Caterpillar Excavator Fault Code SPN3708: Diagnosis and Repair Guide

What is Caterpillar Fault Code SPN3708?

SPN 3708 indicates a problem with the Diesel Exhaust Fluid (DEF) Pump Motor Circuit on Caterpillar excavators equipped with Selective Catalytic Reduction (SCR) emissions systems. This fault code is triggered when the Electronic Control Module (ECM) detects abnormal voltage, current draw, or operational failure in the DEF pump's electrical circuit.

This code specifically monitors the DEF dosing pump motor, which is responsible for injecting precise amounts of diesel exhaust fluid into the exhaust stream to reduce nitrogen oxide (NOx) emissions. On used Caterpillar excavators, this system is critical for meeting EPA Tier 4 Final emissions standards. When SPN3708 is active, the machine may enter derate mode to protect the emissions system, significantly reducing productivity and profitability for operators.

Common Symptoms

When SPN3708 is active on your Caterpillar excavator, you may experience:

• Amber or red warning lamp illuminated on the instrument cluster, specifically the emissions system indicator
• Engine power derate (typically 25-40% power reduction) after a grace period, forcing the machine into limp mode
• DEF system malfunction messages displayed on the monitor screen
• Increased DEF consumption or complete cessation of DEF dosing
• Engine shutdowns or refusal to restart in severe cases with multiple fault code combinations

Potential Causes

The most common technical causes for SPN3708 on used Caterpillar excavators include:

• Failed DEF pump motor due to crystallized DEF contamination or internal electrical short/open circuit
• Damaged wiring harness between the ECM and DEF pump, particularly at known rub points near the exhaust mounting brackets
• Corroded or loose electrical connectors at the DEF pump module (common on machines operated in high-moisture or road salt environments)
• ECM power supply issues, including degraded ground connections or voltage irregularities
• DEF quality problems causing pump seizure—contaminated, frozen, or expired DEF can damage pump internals
• Faulty ECM relay controlling DEF pump power distribution

How to Troubleshoot and Fix Code SPN3708

Step 1: Connect Diagnostic Software Use Caterpillar Electronic Technician (Cat ET) diagnostic software to read active and logged fault codes. Document FMI (Failure Mode Identifier) values—these indicate whether the issue is an open circuit (FMI 5), short to ground (FMI 4), or mechanical failure (FMI 7).

Step 2: Inspect DEF Pump and Wiring Harness Physically examine the DEF pump module located near the SCR catalyst housing. On used excavators, check for:

• Connector corrosion (clean with electrical contact cleaner)
• Damaged pins or loose terminals
• Harness chafing at mounting points—particularly where the harness crosses metal brackets
• DEF crystallization around pump housing (indicates leaks)

Step 3: Perform Electrical Testing With the key OFF and connector disconnected, use a digital multimeter to measure:

• Resistance across DEF pump motor terminals (typically 3-8 ohms; consult service manual)
• Voltage supply at the harness connector with key ON (should read 12-24V depending on system)
• Ground continuity from pump ground wire to chassis (should read less than 1 ohm)

Step 4: Test DEF Pump Operation Using Cat ET, perform a DEF pump function test. Listen for pump motor activation. If the pump runs but SPN3708 persists, suspect ECM internal failure or CAN bus communication issues.

Step 5: Replace Components as Needed If testing confirms pump failure, replace the entire DEF pump assembly (common on excavators with 5,000+ hours). Always use genuine Caterpillar or quality OEM-equivalent parts. For used excavators, verify DEF tank cleanliness before installation to prevent repeat failures.

---

Disclaimer: This guide provides general troubleshooting information. Always consult the official Caterpillar service manual for your specific excavator model. Complex electrical diagnostics should be performed by certified technicians with proper training and equipment.