What is Yanmar General Fault Code P1402?

Fault Code P1402 indicates an EGR (Exhaust Gas Recirculation) valve position sensor circuit malfunction or out-of-range signal in Yanmar diesel engines. This code is triggered when the Engine Control Module (ECM) detects that the EGR valve position sensor is reporting a voltage reading that doesn't match the commanded valve position, or the signal is outside acceptable parameters.

The EGR system is critical for reducing nitrogen oxide (NOx) emissions by recirculating a portion of exhaust gases back into the combustion chamber. In Yanmar excavators, this system directly impacts engine performance, fuel efficiency, and compliance with emission standards. When the ECM cannot accurately monitor EGR valve position, it may force the engine into a derate mode to prevent potential damage or excessive emissions.

Common Symptoms

• Check Engine Light or Malfunction Indicator Lamp (MIL) illuminated on the dashboard
• Noticeable loss of engine power or derate mode activation, limiting maximum RPM
• Rough idling or inconsistent engine performance, particularly under load
• Increased black or gray smoke from the exhaust due to improper EGR function
• Poor fuel economy and sluggish throttle response during excavator operation

Potential Causes

The most common technical reasons for P1402 in used Yanmar excavators include:

• EGR valve position sensor failure due to carbon buildup, heat stress, or internal component wear
• Damaged or corroded wiring harness connections between the sensor and ECM (common in machines with 3,000+ operating hours)
• Shorted, open, or high-resistance circuits in the sensor wiring, often caused by harness chafing near the engine block or exhaust manifold
• Mechanical failure of the EGR valve itself, preventing proper movement and causing sensor reading discrepancies
• ECM calibration issues or software glitches (less common but possible in older control modules)
• Contaminated or stuck EGR valve due to excessive soot accumulation in high-hour machines

How to Troubleshoot and Fix Code P1402

Step 1: Visual Inspection Begin by inspecting the EGR valve and position sensor for obvious physical damage, carbon deposits, or oil contamination. Check all wiring harnesses and connectors for corrosion, fraying, or evidence of rubbing against hot engine components—common failure points in used excavators.

Step 2: Electrical Testing Using a digital multimeter, test the sensor voltage with the ignition on, engine off. The sensor should typically read between 0.5-4.5 volts depending on valve position (consult Yanmar specifications). Check for continuity in the wiring between the sensor connector and ECM pins, and verify ground integrity. Resistance should be near zero ohms on good circuits.

Step 3: Component Testing Manually operate the EGR valve (if accessible) while monitoring sensor voltage changes with diagnostic software or a multimeter. The voltage should change smoothly without dropouts. If the valve is stuck or sensor readings don't correlate with movement, replacement is necessary.

Step 4: Clean or Replace For used machines, thoroughly clean carbon buildup from the EGR valve and passages using appropriate solvents. If sensor or valve components are damaged, replace them with OEM or quality aftermarket parts. Always clear codes and perform a road test under load to verify the repair.

---

Disclaimer: This guide provides general diagnostic information. Always consult Yanmar service manuals and consider professional assistance for complex electrical diagnostics or if you lack experience with diesel engine emission systems.