What is Caterpillar Fault Code SPN1605?

SPN 1605 indicates a problem with the Charge Air Cooler (CAC) Outlet Temperature Sensor circuit. This diagnostic trouble code triggers when the Engine Control Module (ECM) detects an abnormal voltage reading from the sensor that monitors the temperature of compressed air after it passes through the aftercooler/intercooler system.

The Charge Air Cooler reduces the temperature of compressed air from the turbocharger before it enters the engine's combustion chambers. Cooler, denser air improves combustion efficiency, power output, and reduces emissions. The CAC Outlet Temperature Sensor provides critical data to the ECM for fuel management, timing adjustments, and engine protection strategies. On Caterpillar excavators, this sensor failure can significantly impact performance and trigger protective derate modes, especially under heavy load conditions where precise air temperature monitoring is essential.

Common Symptoms

• Check Engine Light or malfunction indicator lamp illuminated on the instrument cluster
• Engine power derate or reduced performance, particularly noticeable during digging or lifting operations
• Increased fuel consumption due to improper fuel-to-air ratio calculations
• Black or excessive exhaust smoke resulting from suboptimal combustion conditions
• Erratic engine behavior during warm-up or load changes as the ECM operates without accurate temperature feedback

Potential Causes

The most common technical failures associated with SPN 1605 on used Caterpillar excavators include:

• Damaged or corroded sensor connector at the CAC outlet location, frequently exposed to moisture and debris
• Broken or shorted wiring harness between the sensor and ECM, particularly at routing points near the engine block or cab where vibration causes wire chafing
• Failed CAC temperature sensor due to thermal cycling fatigue or contamination exposure over thousands of operating hours
• Poor ground connection affecting sensor circuit integrity
• ECM internal fault (rare) affecting the sensor input circuit processing

On older machines, corrosion at connector pins and harness damage from rubbing against brackets are the primary culprits.

How to Troubleshoot and Fix Code SPN 1605

Step 1: Visual Inspection Locate the CAC Outlet Temperature Sensor (typically mounted on the outlet pipe between the charge air cooler and intake manifold). Inspect the wiring harness and connector for physical damage, corrosion, oil contamination, or loose connections. On used excavators, check for harness routing issues where wires may rub against engine components or frame members.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across the sensor terminals. Compare readings against Caterpillar specifications (typically 2,000-3,000 ohms at room temperature for thermistor-type sensors). Check for open circuits or short-to-ground conditions in the harness by measuring continuity from the sensor connector to the ECM pins.

Step 3: Voltage Supply Verification With the ignition on and sensor disconnected, measure reference voltage at the harness connector (should read approximately 5 volts). Low or absent voltage indicates harness damage or ECM issues. Test the ground circuit for proper continuity.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Caterpillar or quality aftermarket sensor. Clean connector terminals with electrical contact cleaner and apply dielectric grease before reconnection. Clear codes using Caterpillar Electronic Technician (ET) software and perform a test cycle.

For used excavators, always inspect connector seals and consider replacing weatherpack connectors that show corrosion, as preventive maintenance against recurring failures.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Caterpillar service manual for your specific machine model and serial number. If you're unfamiliar with electrical diagnostics or high-voltage systems, seek assistance from a certified Caterpillar technician to avoid equipment damage or personal injury.