What is Komatsu PC120-6 Fault Code E318?

Komatsu PC120-6 Fault Code E318 indicates an abnormality in the engine throttle signal circuit — specifically, a fault detected in the potentiometer (throttle dial sensor) signal that communicates the operator's requested engine speed to the Engine Control Module (ECM).

This code is generated when the ECM receives a voltage signal from the throttle volume sensor that falls outside the expected operating range (typically below 0.2V or above 4.8V on the signal wire). On the PC120-6, this sensor is the primary input for engine RPM control, meaning any disruption directly impacts machine productivity and fuel management.

---

Common Symptoms

When E318 is active on a Komatsu PC120-6, operators typically report:

• Engine RPM locks at a fixed low idle and does not respond to throttle dial adjustment
• The monitor panel displays an E318 warning accompanied by an audible alarm
• Sluggish or reduced hydraulic performance, as pump output is tied to engine speed
• Inability to reach full engine power, causing the machine to feel "underpowered" under load
• Intermittent fault behavior — the code may clear temporarily and return under vibration or heat

---

Potential Causes

The following are the most probable technical causes for E318 on a used PC120-6:

• Failed or worn throttle volume sensor (potentiometer) — internal resistive track wear is extremely common on high-hour machines
• Corroded or loose connector at the throttle dial assembly — moisture ingress into the right-side console connector is a known issue on aging PC120-6 units
• Damaged wiring harness between the throttle dial and the ECM, particularly at harness routing points near the cab floor penetration where friction wear occurs
• Open circuit or short to ground on the 5V reference wire or signal return wire
• ECM connector pin corrosion — often overlooked on machines with a history of pressure washing without connector protection

---

How to Troubleshoot and Fix Code E318

Step 1 — Inspect the Throttle Dial Connector First On a used excavator, begin with a physical inspection before touching any electrical components. Disconnect the throttle volume sensor connector (located on the right console inside the cab) and inspect for corrosion, bent pins, or moisture. Clean with electrical contact cleaner and reconnect firmly. Clear the fault code and retest.

Step 2 — Test the Throttle Sensor Output Voltage Using a multimeter, backprobe the signal wire (typically the center wire of the 3-pin connector) with the key ON. Rotate the throttle dial slowly from low to high. You should see a smooth, linear voltage sweep from approximately 0.5V to 4.5V. Any dead spots, jumps, or out-of-range readings confirm a failed potentiometer.

Step 3 — Perform Harness Continuity Check If the sensor tests good, use a multimeter in continuity mode to trace the signal wire from the throttle connector back to the ECM connector. Check specifically where the harness passes through the cab floor grommet — this is the highest-wear friction point on the PC120-6 chassis. Repair any chafed insulation with proper heat-shrink splice connectors.

Step 4 — Use Komatsu KOMTRAX/PC Diagmaster Software If available, connect Komatsu's PC Diagmaster diagnostic tool to read live throttle sensor PID data. This confirms whether the ECM is receiving the corrected signal after repairs, eliminating guesswork before ECM replacement.

Step 5 — Replace Components in Order Replace the throttle volume sensor first (it is a low-cost component). Only after confirming a clean harness and good connector integrity should ECM replacement be considered.

---

Disclaimer: This guide is intended for reference by qualified diesel mechanics and experienced equipment operators. Always consult the official Komatsu PC120-6 Shop Manual and follow proper lockout/tagout procedures before performing electrical diagnostics. If you are unsure of any step, contact a certified Komatsu dealer or licensed heavy equipment technician.