Why Your Truck Keeps Throwing DEF Codes — And What You Can Do About It

Why Your Truck Keeps Throwing DEF Codes — And What You Can Do About It

If your dash is lighting up with DEF-related fault codes, you're not alone. Thousands of diesel equipment owners run into DEF system issues every year — especially in colder climates, with poor-quality DEF, or after years of hard use.

But here’s the good news: most DEF codes boil down to a few common issues. In this guide, we’ll break down the most frequent DEF system trouble codes, what causes them, and how to fix them without spending a fortune.

🔍 Common DEF System Codes and What They Mean

Below are the most frequently reported DEF-related fault codes across heavy-duty diesel engines:

1. SPN 5246 / FMI 0-15

This generic “SCR malfunction” code often appears alongside other more specific codes. It usually signals that your DEF system isn’t operating correctly — or has been disabled by the ECM due to ongoing faults.

Possible Causes:

  • DEF pump failure

  • Faulty NOx sensors

  • Crystallized DEF lines

  • Incorrect fluid level or quality

2. SPN 4334 / FMI 7

Indicates low pressure in the DEF supply line, typically meaning the DEF pump isn’t producing enough pressure to dose fluid correctly.

Fix:

  • Check for clogged filters or frozen DEF

  • Inspect pump performance — if in doubt, replace with a known-good aftermarket pump

3. SPN 3364 / FMI 2 or 9

This code signals a NOx sensor issue, often due to sensor failure, contaminated DEF, or ECM communication errors.

Fix:

  • Replace the sensor with a high-quality aftermarket or OEM equivalent

  • Verify CAN bus signal integrity if issues persist

4. SPN 5246 / SPN 3362 Cluster

A mix of codes pointing to ongoing DEF dosing failure. If ignored, these codes will eventually derate your engine, forcing it to idle or run at limited RPM.

Fix Immediately:

  • Clear codes after inspection and repair

  • Replace DEF pump, header, or injector if components are faulty

❄️ DEF System Issues in Cold Weather

In Canada and the northern U.S., frozen DEF lines and pumps are a major cause of mid-winter DEF codes. If your truck starts throwing faults after a cold snap, try these steps:

  • Park indoors or use a DEF tank heater

  • Inspect the DEF pump for cracks or ice blockage

  • Replace damaged lines or sensors ASAP

🛠️ How to Fix DEF System Issues Fast

Here’s what to do if you’re dealing with DEF system codes:

  1. Use diagnostics to read and record active and inactive fault codes

  2. Inspect your DEF tank, lines, pump, and sensors

  3. Replace failed parts with quality components

  4. Clear codes with your diagnostic software after repairs

  5. Test drive to ensure the codes don’t return

🛒 Need Reliable DEF System Parts?

At AftertreatmentParts.com, we carry:

  • DEF Pumps

  • NOx Sensors

  • DEF Headers

  • DEF Injectors

  • Wiring harnesses and fittings

All backed by fast shipping, a 365-day warranty (on most items), and no hidden fees for Canadian or U.S. buyers.

🚛 Don’t Let DEF Codes Park Your Truck

We know how important uptime is — whether you’re running a single truck or a fleet. If your machine is throwing DEF codes, don’t waste time at the dealer.

👉 Visit AftertreatmentParts.com to find the parts you need — in stock and ready to ship.

📞 Have questions? Call us at 1-587-438-8819 — we’ll help you get back on the road fast.

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