How to Troubleshoot SPN 3361 FMI 4: Low DEF Pressure Fault Code

How to Troubleshoot SPN 3361 FMI 4: Low DEF Pressure Fault Code

How to Troubleshoot SPN 3361 FMI 4: Low DEF Pressure Fault Code

Diesel engines equipped with Selective Catalytic Reduction (SCR) systems rely on Diesel Exhaust Fluid (DEF) to reduce nitrogen oxide (NOx) emissions. When the SPN 3361 FMI 4 fault code appears, it indicates a problem with low DEF pressure, which can lead to engine derates, increased emissions, and possible SCR system failure.

In this blog post, we’ll walk through the potential causes, troubleshooting steps, and solutions for fixing a low DEF pressure fault on heavy-duty trucks, construction equipment, and other off-road machinery.

What Does SPN 3361 FMI 4 Mean?

This fault code means the DEF pressure is below the required level for proper system operation. The SCR system depends on a precise amount of DEF being injected into the exhaust stream, and low pressure can lead to poor NOx reduction, increased emissions, and potential engine derate.

Common Symptoms:

  • Engine derate (reduced power or speed limitation)

  • DEF warning light or check engine light

  • Excessive DEF consumption or no DEF being used

  • White smoke or ammonia smell from exhaust

  • DEF crystallization in lines or around the injector

Common Causes of Low DEF Pressure

Several factors can contribute to a low DEF pressure fault. Here are the most common:

  1. DEF Pump Failure – The pump may be clogged, weak, or completely inoperative.

  2. Restricted or Leaking DEF Lines – Crystallized DEF can block lines or leaks may cause pressure loss.

  3. DEF Filter Clogged – The DEF filter may be overdue for replacement.

  4. DEF Injector Malfunction – A stuck-open injector can cause pressure loss.

  5. DEF Tank Level or Quality Sensor Issue – Incorrect DEF mixture (water contamination) or sensor failure.

  6. Electrical or Software Issues – Faulty sensors, wiring issues, or outdated ECM software.

Step-by-Step Troubleshooting Guide

If you're experiencing SPN 3361 FMI 4, follow these steps to diagnose and fix the issue:

Step 1: Check for Additional Fault Codes

  • Connect a diagnostic tool (such as Cummins Insite, Detroit Diesel Diagnostic Link, or JPRO) to scan for other codes.

  • If other DEF or SCR-related codes are present, address them first.

Step 2: Inspect DEF Tank and Fluid Quality

  • Ensure the DEF tank is at least 30% full.

  • Check for contamination – DEF should be clear, not cloudy.

  • If contamination is suspected, drain the tank and refill with fresh DEF.

Step 3: Inspect DEF Lines and Fittings

  • Look for any leaks or cracks in the DEF supply lines.

  • Check for crystallized DEF buildup, which can block flow.

  • If crystallization is found, clean or replace affected lines.

Step 4: Test the DEF Pump

  • Perform a manual DEF pressure test with a scan tool.

  • If the pump is weak or not building sufficient pressure, it may need to be replaced.

Step 5: Check and Replace the DEF Filter

  • Locate the DEF filter (usually near the DEF pump) and inspect for clogging.

  • If clogged, replace it with a new OEM or high-quality aftermarket filter.

Step 6: Inspect the DEF Injector

  • Remove the DEF injector and inspect for crystallization or damage.

  • Clean it using warm distilled water, or replace it if defective.

Step 7: Verify Electrical Connections

  • Inspect the DEF pressure sensor wiring for corrosion or loose connections.

  • Test the sensor using a multimeter to check for proper voltage.

Step 8: Reset the Fault Code and Test Drive

  • After completing repairs, clear the fault codes using a diagnostic tool.

  • Run the engine and monitor DEF pressure in real time.

  • Perform a forced regen if needed to reset the SCR system.

When to Seek Professional Help

If you've gone through these troubleshooting steps and still experience low DEF pressure, you may need dealer-level diagnostics or SCR system repairs. A professional technician can:

  • Perform advanced DEF pump testing

  • Update the Engine Control Module (ECM) software

  • Run manufacturer-specific tests to pinpoint hidden issues

Final Thoughts

A low DEF pressure fault can cause major performance issues and even force an engine derate, leading to downtime and expensive repairs. By following this guide, you can diagnose and fix the problem quickly, ensuring your equipment stays operational and compliant with emissions regulations.

If you're looking for high-quality aftertreatment parts, including DEF pumps, filters, and injectors, check out AfterTreatmentParts.com. We specialize in heavy-duty emissions components at competitive prices!

Need Help?

If you have questions about DEF system issues or need parts, leave a comment below or contact our team—we’re here to help!

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