What Happens If You Run Out of DEF? (And How to Get Moving Again)
Diesel Exhaust Fluid (DEF) is essential for your truck or equipment’s emissions system — but what happens when you run out?
Whether it’s by accident or a faulty DEF level sensor giving you false readings, running out of DEF while on the road can lead to engine power loss, forced shutdown, or even expensive towing and downtime.
In this post, we’ll explain exactly what happens when your DEF tank goes empty, how different truck manufacturers handle it, and what you can do to get back up and running fast.
🚨 What Happens When You Run Out of DEF?
Most diesel trucks and off-road equipment built after 2010 are equipped with Selective Catalytic Reduction (SCR) systems, which use DEF to reduce NOx emissions.
When the DEF tank runs dry, the system will trigger a series of escalating warnings and consequences:
❗ 1. Warning Lights and Messages
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DEF low warning on dash
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Countdown of miles or engine hours remaining
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Emissions system warning indicator
⏳ 2. Engine Derate
If the DEF tank isn’t refilled, the ECM will limit engine power to encourage compliance.
Common symptoms:
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Reduced torque and acceleration
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Max speed limited (often 8–30 km/h or 5–20 mph)
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Poor fuel economy
⛔ 3. No Start or Shutdown Condition
Once the system detects extended operation with no DEF, some trucks will:
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Disable the ability to restart after shutdown
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Limit RPM to idle only
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Require dealer intervention to clear codes and restore normal function
🧠 What Manufacturers Do When DEF Runs Out
Each OEM handles it differently, but all follow EPA enforcement protocols — meaning your engine must enter a derate or shutdown scenario once DEF is exhausted.
🛠️ How to Get Back on the Road
If you’ve run out of DEF, here’s what to do:
✅ Step 1: Add Fresh DEF
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Use only ISO 22241-compliant DEF
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Add at least 2–5 gallons (8–20L) — some systems require a minimum level before allowing reset
✅ Step 2: Cycle the Key
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Turn the ignition off for 2 minutes
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Restart the engine — some systems will auto-clear the derate once DEF is detected
✅ Step 3: Monitor for Remaining Fault Codes
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If derate or shutdown persists, you may need diagnostic software to:
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Clear inactive codes
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Confirm DEF pressure and level sensor data
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Force an SCR reset or perform a DEF system test
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✅ Step 4: Replace Faulty DEF Components (If Needed)
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If adding DEF doesn’t resolve the issue, you may have a:
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Faulty DEF level sensor
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Failed DEF pump
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Crystallized DEF line or blocked injector
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💡 Can You Bypass the DEF System?
Short answer: No.
Bypassing the DEF system is:
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Illegal in Canada and the U.S.
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Detectable by ECM inspection
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Grounds for heavy fines and lost warranty coverage
Instead, fix the issue properly with quality parts and avoid costly legal and mechanical consequences.
🛒 Need DEF System Parts Fast?
At AftertreatmentParts.com, we stock:
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DEF Pumps
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DEF Headers / Level Sensors
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NOx Sensors
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DEF Injectors and Hoses
✅ Fast shipping to Canada & U.S.
✅ OEM-quality parts at discounted prices
✅ 365-day warranty on most components
👉 Shop DEF Parts Now
📞 Have questions? Call us at [1 587 438 8819]
🧭 Final Thoughts
Running out of DEF can bring your operation to a halt, but with the right steps, you can be back on the road quickly. Don’t ignore DEF warnings — and always keep spare DEF on hand, especially on long hauls or in remote areas.
For DEF system troubleshooting help, part recommendations, or bulk orders, reach out to our team — we’re here to keep your equipment running reliably.