What is Hitachi ZX170W-3 Fault Code P1112 (105-4)?

Fault Code P1112 (105-4) on the Hitachi ZX170W-3 wheeled excavator indicates an Intake Air Temperature (IAT) Sensor Circuit High Voltage condition. This diagnostic trouble code is triggered when the Engine Control Module (ECM) detects an abnormally high voltage signal from the intake air temperature sensor, typically exceeding 4.5-4.9 volts for more than 0.5 seconds.

The IAT sensor plays a critical role in engine performance by measuring incoming air temperature and sending this data to the ECM. The ECM uses this information to calculate proper fuel injection timing and air-fuel ratios. On used ZX170W-3 machines, this sensor failure can significantly impact fuel efficiency, engine power output, and emissions compliance, making prompt diagnosis essential.

Common Symptoms

• Check Engine Light or warning lamp illuminated on the dashboard display
• Rough idle or irregular engine running, particularly during cold starts
• Reduced engine power and noticeable loss of hydraulic performance during digging operations
• Increased fuel consumption due to ECM defaulting to pre-programmed temperature values
• Black smoke from exhaust during acceleration as the engine runs rich without accurate air temperature data

Potential Causes

The most common causes for Code P1112 on used Hitachi ZX170W-3 excavators include:

• Open circuit in the IAT sensor wiring harness, often caused by wire breakage near flex points around the air intake manifold
• Corroded or loose connector pins at the IAT sensor plug—extremely common on machines operating in wet or corrosive environments
• Failed IAT sensor with internal resistance breakdown (sensor resistance should be approximately 2.3-3.0 kΩ at 68°F/20°C)
• Damaged wiring insulation where the harness rubs against engine components or frame structures
• ECM internal fault (rare, but possible on high-hour used machines with voltage spike history)

How to Troubleshoot and Fix Code P1112 (105-4)

Step 1: Visual Inspection Locate the IAT sensor on the intake manifold (typically integrated into the air intake tube). Inspect the sensor connector for corrosion, bent pins, moisture intrusion, or physical damage. On used excavators, check the wiring harness routing for signs of rubbing, especially where it passes near the engine block or crosses vibration points.

Step 2: Sensor Resistance Testing Disconnect the IAT sensor connector and use a digital multimeter set to ohms. Measure resistance across the sensor terminals. Compare readings against the temperature-resistance chart (typically 2.3-3.0 kΩ at 68°F). If resistance is infinite (open circuit) or extremely low, replace the sensor.

Step 3: Circuit Voltage Testing With the ignition on and sensor disconnected, check the signal wire voltage at the harness connector. Voltage should read approximately 5 volts (reference voltage from ECM). If voltage reads battery voltage (12-24V) or fluctuates erratically, inspect for short circuits to power or damaged ECM circuits.

Step 4: Harness Continuity Check Test continuity between the sensor connector and ECM pins using wiring diagrams. On used machines, pay special attention to harness sections that flex during operation. Repair or replace damaged sections with proper gauge wire and weatherproof connectors.

Step 5: Clear Code and Monitor After repairs, use Hitachi Dr.EX diagnostic software or compatible scan tool to clear the fault code. Operate the machine through various temperature conditions and monitor for code recurrence.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Hitachi service manual for your specific machine serial number and consider professional diagnostic assistance for complex electrical issues, especially on used equipment with unknown service history.