What is Caterpillar Fault Code 137?

Caterpillar Fault Code 137 indicates an Inlet Air Temperature (IAT) Sensor Circuit High Voltage condition. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally high voltage signal from the intake air temperature sensor, typically above 4.5 volts, suggesting either an open circuit or sensor failure.

The IAT sensor plays a critical role in monitoring the temperature of air entering the engine's combustion chamber. The ECM uses this data to calculate proper fuel injection timing, air-fuel ratios, and adjust turbocharger boost levels. On Caterpillar excavators, accurate intake air temperature readings are essential for optimal engine performance, fuel efficiency, and emissions control. When this sensor circuit fails, the ECM operates on default values, which can compromise engine power and increase fuel consumption, particularly problematic in demanding excavation applications.

Common Symptoms

When Fault Code 137 is active, operators may experience:

• Check Engine Light or Diagnostic Warning Lamp illuminated on the instrument cluster
• Noticeable loss of engine power or reduced throttle response, especially under load
• Increased fuel consumption due to the ECM running in failsafe mode with default air temperature values
• Hard starting conditions in cold weather, as the ECM cannot properly compensate for actual air temperature
• Black smoke from the exhaust during acceleration, indicating improper fuel mixture calculations

Potential Causes

The most common technical reasons for Code 137 on used Caterpillar excavators include:

• Open circuit in the IAT sensor wiring harness, often caused by wire chafing near engine vibration points or heat sources
• Failed IAT sensor with internal resistance breakdown, common in machines with 5,000+ operating hours
• Corroded or damaged connector pins at the sensor or ECM connection points, particularly in machines exposed to moisture or harsh environments
• Damaged wiring insulation where the harness routes near sharp edges or moving components in the intake system
• ECM internal circuit failure (rare, but possible in older machines with electrical system stress)

How to Troubleshoot and Fix Code 137

Step 1: Visual Inspection Begin by inspecting the IAT sensor and its wiring harness for obvious damage. The sensor is typically located in the air intake manifold or inlet piping. Check for cracked connectors, corroded terminals, or abraded wiring, especially where the harness contacts metal surfaces. On used excavators, pay special attention to areas where vibration causes harness friction against brackets or engine components.

Step 2: Electrical Testing Disconnect the IAT sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At 68°F (20°C), resistance should typically read between 2,000-3,000 ohms (consult your specific service manual for exact specifications). Next, check for continuity between the sensor connector and the ECM harness connector. Any reading of infinite resistance indicates an open circuit requiring harness repair or replacement.

Step 3: Voltage Circuit Testing With the key in the ON position (engine off), backprobe the sensor connector and measure voltage on the signal wire. You should see approximately 5 volts reference voltage from the ECM. If voltage reads above 4.8 volts with the sensor disconnected, this confirms a high voltage condition. Check the ground circuit for proper continuity to the ECM (should read less than 0.5 ohms resistance).

Step 4: Component Replacement and Verification If testing confirms sensor failure, replace the IAT sensor with a genuine Caterpillar or OEM-equivalent part. After replacement, clear the fault code using Caterpillar Electronic Technician (Cat ET) diagnostic software or an appropriate diagnostic tool. Run the engine through various load conditions and verify the code does not return. For used machines, consider applying dielectric grease to connector terminals to prevent future corrosion.

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Disclaimer: This guide provides general diagnostic information for Caterpillar Fault Code 137. Always consult your machine's specific service manual and consider professional diagnostic assistance from a certified Caterpillar technician for complex electrical issues or if you're uncertain about any procedure.