay particular attention to harness routing near moving components and areas exposed to heat or hydraulic fluid contamination.

Step 2: Connector and Ground Verification Disconnect the TPS connector and inspect both male and female terminals for corrosion, bent pins, or moisture intrusion. Clean contacts with electrical contact cleaner if necessary. Verify the ground circuit has less than 0.5 ohms resistance using a digital multimeter (DMM).

Step 3: Voltage Testing With the ignition key ON and engine OFF, backprobe the TPS connector to measure the reference voltage supply from the ECM (should be approximately 5 volts). Move the throttle lever through its full range while monitoring the signal voltage, which should sweep smoothly from approximately 0.5V to 4.5V. Any dropout, spiking, or failure to reach minimum voltage confirms sensor or circuit failure.

Step 4: Resistance Testing If voltage readings are inconclusive, disconnect the sensor and measure resistance across the TPS terminals according to Caterpillar specifications (typically 500-5000 ohms depending on throttle position). Compare readings to factory specifications using Caterpillar Electronic Technician (Cat ET) software.

Step 5: Component Replacement If testing confirms sensor failure, replace the throttle position sensor with a genuine Caterpillar or quality OEM equivalent part. For used machines, also replace the connector pigtail if any corrosion is present to prevent future failures. After replacement, clear codes using diagnostic software and perform a throttle calibration procedure per manufacturer guidelines.

---

Disclaimer: This guide provides general troubleshooting information. Always consult the official Caterpillar service manual for your specific model and serial number. If you lack proper diagnostic tools or experience with high-pressure fuel systems and electronic controls, consult a certified Caterpillar technician to avoid injury or further machine damage.