What is Yanmar General Fault Code P0122-4?

Fault Code P0122-4 indicates a Throttle Position Sensor (TPS) "A" Circuit Low Input with a severity level 4 condition. This diagnostic trouble code triggers when the Engine Control Module (ECM) detects an abnormally low voltage signal from the throttle position sensor, typically below 0.17 volts for an extended period during engine operation.

The throttle position sensor is critical for Yanmar diesel engines as it communicates accelerator pedal position to the ECM, which then calculates proper fuel delivery and engine response. On excavators, this directly affects engine RPM control, hydraulic pump speed, and overall machine performance. The "-4" suffix indicates this is a moderately severe fault that may trigger engine derate or protective modes if left unresolved.

Common Symptoms

When P0122-4 is active on a Yanmar excavator, operators typically experience:

• Check Engine Light or malfunction indicator lamp illuminated on the instrument cluster
• Reduced engine power or inability to reach full RPM, limiting hydraulic system performance
• Poor throttle response with delayed or erratic acceleration when operating control levers
• Engine may run at idle-only mode or enter a protective derate, restricting operational capability
• Intermittent stalling or rough idle conditions, particularly during cold starts

Potential Causes

The most common technical causes for P0122-4 on used Yanmar excavators include:

• Throttle position sensor failure due to internal wear, carbon buildup, or component degradation over time
• Damaged or corroded wiring harness between the TPS and ECM, especially at connector pins exposed to moisture
• Poor ground connection at the sensor mounting point or ECM ground circuit
• Chafed or pinched wiring near the fuel injection pump or engine harness routing points (common wear areas on older machines)
• Low reference voltage supply from the ECM (typically 5-volt supply circuit) due to ECM issues or circuit shorts
• Corroded connector terminals at the TPS plug, common in machines with high operating hours in wet conditions

How to Troubleshoot and Fix Code P0122-4

Step 1: Visual Inspection Begin by inspecting the throttle position sensor and its wiring harness for obvious damage. Check connector pins for corrosion, bent terminals, or moisture intrusion—common issues on used excavators. Examine the harness routing from the TPS to the ECM for chafing points, particularly where wires contact metal brackets or the engine block.

Step 2: Electrical Testing Using a digital multimeter, backprobe the TPS connector with the ignition on (engine off). Verify the 5-volt reference voltage supply from the ECM. Check the ground circuit for continuity (should read less than 0.5 ohms to chassis ground). Measure the signal voltage while slowly moving the throttle—it should sweep smoothly from approximately 0.5V to 4.5V without dropouts.

Step 3: Component and Circuit Testing If voltage readings are abnormal, disconnect the TPS and measure resistance across sensor terminals per Yanmar specifications (typically 1-5k ohms depending on position). Test the wiring harness for shorts to ground or open circuits with the sensor disconnected. For used machines, pay special attention to harness flex points that may have developed internal wire breaks.

Step 4: Replacement and Verification If the sensor tests faulty, replace the throttle position sensor with an OEM or quality aftermarket unit, ensuring proper calibration. Clean all connector terminals with electrical contact cleaner before reassembly. After replacement, clear codes using Yanmar diagnostic software or a compatible scan tool, then perform a test run to verify proper operation and confirm code does not return.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Yanmar service manual for your specific model and consider professional assistance from a certified technician for complex electrical diagnostics or if you're unfamiliar with excavator systems.