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How Long Does DEF Last in 6.7 Powerstroke? SCR Explained

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Due to the ever-increasing exhaust emission standards and regulations, a once simple and rudimentary diesel engine is now jam-packed with more technology than the rocket that got us to the Moon. Naturally, it’s easy to get confused over all the technical mumbo-jumbo that surrounds it.

One of the most confusing things that 6.7 Powerstroke owners have to face is the Diesel Exhaust Fluid (DEF). So, in this article, we’ll take a quick look into how it works, why it’s needed, and most importantly – how long the DEF in the 6.7 Powerstroke lasts on average. 

What Is DEF and What Does It Do?

In a nutshell, DEF is a urine-smelling mixture of 1/3 urea and 2/3 de-ionized water that is sprayed in your truck’s exhaust to further reduce Nitrogen Oxide emissions. Fun fact: it’s an excellent fertilizer, and if you’ll accidentally spill some on your lawn – you’ll definitely know it in a month.

What pushed diesel engine manufacturers to use such technology is the US NOx emission standard of 0.20 grams per brake horsepower hour, which became mandatory for all diesel engines after 2010. In fact, this technology is called Selective Catalytic Reduction (SCR) and it has been around for decades, even before 2010. It was primarily used in coal and gas-fired power plants to reduce NOx emissions into the atmosphere.

How DEF & SCR Works on the 6.7 Powerstroke

The whole process of reducing NOx emissions inside the 6.7 Powerstroke is pretty simple.

First, exhaust gases leave the engine and pass through the Catalytic Converter – which filters out the initial NOx.

Next, the SCR system constantly monitors the amount of NOx at the inlet and adjusts the amount of DEF that is sprayed into the exhaust accordingly.  The urea inside DEF reacts with the remaining Nitrogen Oxides coming out of the Catalytic Converter and converts it into nitrogen and water – both completely harmless to the Earth’s atmosphere.

On the outlet side, an additional outlet NOx sensor monitors the efficiency of the chemical reaction and warns the driver if the system isn’t working correctly – that’s where the P20EE – SCR NOx Catalyst Efficiency Bellow Threshold (Bank 1) comes from.

How Long Does DEF Last in the 6.7 Powerstroke?

The number of miles that the DEF will last in your 6.7 Powerstroke largely depends on your driving habits and how often you’re towing heavy.  However, in most cases, a single tank of DEF will last between 5000 to 6000 miles on the 6.7 Powerstroke

But if you want to get specific, you can actually calculate your average DEF requirements since DEF is usually consumed at a rate of 2-3% relative to the amount of diesel that you use.

For example, on a long-bed Ford F350 with a 38-gallon fuel tank, you’ll use up around 0.76 to 1.14 gallons of DEF, every time your fuel tank goes from full to empty.

How & When to Fill up DEF

Filling up your truck with DEF is extremely easy.

Just open up the fuel door, find and unscrew the small blue DEF filler cap and fill it up with DEF. You can fill it until it’s completely full, but you can also estimate the amount of DEF you’ll have to fill by knowing that it’s a 5-gallon tank, and that once you get the “Less than half full” message on your dash, you can safely put a 2.5-gallon jug in it.

As for when to fill your 6.7 Powerstroke with DEF, you can use the DEF gauge on the dash to know approximately how much DEF is in the tank, or you can calculate it.

Taking the example above, if you own a truck with a 38-gallon tank, you’ll use up around 2-3% of DEF every fill-up – that’s around 0.76 to 1.14 gallons of DEF. Therefore, you should fill up on DEF every third or fourth time you fill up your truck with Diesel.


Modern high-performance diesel engines, like the 6.7 Powerstroke, are pretty complex. They’re fitted with a bunch of new emission reduction technologies, that need regular maintenance – DEF & SCR is one of them.

Luckily, it’s pretty easy to maintain the SCR system on a 6.7 Powerstroke – all you need to do is to keep your DEF tank full, which lasts for 5000-6000 miles.


Diesel Exhaust Fluid: Fastest Growing Non-Fertilizer Use for Urea – IHRDC

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