Caterpillar Fault Code 678-3: Meaning, Symptoms, and Fix

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What is Caterpillar Fault Code 678-3?

Caterpillar Fault Code 678-3 indicates a Voltage Above Normal condition on the Throttle Position Sensor (TPS) circuit — specifically on the sensor signal wire returning to the Engine Control Module (ECM).

This code is triggered when the ECM detects that the voltage signal from the throttle position sensor has exceeded the acceptable upper threshold, typically above 4.8V DC on a standard 5V reference circuit.

On Caterpillar excavators, the throttle position sensor directly governs engine RPM response and hydraulic pump demand management. An unreliable signal can cause the machine to behave unpredictably under load, making this fault code a critical operational concern.

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Common Symptoms

When Fault Code 678-3 is active, operators typically report the following:

• Erratic or unresponsive engine RPM — the engine may surge, hunt, or fail to respond to throttle dial input
• Illuminated warning lamp on the monitor panel with a system alert message
• Engine derate mode activation, limiting power output to protect drivetrain components
• Reduced hydraulic performance, including sluggish boom, arm, or bucket response
• Fault code stored in ECM memory, even after the machine is cycled off and restarted

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Potential Causes

Several technical conditions can trigger this fault, particularly on used or high-hour excavators:

• Failed or drifting throttle position sensor — internal resistive element degradation is common after 5,000+ hours
• Short to voltage on the signal wire — the signal wire contacting a 12V or 24V source due to harness chafing
• Corroded or backed-out connector pins at the TPS connector or ECM harness plug
• Damaged 5V reference circuit inside the ECM, causing a false high-voltage reading
• Previous improper repairs — spliced wiring or non-OEM sensors installed by prior owners

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How to Troubleshoot and Fix Code 678-3

Step 1 — Connect Diagnostic Software Use Caterpillar ET (Electronic Technician) software with a valid communication adapter to read active and logged fault codes. Check the TPS voltage reading in real time under the "Status" screen. A healthy sensor should read between 0.5V and 4.5V across the throttle range.

Step 2 — Inspect the Harness and Connectors On used excavators, physically trace the throttle sensor harness from the sensor to the ECM. Look for rub points near frame edges, zip tie wear, and heat-damaged insulation. Inspect the connector pins for corrosion, green oxidation, or spread terminals. Clean with electrical contact cleaner before drawing conclusions.

Step 3 — Test the Sensor and Circuit Using a digital multimeter, back-probe the signal wire at the TPS connector with the key ON. Confirm the 5V reference is present on the reference pin. If signal voltage reads above 4.8V with the sensor disconnected, suspect a short to voltage in the harness — not the sensor itself. Replace only after confirming the circuit is clean.

Step 4 — Replace if Confirmed Faulty Install an OEM Caterpillar throttle position sensor to ensure correct calibration range. Clear codes with Cat ET and perform a full throttle sweep test to verify resolution.

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Disclaimer: This guide is intended for informational purposes and general diagnostic direction. Always consult a certified Caterpillar dealer or licensed heavy equipment mechanic before performing repairs, especially on machines under warranty or in safety-critical applications.