What is Yanmar General Fault Code P1620-12?

Fault Code P1620-12 indicates a short circuit to ground in the Intake Air Heater (Glow Plug) Relay Control Circuit. This diagnostic trouble code (DTC) means the Engine Control Module (ECM) has detected an abnormal electrical pathway where current is flowing directly to ground instead of through the intended relay circuit.

In Yanmar diesel engines, the intake air heater system (commonly called glow plugs) is critical for cold-start performance and efficient combustion during warm-up cycles. The ECM controls power delivery to the intake air heater relay, which then energizes the glow plugs. When the control circuit shorts to ground (indicated by the "-12" modifier), the ECM cannot properly regulate heater operation, potentially causing starting difficulties and increased engine wear during cold conditions. This is particularly important in used excavators where wiring degradation and connector corrosion are common issues.

Common Symptoms

• Check Engine Light (CEL) illuminated on the instrument panel with stored fault code
• Difficult or extended cranking during cold starts, especially in temperatures below 50°F (10°C)
• White or gray smoke during initial startup due to incomplete combustion
• Rough idle for the first few minutes of operation until engine reaches operating temperature
• ECM may disable intake air heater operation entirely as a protective measure, preventing the relay from energizing

Potential Causes

The most common technical causes for P1620-12 in used Yanmar excavators include:

• Damaged wiring harness with insulation worn through, especially at harness routing points near the engine block, frame rails, or hydraulic lines where vibration causes rubbing
• Corroded or moisture-infiltrated connectors at the intake air heater relay or ECM connection points
• Failed intake air heater relay with internal short circuit to the mounting bracket or chassis ground
• Chafed or pinched wires in the engine harness, particularly in areas where previous maintenance may have disturbed factory routing
• ECM internal driver circuit failure (less common but possible in high-hour machines)
• Aftermarket electrical modifications or improper repairs creating unintended ground paths

How to Troubleshoot and Fix Code P1620-12

Step 1: Visual Inspection and Connector Check Begin by thoroughly inspecting the intake air heater relay location and all associated wiring. Remove the relay and examine the connector for corrosion, bent pins, or moisture. Check the entire engine harness for obvious damage, particularly where it contacts metal surfaces or moves with engine vibration. On used excavators, pay special attention to harness zip-tie points and areas where previous repairs may have been performed.

Step 2: Electrical Testing with Multimeter Using a digital multimeter (DMM), perform resistance checks on the relay control circuit. With the relay removed and ignition off, measure resistance between the ECM relay control pin and chassis ground—you should see infinite resistance (open circuit). If you measure continuity (less than 5 ohms), you've confirmed a short to ground. Test the relay itself by checking coil resistance (typically 70-90 ohms for Yanmar relays) and verifying no continuity between control terminals and the relay case.

Step 3: Harness Repair and Component Replacement If you've identified damaged wiring, repair using OEM-spec wire with proper heat-shrink connections and ensure correct harness routing away from heat and abrasion points. Replace corroded connectors with new sealed units. If the relay is faulty, install an OEM Yanmar replacement—aftermarket relays may have incorrect specifications. After repairs, clear the fault code using Yanmar diagnostic software or a compatible scan tool, then perform a cold-start test to verify proper intake air heater operation.

Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the official Yanmar service manual for your specific model and serial number. If you're uncomfortable with electrical diagnostics, seek assistance from a certified Yanmar technician to prevent further damage to ECM components.