What is Hitachi Excavator Fault Code P1625?

Fault Code P1625 indicates a malfunction in the Throttle Position Sensor (TPS) Circuit B - High Input, signaling that the Engine Control Module (ECM) is receiving an abnormally high voltage signal from the secondary throttle position sensor circuit. This diagnostic trouble code specifically relates to the electronic throttle control system that monitors the engine's throttle valve position and governs engine speed accordingly.

In Hitachi excavators, the throttle position sensor uses dual circuits (A and B) for redundancy and safety. When Circuit B registers voltage readings above the manufacturer's specified threshold—typically above 4.5-4.8 volts—the ECM logs P1625 and may trigger protective measures. This fault is critical because the throttle control system directly affects engine performance, fuel delivery, and the excavator's ability to respond to operator commands. On used machines, this code often surfaces due to sensor degradation, wiring deterioration, or connector corrosion accumulated over thousands of operating hours.

Common Symptoms

• Check Engine Light or malfunction indicator lamp illuminated on the instrument panel
• Engine derate mode activated, limiting maximum engine RPM to protect the powertrain
• Erratic throttle response or delayed acceleration when moving control levers
• Rough idle conditions or engine hunting (RPM fluctuating without operator input)
• Reduced hydraulic performance during digging or lifting operations due to insufficient engine power

Potential Causes

The most frequent technical causes for P1625 on used Hitachi excavators include:

• Throttle Position Sensor failure due to internal wear, contamination, or resistor track degradation
• Wiring harness damage at common rub points near the engine mount or firewall where vibration causes insulation breakdown
• Connector corrosion at the TPS plug or ECM connections, creating high resistance or intermittent open circuits
• ECM power supply issues, including damaged ground connections or voltage spikes from faulty alternators
• Short circuits to power in the sensor signal wire, often from pinched harnesses during previous repairs

How to Troubleshoot and Fix Code P1625

Step 1: Visual Inspection Begin by thoroughly inspecting the throttle position sensor and its wiring harness. Check for physical damage, frayed wires, or oil contamination on connectors. On used excavators, pay particular attention to harness routing where it contacts frame members or the engine block—these are common failure points.

Step 2: Electrical Testing Using a digital multimeter, disconnect the TPS connector and measure the reference voltage from the ECM (should be approximately 5 volts). Then check the signal wire resistance between the sensor and ECM—readings above 10 ohms indicate wiring problems. Measure the sensor's output voltage while manually moving the throttle linkage; Circuit B should sweep smoothly from approximately 0.5V to 4.5V without spikes or dropouts.

Step 3: Component Replacement If voltage readings exceed specifications or the sensor shows erratic behavior, replace the throttle position sensor with OEM or equivalent parts. Before installation, clean all connector pins with electrical contact cleaner and apply dielectric grease to prevent future corrosion. After replacement, clear codes using Hitachi diagnostic software (Dr.EX or compatible tools) and perform a throttle relearn procedure per manufacturer specifications.

Step 4: Verification Operate the excavator through full work cycles while monitoring live sensor data to confirm stable voltage readings and proper throttle response.

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Disclaimer: This guide provides general troubleshooting information. Complex electrical diagnostics should be performed by qualified technicians with proper equipment. Always consult the machine's service manual and follow safety protocols when working on heavy equipment.