What is Caterpillar Fault Code SPN173?

SPN173 indicates an Exhaust Gas Temperature (EGT) sensor circuit malfunction in Caterpillar diesel engines. This Suspect Parameter Number (SPN) is triggered when the Engine Control Module (ECM) detects abnormal voltage readings from the exhaust gas temperature sensor, typically located in the exhaust manifold or turbocharger outlet. The EGT sensor plays a critical role in monitoring exhaust temperatures to protect the aftertreatment system, prevent engine overheating, and optimize fuel efficiency. For used excavators, this code is particularly important because excessive exhaust temperatures can cause catastrophic damage to the diesel particulate filter (DPF), turbocharger, and cylinder head components.

Common Symptoms

When SPN173 is active on your Caterpillar excavator, you may experience:

• Check Engine Light or malfunction indicator lamp (MIL) illuminated on the instrument cluster
• Engine derate or reduced power output to prevent thermal damage, especially during high-load operations
• Failed DPF regeneration cycles or regeneration warning lights appearing more frequently
• Abnormally high or erratic exhaust temperature readings displayed on the monitor (if equipped)
• Black smoke from the exhaust during acceleration, indicating incomplete combustion compensation

Potential Causes

The most common technical reasons for SPN173 appearing on used Caterpillar excavators include:

• Damaged or corroded EGT sensor wiring harness, particularly at rub points near the exhaust manifold where heat and vibration cause insulation breakdown
• Failed exhaust gas temperature sensor due to thermal cycling fatigue or internal element deterioration (common after 5,000+ operating hours)
• Corroded or loose electrical connectors at the sensor or ECM, especially on machines operated in harsh or marine environments
• Open circuit or short to ground in the sensor signal wire caused by pinched harness during previous repairs
• ECM calibration issues or software glitches requiring a flash update (less common but possible on older controller versions)

How to Troubleshoot and Fix Code SPN173

Step 1: Visual Inspection Begin by inspecting the EGT sensor and its wiring harness for obvious physical damage. On used excavators, pay special attention to harness routing near the exhaust manifold—look for melted insulation, chafing against metal components, or brittle wires from heat exposure. Check the sensor connector for corrosion, moisture intrusion, or bent pins.

Step 2: Electrical Testing Using a digital multimeter (DMM), disconnect the EGT sensor and measure resistance across the sensor terminals. Typical resistance values range from 50-150 ohms at room temperature (consult your specific service manual). Next, check for continuity from the sensor connector back to the ECM on the signal and ground wires—any reading above 5 ohms indicates excessive resistance requiring harness repair.

Step 3: Voltage Circuit Verification With the ignition key on and sensor disconnected, measure voltage at the harness connector. You should see approximately 5 volts on the signal wire (reference voltage from ECM). If voltage is absent or incorrect, trace the circuit for shorts or check ECM pin integrity—common on high-hour used machines.

Step 4: Component Replacement If testing confirms sensor failure, replace the EGT sensor with OEM or quality aftermarket parts. For used excavators, always apply anti-seize compound to sensor threads and use new copper crush washers to ensure proper sealing and heat transfer. Clear codes using Caterpillar Electronic Technician (Cat ET) diagnostic software and perform a regeneration cycle to verify repair.

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Disclaimer: This guide provides general troubleshooting information for SPN173 on Caterpillar excavators. Always consult the manufacturer's service manual for your specific model and serial number. If you're uncomfortable performing electrical diagnostics, contact a certified Caterpillar technician to prevent further damage to emission control systems.