What is Yanmar General Fault Code P1402-4?

Fault Code P1402-4 indicates an EGR (Exhaust Gas Recirculation) valve position sensor circuit intermittent signal or moderately severe malfunction. This code specifically relates to the EGR system's inability to accurately report valve position to the Engine Control Module (ECM), affecting emissions control and combustion efficiency.

In Yanmar diesel engines, the EGR valve recirculates a portion of exhaust gases back into the intake manifold to reduce combustion temperatures and lower NOx emissions. The position sensor provides critical feedback to the ECM about valve opening percentage. When the ECM detects an intermittent or erratic signal (severity level 4), it cannot properly regulate EGR flow, potentially causing performance issues and triggering derate modes to protect the engine from damage.

Common Symptoms

• Check engine light or malfunction indicator lamp (MIL) illuminated on the instrument panel
• Noticeable loss of engine power or activation of derate mode limiting maximum RPM
• Rough idling or irregular engine operation, especially during warm-up cycles
• Increased black smoke from the exhaust due to improper air-fuel mixture
• Poor throttle response or hesitation during acceleration under load

Potential Causes

The most common technical reasons for P1402-4 on used Yanmar excavators include:

• Corroded or loose electrical connectors at the EGR valve position sensor (common in machines with high hours or exposure to moisture)
• Damaged wiring harness due to rubbing against engine components or heat exposure near the exhaust manifold
• Failed EGR valve position sensor itself, often due to carbon buildup or internal wear
• Intermittent short circuits or open circuits in the sensor signal wire
• Carbon deposits on the EGR valve stem causing mechanical binding and erratic position readings
• ECM software issues or corrupted calibration data (less common but possible in older units)

How to Troubleshoot and Fix Code P1402-4

Step 1: Visual Inspection Begin by locating the EGR valve assembly on the intake side of the engine. Inspect the wiring harness and connector for obvious damage, corrosion, or oil contamination. On used excavators, pay special attention to harness routing where it may contact hot exhaust components or moving parts. Clean all connectors with electrical contact cleaner and ensure tight connections.

Step 2: Electrical Testing Using a digital multimeter, check the position sensor voltage with the ignition on, engine off. Typical reference voltage should read approximately 5 volts at the sensor connector. Check ground continuity (should be less than 1 ohm resistance) and inspect the signal wire for shorts to ground or power. Consult Yanmar's service manual for exact specifications for your engine model.

Step 3: Mechanical and Sensor Verification Remove the EGR valve and inspect for excessive carbon buildup on the valve and stem. Clean thoroughly with appropriate EGR cleaner. Test the position sensor by measuring resistance changes as you manually move the valve (if possible). Replace the sensor if readings are erratic or out of specification. For used machines, consider replacing both the valve and sensor together if components show significant wear.

Step 4: Clear Code and Test After repairs, use Yanmar diagnostic software or a compatible scan tool to clear the fault code. Perform a road test under varying loads and monitor for code recurrence. The intermittent nature (level 4) may require extended testing to confirm the fix.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Yanmar service manual for your specific engine model and consider professional assistance from certified Yanmar technicians for complex repairs or if you're unfamiliar with diesel engine diagnostics.