What is Hitachi Excavator Fault Code P1173?

Fault Code P1173 indicates a malfunction in the Fuel Pressure Regulator Control Circuit, specifically detecting an abnormally high voltage or open circuit condition in the fuel system's electronic control pathway. This diagnostic trouble code (DTC) is generated when the Electronic Control Module (ECM) detects voltage readings outside the manufacturer's specified range (typically above 4.5 volts) in the fuel pressure regulator's command circuit.

This code directly affects the engine's ability to maintain proper fuel rail pressure, which is critical for optimal combustion efficiency, power output, and emissions control. In Hitachi excavators, the fuel pressure regulator works in tandem with the common rail fuel injection system to ensure precise fuel delivery under varying load conditions. When P1173 triggers, the ECM may enter a protective derate mode to prevent engine damage from improper fuel pressurization.

Common Symptoms

• Check Engine Light or malfunction indicator lamp illuminated on the instrument cluster
• Noticeable loss of engine power or failure to reach maximum RPM under load
• Rough idle, engine misfiring, or difficulty starting, especially when cold
• Black smoke from exhaust indicating incomplete fuel combustion
• Engine automatically limiting to reduced power mode (derate) to protect components

Potential Causes

The P1173 code in used Hitachi excavators typically stems from these technical failures:

• Damaged wiring harness or corroded connectors between the ECM and fuel pressure regulator, particularly at flex points near the engine mount
• Failed fuel pressure regulator solenoid due to internal coil breakdown or mechanical wear
• Open circuit caused by broken wires, often from vibration fatigue in high-hour machines
• Faulty ECM with internal driver circuit failure (less common but possible in older units)
• Poor ground connections at the fuel system harness grounding points
• Previous repair attempts using incorrect connector pins or inadequate weatherproofing

How to Troubleshoot and Fix Code P1173

Step 1: Visual Inspection Begin with a thorough inspection of the fuel pressure regulator wiring harness. On used excavators, focus on areas where the harness routes near hot exhaust components or rubs against the engine block. Check all connectors for corrosion, bent pins, or moisture intrusion. Clean connections with electrical contact cleaner and inspect for proper sealing.

Step 2: Electrical Testing Using a digital multimeter, disconnect the fuel pressure regulator connector and measure resistance across the solenoid terminals. Typical specifications range from 2-8 ohms (consult your service manual for exact values). Next, check for continuity between each wire and the ECM connector. Any reading of infinite resistance indicates an open circuit requiring harness repair or replacement.

Step 3: Voltage Supply Check With the ignition on (engine off), backprobe the regulator connector to verify the ECM is providing proper reference voltage (typically 5 volts) and ground. If voltage exceeds 4.8 volts continuously, suspect ECM driver circuit failure. If no voltage is present, trace the power supply circuit back to the ECM.

Step 4: Component Replacement If wiring and voltage checks pass specifications, replace the fuel pressure regulator assembly. For used machines with over 8,000 hours, consider replacing associated fuel system seals and O-rings simultaneously to prevent future leaks.

Professional Tip: Before ordering expensive components, connect Hitachi diagnostic software (Dr.EX or equivalent) to monitor live fuel pressure data and regulator command signals, which can pinpoint intermittent failures that static testing may miss.

---

Disclaimer: This guide provides general troubleshooting information. Always consult your Hitachi service manual for model-specific procedures and specifications. Complex fuel system repairs should be performed by qualified diesel technicians with proper diagnostic equipment.