What is Caterpillar 336D2 Fault Code 39:1658-8?

Fault Code 39:1658-8 indicates an abnormally high electrical signal detected in the Diesel Exhaust Fluid (DEF) temperature sensor circuit. This code specifically means the Electronic Control Module (ECM) has detected voltage above normal operating parameters in the exhaust aftertreatment system's DEF temperature sensor, which monitors the temperature of the diesel exhaust fluid before injection.

This code is critical for the Caterpillar 336D2's emissions control system. The DEF temperature sensor provides essential data to the ECM for proper Selective Catalytic Reduction (SCR) operation. When this sensor reports incorrect high voltage, the system cannot accurately control DEF injection rates, potentially causing emissions compliance failures and triggering engine derate modes to protect components.

Common Symptoms

When fault code 39:1658-8 is active on your 336D2 excavator, you may experience:

• Amber or red malfunction indicator lamp (MIL) illuminated on the dashboard
• Engine power derate after a warning period, reducing machine performance by 25-40%
• DEF system warning messages displayed on the monitor screen
• Poor fuel economy due to improper SCR system operation
• Failed emissions tests or inability to regenerate the diesel particulate filter properly

Potential Causes

The most common technical reasons for code 39:1658-8 on used Caterpillar excavators include:

• Damaged or corroded wiring harness connections at the DEF temperature sensor (common wear point near the DEF tank mounting bracket)
• Failed DEF temperature sensor due to crystallized DEF contamination or internal circuit failure
• Chafed wiring where the harness passes through frame rails or rubs against hydraulic lines
• Corroded connector pins at the sensor or ECM connections from moisture intrusion
• Short circuit to positive voltage in the sensor harness caused by damaged insulation
• ECM internal fault (rare, but possible in high-hour used machines)

How to Troubleshoot and Fix Code 39:1658-8

Step 1: Visual Inspection Begin by thoroughly inspecting the DEF temperature sensor harness from the sensor (located on the DEF supply line near the tank) to the ECM. On used excavators, check for harness wear at known rub points—especially where wiring passes near the hydraulic pump or frame crossmembers. Look for damaged insulation, melted wire covering, or corroded connectors.

Step 2: Electrical Testing Disconnect the DEF temperature sensor connector and use a digital multimeter to check for proper circuit resistance. The sensor should typically read 2,000-3,000 ohms at room temperature (consult your service manual for exact specifications). Check for short circuits to power by measuring voltage at the disconnected sensor harness connector with the key on—you should see 0V. If voltage is present, trace the harness for damage.

Step 3: Sensor and Harness Verification Using Caterpillar Electronic Technician (Cat ET) diagnostic software, monitor live sensor data while gently moving the harness. Erratic readings indicate intermittent wiring faults. Replace the DEF temperature sensor if resistance values are out of specification. For used machines, always inspect and clean all connector terminals with electrical contact cleaner and apply dielectric grease before reassembly to prevent future corrosion.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Caterpillar service manual for your specific machine serial number and consider professional diagnostic assistance for complex electrical issues.