"5mph Max, Service DEF System" - How to Reset This on Trucks?

As you drive your truck one day, you notice the ''5mph Max, Service DEF System'' message appear on the dashboard. And true to its name, your truck refuses to pick up the pace. No matter how hard to push the gas pedal, nothing changes. What can you do to fix this?

We've prepared this article to teach you the following:

• What the ''5mph Max, Service DEF System'' message means
• What DEF is
• How long DEF lasts
• What causes the ''5mph Max, Service DEF System'' message to appear

What does the ''5mph Max, Service DEF System'' message mean?

No one likes seeing a message appear on their truck's dashboard. And while ''Service DEF System'' isn't a sign of a severe issue, you should still address it immediately.

This message points to low levels of DEF, which stands for Diesel Exhaust Fluid. This fluid is necessary for Selective Catalytic Reduction (SCR), a system that purifies the exhaust fumes as they enter your truck.

All trucks made after 2010 have to comply with the EPA rules. That's why manufacturers equip them with the said system. SCR traps toxic nitrogen oxides and converts them to a harmless mixture of water and nitrogen. However, this system can't run without DEF.

When the DEF levels get too low, this purification system won't work correctly. All trucks come with an electronic safety system that limits your maximum speed when this happens. The most common maximum speed is 5 mph.

Low DEF levels won't affect your engine performance. However, that won't help you much if you have to drive at 5 mph per hour. Thus, you must act quickly when this happens to you.

What is DEF?

DEF stands for Diesel Exhaust Fluid. It is a mixture of 32% urea and 67.5% de-ionized water. This substance is an essential component of all modern diesel trucks. It reduces the number of harmful nitrogen oxides your truck creates.

DEF is a cheap and widely available substance, and you should be able to find it at most truck stops. We recommend carrying extra with you on your trips in case it starts running low. When this happens, your truck's speed will become severely reduced.

The mixture is nontoxic, and you don't need any safety equipment while handling it.

Since it contains mostly water, it's sensitive to physical factors such as temperature and humidity. For instance, it freezes at temperatures below 12°F. And while freezing doesn't affect its performance, it still poses a problem. DEF can expand by 7% of its volume when it freezes, which can crack contains. You should thus not keep the container filled to the brim.

Def also degrades when you store it at temperatures above 85°F. This can reduce both its lifespan and performance. It also evaporates if you keep the container open.

Humidity can dilute the mixture if it enters the container, which also lowers its performance.

Well-kept DEF is always a clear liquid.

How long can you store DEF?

Like most substances, DEF doesn't last forever. The urea degrades over time, which reduces its overall effectiveness. However, it should last up to 2 years if you store it properly.

Make sure you keep it away from direct sunlight and under 85°F. Don't use it past the expiration date, as this can lead to reduced performance and even SCR malfunction.

What caused ''5mph Max, Service DEF System'' message? 5 common reasons

The ''5mph Max, Service DEF System'' message always points to an issue with the SCR system. It means the system doesn't have enough DEF to function properly. If it runs too low, safety measures will activate to prevent air pollution. This will restrict your vehicle speed to a maximum of 5 mph.

However, many factors can compromise the levels of DEF in your truck. Here are 5 common reasons why it happens:

1) The levels drop because of poor maintenance

No truck fluid lasts forever, not even DEF. This means you have to refill it regularly to keep your truck functional. However, it's often forgotten about.

When the fluid runs low, replenishing is often enough to fix the issue, as DEF leaks are extremely rare.

But how much DEF should you add to the tank?

You should strive to add at least 2% of the maximum diesel volume to the tank. For instance, a 100-gallon tank would need 2 gallons of the tank to function properly.

Each gallon of DEF should last from 300 to 500 miles, depending on factors like acceleration and idling. However, you should always try to keep the levels stable instead of waiting until they run dry.

2) The fluid's been contaminated.

Although contaminants don't reduce the levels of DEF, they decrease their effective volume. The diluted DEF thus won't work properly.

Contamination usually happens because of a dirty fill cap. DEF picks up dirt and grime as you pour it, which reduces performance. You should thus keep the area around your fill cap as clean as possible while handling refills.

Unfortunately, you have no real way of checking whether your DEF's been contaminated or not. This is only possible with a refractometer, which can be an expensive investment. If you have one, you can use it to check the state of your DEF. The ideal urea concentration should lie between 32.5% to 37%. Both high and low amounts of urea will cause your SCR to malfunction.

If you suspect contamination, you'll have to flush the entire system and clean it. We recommend leaving this task to a mechanic.

Make sure you don't store your DEF in used containers, either. These usually carry a lot of contaminants even if you can't see them - stick to the product container if you can.

3) It's been mixed with diesel

Many new truck owners make the mistake of pouring DEF into the fuel tank. DEF is water-based, while diesel is an oil-based organic compound. Thus, you should never mix them.

Water contamination will severely damage your truck if it enters a running engine. The two filler caps are right next to each other, which can be confusing for new owners.

However, you can tell them apart easily. The DEF cap is usually blue. Moreover, it's much smaller than the diesel cap.

Don't drive your truck if you accidentally mix these two substances. Doing so will cause irreparable damage. The only solution is to flush both tanks, which is something best left to a mechanic.

4) It crystalized

Crystallization is another issue that can lower the effective levels of DEF. This usually happens when you add too much fluid into the tank. The excess fluid will solidify and form crystals within the tanks. These can cover the exhaust and other components of the SCR, which leads to many secondary problems.

Crystallization can also happen if you add hard water to the tank. This is much less likely since most DEF brands already come as solutions.

Unfortunately, you can't do anything about crystallization yourself. You'll need a professional mechanic to test and flush the system. Doing so should reset the ''5mph Max, Service DEF System'' message.

5) It's an electrical issue

If you can't find any issue with your DEF levels, your truck probably has an electric malfunction.

The warning light generally turns on at 10%. It starts flashing at 5%. At 2.5%, the light emits constant light. The ''5mph Max, Service DEF System'' comes on when it runs out completely.

Sadly, you can't fix issues with the computer. If none of the above methods reset the message, you'll have to take your truck to a mechanic.

Our final thoughts on resetting ''5mph Max, Service DEF System'' message

The ''5mph Max, Service DEF System'' message appears when your Diesel Exhaust Fluid runs low. DEF is essential for filtering the toxic nitrogen oxides from exhaust fumes. When the fluid runs low, your track activates a safety measure that lowers your maximum speed. This can be a nuisance if it happens on a highway.

The most common reason why the ''5mph Max, Service DEF System'' message comes up is that you forgot to replenish DEF during the last maintenance. Simply refilling it up to the recommended levels should be enough to fix the issue.

The message can also appear if you don't clean the filler cap often. The fluid can wash down dirt as you pour it into the tank. These contaminants then lower its performance, causing the message to appear.

New truck owners may accidentally pour DEF into their diesel reservoirs. Driving the truck in this condition can lead to severe engine damage.

DEF can also form crystals in high concentrations. This happens when you use too much of it. The crystal can flog filters and injectors, which can lead to other issues with the system.

And finally, the ''5mph Max, Service DEF System'' may appear because of an electrical issue. The computer may falsely read the status of your DEF levels, prompting the message. In this case, the message won't go away until a mechanic resets the entire system.