Yanmar Excavator Fault Code P1125-1: Meaning, Symptoms & Fix

---

What is Yanmar General Fault Code P1125-1?

Yanmar fault code P1125-1 indicates a low voltage or out-of-range signal fault on the Throttle Position Sensor (TPS) or Accelerator Position Sensor (APS) circuit — specifically, the sensor output voltage has dropped below the acceptable threshold as defined by the engine's Electronic Control Module (ECM).

This code is triggered when the ECM detects that the analog voltage signal returning from the throttle/accelerator sensor falls below approximately 0.2V–0.5V during normal operating conditions, indicating an open circuit, short to ground, or sensor failure.

On Yanmar-powered equipment, throttle position data is critical. The ECM relies on this signal to calculate precise fuel delivery, engine speed control, and hydraulic pump output. A corrupted or missing signal can trigger protective derate strategies that significantly impact machine productivity.

---

Common Symptoms

When fault code P1125-1 is active, operators typically report the following:

• Engine warning light illuminated on the instrument cluster
• Engine speed stuck at low idle regardless of throttle dial input
• Reduced hydraulic response — boom, arm, and bucket feel sluggish or unresponsive
• Machine enters limp-home mode, limiting RPM to protect the drivetrain
• Erratic or hunting engine RPM at certain throttle positions

---

Potential Causes

The following are the most common technical causes for this fault, especially on used or older Yanmar equipment:

• Faulty Accelerator/Throttle Position Sensor — internal resistance drift or potentiometer wear
• Damaged wiring harness — chafing, pinching, or cuts near the engine bay firewall or frame routing points
• Corroded or loose connector pins at the sensor plug or ECM connector
• Short to ground in the signal wire (Pin A or signal return circuit)
• Failed 5V reference voltage supply from the ECM to the sensor
• Water intrusion into the sensor body or connector, common in older machines with deteriorated seals

---

How to Troubleshoot and Fix Code P1125-1

Step 1 — Visual Inspection First Before using any diagnostic tools, physically inspect the throttle/accelerator sensor harness from the sensor body back to the ECM connector. On used excavators, pay close attention to known rub points near the engine mount brackets, fuel lines, and cab floor penetrations. Look for frayed insulation, corrosion, green oxidation on pins, or heat damage.

Step 2 — Check the 5V Reference Voltage Using a digital multimeter, backprobe the sensor connector with the key ON and engine OFF. Measure voltage between the 5V reference pin and the ground pin. You should read 4.8V–5.2V. A reading significantly below this indicates an ECM reference circuit fault or a short to ground pulling the voltage down.

Step 3 — Test Sensor Output Voltage With the key ON, measure the signal pin voltage while slowly rotating the throttle dial. Voltage should sweep smoothly from approximately 0.5V (low idle) to 4.5V (full throttle). A flat, jumping, or out-of-range reading confirms sensor failure.

Step 4 — Inspect ECM Connector If wiring and sensor check out, inspect the ECM connector for bent pins, corrosion, or moisture. Use electrical contact cleaner and a pin probe tool to verify terminal tension. On used machines, this connector is frequently overlooked and is a common root cause.

Step 5 — Replace and Verify Replace the faulty sensor or damaged harness section using OEM or equivalent quality parts. Clear the fault code using Yanmar diagnostic software (YDT — Yanmar Diagnostic Tool) or a compatible J1939/CAN bus scan tool, then perform a full throttle sweep test to confirm resolution.

---

Disclaimer: This guide is intended for informational and diagnostic reference purposes only. Always consult a certified Yanmar dealer or licensed heavy equipment mechanic before performing repairs, especially on machines under warranty or operating in safety-critical applications. Incorrect repairs may cause further damage or void existing service agreements.