What is Hitachi ZX350K-5G Fault Code 16605-2?

Fault Code 16605-2 indicates a malfunction in the DEF (Diesel Exhaust Fluid) pressure sensor circuit, specifically a voltage signal that is too low or intermittent. This code is part of the Selective Catalytic Reduction (SCR) system that monitors DEF pressure to ensure proper injection into the exhaust stream for emissions compliance.

On the Hitachi ZX350K-5G excavator, this fault directly affects the SCR aftertreatment system, which is critical for meeting Tier 4 Final emissions standards. The DEF pressure sensor monitors fluid pressure in the supply line between the DEF tank and the dosing injector. When the Engine Control Module (ECM) detects voltage below the expected threshold (typically below 0.5V when specification calls for 0.5-4.5V), it triggers this fault code. This is particularly important because improper DEF delivery can cause the machine to enter derate mode, significantly reducing productivity on the jobsite.

Common Symptoms

• Yellow engine warning light illuminated on the instrument panel with possible SCR system message
• Engine power derate after a predetermined grace period (typically 50-100 operating hours), limiting machine to 50-75% power
• DEF consumption abnormalities, either unusually high or no consumption at all
• Intermittent fault code that appears and disappears, especially during cold starts or after extended idle periods
• Rough engine performance or increased black smoke due to improper SCR function

Potential Causes

The most common technical causes for Code 16605-2 on used ZX350K-5G excavators include:

• Corroded or damaged connector at the DEF pressure sensor (located near the DEF tank or supply module)
• Wiring harness chafing at known rub points along the chassis rail where DEF lines run
• Failed DEF pressure sensor due to crystallized DEF contamination or internal sensor degradation
• Low supply voltage from the ECM due to poor ground connections or ECM power supply issues
• Frozen or contaminated DEF causing abnormal pressure readings (common in machines stored outdoors)
• Damaged DEF supply module or clogged filter screens affecting pressure regulation

How to Troubleshoot and Fix Code 16605-2

Step 1: Visual Inspection and Connector Check Begin by locating the DEF pressure sensor on the supply module near the DEF tank. Inspect the electrical connector for corrosion, bent pins, or moisture intrusion—common issues on used excavators exposed to harsh environments. Clean the connector with electrical contact cleaner and check for proper seal integrity. Examine the wiring harness from sensor to ECM for obvious damage, particularly where it routes along frame rails.

Step 2: Electrical Testing with Multimeter Disconnect the sensor and measure supply voltage at the harness connector with ignition on. You should see approximately 5 volts on the signal supply wire. Next, measure sensor resistance across the sensor terminals (consult service manual for specifications, typically 200-500 ohms). Check ground continuity between the sensor ground pin and battery negative—resistance should be less than 1 ohm.

Step 3: DEF System Pressure and Quality Check Using Hitachi diagnostic software (Dr.EX) or a compatible J1939 scan tool, monitor live DEF pressure readings while the system is operating. Normal pressure ranges from 30-87 PSI during dosing. Check DEF quality using a refractometer (should read 32.5% concentration) and inspect for contamination or crystallization. On used excavators, drain a sample from the tank to check for sediment.

Step 4: Component Replacement and Verification If electrical tests pass but the code persists, replace the DEF pressure sensor (Hitachi part number may vary by serial number). For used machines, also replace the DEF supply filter as preventive maintenance. After replacement, clear codes using diagnostic software and perform a SCR system function test to verify proper operation. Monitor for code recurrence over several operating cycles.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Hitachi service manual for your specific serial number and consider professional diagnosis for complex SCR system issues. Improper repairs to emissions systems may violate environmental regulations.