Caterpillar Excavator SPN 3371 Fault Code: Complete Diagnostic Guide

What is Caterpillar Fault Code SPN 3371?

SPN 3371 indicates a problem with the DEF (Diesel Exhaust Fluid) Pump Motor Circuit, specifically detecting abnormal current or voltage conditions in the dosing pump control circuit. This fault is part of Caterpillar's emissions control system and directly affects the Selective Catalytic Reduction (SCR) system performance.

This code triggers when the Engine Control Module (ECM) detects that the DEF pump motor is drawing incorrect amperage or experiencing electrical irregularities. For Caterpillar excavators equipped with Tier 4 Final engines, this is a critical emission system fault. If left unresolved, the machine will eventually enter derate mode, limiting engine power to 50% or forcing a shutdown to ensure EPA compliance.

Common Symptoms

When SPN 3371 is active, operators typically experience:

• Amber or red Check Engine Light (CEL) illuminated on the instrument cluster
• Warning messages on the monitor display regarding "Exhaust System Fault" or "DEF System Malfunction"
• Engine power derate after a set number of operating hours (typically 50-100 hours warning period)
• DEF consumption irregularities – either excessive usage or no consumption at all
• Difficulty starting in cold weather conditions or extended cranking periods

Potential Causes

The most common technical reasons for SPN 3371 on used Caterpillar excavators include:

• Failed DEF dosing pump motor due to crystallized DEF contamination or internal motor failure
• Corroded or damaged wiring harness connections at the DEF pump connector (common on machines 3+ years old)
• Shorted or open circuits in the pump supply wiring, often caused by harness chafing near the chassis or exhaust components
• ECM internal fault or software calibration issue requiring reprogramming
• Poor quality or contaminated DEF fluid causing pump seizure or electrical resistance changes
• Low battery voltage or charging system problems affecting pump operation

How to Troubleshoot and Fix Code SPN 3371

Step 1: Visual Inspection Start by inspecting the DEF tank, pump assembly, and all related wiring harnesses. On used excavators, check for corrosion on the DEF pump connector pins and inspect harness routing for friction points against the frame or exhaust system. Look for crystallized DEF residue around the pump housing indicating leaks.

Step 2: Electrical Testing Using a digital multimeter (DMM), disconnect the DEF pump connector and check for proper supply voltage (typically 12V with key on). Test circuit resistance between the ECM and pump connector – should be less than 5 ohms. Check for short-to-ground conditions on both motor supply wires. Use Caterpillar Electronic Technician (Cat ET) software to perform active tests and monitor live pump current draw.

Step 3: Component Testing With ignition off, measure DEF pump motor resistance across terminals – typical specification is 3-8 ohms depending on model. If resistance is infinite (open) or near zero (shorted), replace the pump. Before installing new components on used machines, flush DEF lines thoroughly and inspect the DEF tank pickup screen for debris.

Step 4: System Verification After repairs, clear codes using Cat ET diagnostic software and perform a complete SCR system performance test. Verify proper DEF injection rates at various engine loads and confirm no other related emission codes appear.

Disclaimer: This guide provides general diagnostic information. Always consult your Caterpillar service manual and consider professional assistance for complex electrical diagnostics. Improper repairs to emission systems may result in regulatory non-compliance.