4 Common DEF Problems Every Diesel Owner Should Know How to Fix

Experiencing power loss, decreased fuel economy, or unexpected shutdowns in your diesel vehicle? That DEF warning light on your dashboard might be trying to tell you something important. Let’s dive into what’s happening and how you can solve it without expensive trips to the repair shop.

What is DEF and Why Does it Matter?

Diesel Exhaust Fluid (sometimes called AdBlue®) isn’t just another fluid in your vehicle – it’s essential for modern diesel engines to meet emission standards. DEF works with your vehicle’s Selective Catalytic Reduction (SCR) system to convert harmful nitrogen oxides in your exhaust into harmless nitrogen and water vapor.

Think of DEF as your diesel’s environmental partner, helping it run cleaner while maintaining performance. The fluid consists of 32.5% urea and 67.5% deionized water – a precise ratio that gives it specific properties designed for optimal performance.

Problem #1: Low DEF Levels

The Warning Signs: That DEF light on your dashboard is usually your first clue. You might also notice your vehicle entering reduced power mode if levels get critically low.

The Solution:

• Regularly check and refill your DEF tank, ideally when refueling

• Look for the DEF filler port (typically near your fuel fill opening on pickups)

• Always use high-quality DEF that displays the API Certification Mark

• Check for leaks if you find yourself refilling more frequently than expected

Keep in mind that your vehicle consumes more DEF during heavy towing or high-speed driving, so plan accordingly if you’re using your diesel for work purposes.

Problem #2: Crystal Deposits in Your DEF System

The Warning Signs: Reduced performance, fault codes (especially P20EE), and system malfunctions can all point to crystal buildup in your DEF system.

The Solution:

• Use a specialized system cleaner like Rislone DEF Crystal Clean® to dissolve existing crystal deposits

• Follow up with Rislone DEF Treatment to prevent new crystal formations

• These products can be added directly to your DEF tank and won’t void your warranty

• Regular treatment helps prevent costly system repairs down the road

The white crystal deposits form when water evaporates from DEF, leaving behind urea crystals that can clog components throughout the system. Cleaning these deposits restores proper function without expensive mechanical repairs that can often cost $1,500+.

Problem #3: DEF Contamination

The Warning Signs: Erratic performance, warning lights, and potential system damage can result from contaminated DEF.

The Solution:

• Always clean the area around your DEF fill cap before refilling

• Use only dedicated funnels and containers for DEF

• Store DEF properly – away from direct sunlight in a cool, dry place

• Look for API-certified DEF that’s clear and free of particles

• Never mix other fluids with DEF, even accidentally

If other fluids have been accidentally added to your DEF tank, don’t start your vehicle. Have the system professionally drained and inspected to prevent potentially extensive damage.

Problem #4: Frozen DEF

The Warning Signs: DEF warning lights in very cold weather, even with adequate fluid levels.

The Solution:

• Remember that DEF freezes at 12°F (-11°C)

• Allow your engine to idle to activate the DEF tank heater

• Don’t add antifreeze or other chemicals to prevent freezing – they’ll damage the system

• Avoid filling the DEF tank completely in freezing weather – the fluid expands by 7% when frozen

• Don’t worry about performance once thawed-freezing doesn’t damage the fluid itself

Most vehicles are designed to operate normally while frozen DEF thaws, though your warning light may remain illuminated temporarily.

Protecting Your Diesel Investment

Your diesel vehicle represents a significant investment, and proper DEF management is essential for its longevity and performance. The SCR system components are expensive to replace, making prevention much more cost-effective than repair.

By keeping an eye on your DEF levels, using quality fluid, and addressing issues promptly, you’ll maximize your diesel’s performance, maintain fuel efficiency, and avoid those frustrating breakdowns and repair bills.

Remember – what might seem like a minor issue with your DEF system today could lead to significant problems tomorrow. A little DEF maintenance now saves a lot of trouble later.

This post was prepared with technical information from Rislone, a global leader in diesel additives and engine treatment solutions since 1921. Rislone’s DEF solutions are entirely made in the USA and designed to work with all UREA DEF/AdBlue® fluids without voiding manufacturers’ warranties.

AdBlue® is a registered trademark of VDA Verband der Automobilindustrie.