If your diesel exhaust fluid (DEF) light is on, you need to solve this issue as quickly as possible. Your selective catalytic reduction (SCR) system’s warning light helps you recognize and solve potential problems before major damage occurs. Here’s how these systems work and how to solve the problem.
What Does the DEF Warning Light Mean?
The DEF warning light generally indicates low diesel engine fluid levels, but it may also signal a secondary problem with the system.
SCR systems reduce nitrogen oxide emissions using ammonia, but cannot perform this vital function without diesel exhaust fluid. The system design ensures you cannot continue driving long-term without refilling the tank or repairing the problem. After a specific period, the vehicle will enter limp mode or reduce power to the engine.
Common Causes of a DEF Warning Light
Whether you’re experiencing an AdBlue/DEF fault on your Audi A4 or a warning light on another vehicle, your DEF light generally comes on due to one of a few reasons:
Low Diesel Exhaust Fluid Levels
A low fluid level is the most common activator of a DEF warning light. It simply means that it’s time to refill your vehicle’s DEF tank. If refilling the tank and recalibrating the system doesn’t solve the problem, there’s a secondary issue. A high-quality diesel exhaust fluid is essential, since lower-quality fluids come with unique risks and may not produce the desired result.
Contaminated Diesel Exhaust Fluid
If your DEF system is contaminated with dust and debris, it can abrade system components and cause the DEF warning light to come on. Adding the wrong fluid type to the DEF tank also qualifies as contamination and may damage the system. To avoid this, store the fluid in a sealed container and clean the area around the DEF fill cap before adding fluid. You’ll also want to ensure the cap is securely closed to prevent contamination entry.
Crystal Formation
Your SCR system is sensitive to urea crystal formation, and this crystal buildup may lead to clogging or similar concerns. Your diesel engine fluid consists of 32.5% urea and 67.5% deionized wate . If the water evaporates, the ratio shifts, resulting in urea crystal formation. Excessive water content may also contribute to crystallization.
Frozen DEF Fluid
In a running vehicle, freezing isn’t a concern since the SCR system automatically heats the fluid when temperatures dip below a certain threshold. However, freezing occurs in a stored or parked vehicle at temperatures below 12°F. If the DEF fluid freezes, idling the car for a while should thaw it out.
Most SCR systems have the features needed to operate as DEF thaws. The warning light comes on until the fluid thaws. After thawing, the system should operate normally.
What to Do When the DEF Warning Light Comes On
When the DEF warning light comes on, determine whether the fluid is frozen. If the car has been parked for a few hours in cold temperatures, idle the vehicle for a few minutes, and the fluid should thaw, fixing the problem.
If the fluid isn’t frozen, check the diesel exhaust fluid levels. Most of the time, refilling the tank with premium exhaust fluid should solve the problem. However, resetting the low DEF warning light requires additional input. Drive the vehicle at a consistently slow, steady speed until the system recalibrates.
If the system doesn’t recalibrate after this process, it may be experiencing crystal buildup. Invest in a product like Premium DEFender™ that cleans crystal formation, clears related trouble codes and protects the system against recurring problems.
If none of these solutions resolves the problem, your system is likely contaminated and needs to be drained, flushed and refilled.
DEF System Limp Mode
If your DEF warning light is on, the SCR system either derates your engine — meaning it slowly reduces power — or enters limp mode, when the system cuts engine power and speed. Newer systems have the slow deration system, while older systems enter limp mode. This power reduction approach reduces the risk of causing additional damage due to low fluid levels or system malfunctions.
No matter which diesel you drive, the system will eventually deactivate and cut all power to the engine. If your vehicle enters limp mode — or preferably before then — apply the solutions mentioned earlier to restore your system to full function.
How to Prevent the DEF Warning Light from Coming Back
Preventing a recurring DEF warning light or similar problem is simple. In addition to filling the tank regularly, use high-quality DEF fluid, store it in a cool place away from direct sunlight, perform regular maintenance and use a trustworthy cleaning agent.
A product like Premium DEFender simplifies DEF maintenance by removing existing crystal buildup and preventing recurrence. This approach improves vehicle performance, and a functioning SCR system significantly reduces emissions.
FAQs About the DEF Warning Light
Looking for answers fast? Review our FAQs.
Can additives fix a DEF light issue?
Additives often can fix a DEF light issue, they can help to solve crystallization issues. A targeted product like Premium DEFender cleans, protects and enhances your DEF system to improve results and eliminate crystallization issues.
Can I drive with the DEF warning light on?
Yes, you can, but not for long. The diesel exhaust fluid light on your dashboard is an initial indicator. If you continue to drive your car or truck, the light will start flashing and the system will cut power to the engine. This light is a high-priority indicator, and you should address the issue promptly.
What happens if I ignore the DEF light?
If you ignore the DEF warning light, the system will enter limp mode. Your vehicle will have reduced engine power and limited speed. If you continue driving the vehicle in limp mode, the DEF light starts blinking, and the engine will eventually power down.
How do I reset the DEF light after filling the tank?
After refilling the DEF tank, start the engine and drive slowly and steadily for several minutes. A few minutes of driving will recalibrate the system. If it doesn’t, you may need to use an OBD-II scanner to identify the cause of the problem.
Repair Your DEF Warning Light for Good With Hot Shot’s Secret
From the DEF light on the dashboard of your Audi A3 to a heavier-duty vehicle, solving a problem with this system generally follows similar principles. Most problems result from contamination, crystal buildup or an empty DEF tank. Order Hot Shot’s Secret Premium DEFender to maintain your vehicle’s SCR system and eliminate future crystallization issues.