How To Check Ford F350 DEF Fluid Level?

Diesel exhaust fluid (DEF) is an important fluid used in diesel engines to reduce harmful emissions. All Ford F350 trucks with diesel engines built since 2011 use DEF. Checking and maintaining proper DEF levels is crucial to keep your truck running smoothly. But how exactly do you check the DEF fluid level in a Ford F350?

What is DEF and Why is it Important?

DEF is a liquid solution composed of urea and deionized water. It is sprayed into the exhaust stream of diesel engines to break down dangerous NOx emissions into harmless nitrogen and water vapor.

All diesel vehicles built after 2010 use selective catalytic reduction (SCR) systems that require DEF. SCR technology can reduce NOx emissions by up to 90%.

Maintaining proper DEF levels is critical for SCR systems to function correctly. Running low on DEF can lead to a range of issues:

  • Reduced engine power
  • Increased exhaust emissions
  • Engine damage over time

Monitoring and refilling DEF regularly is vital for diesel engine health, performance, and emissions compliance.

Locating the DEF Tank

The first step in checking DEF levels is locating the DEF tank. In Ford F350 trucks, the tank is usually found on the driver's side of the vehicle near the regular diesel fuel tank.

On Super Duty F350 models, the DEF tank is mounted on the inboard side of the frame rail behind the fuel tank. It is rectangular in shape with a blue cap on top.

Some key things to look for when locating the DEF tank:

  • Check driver's side behind the fuel filler door
  • Look for a rectangular blue tank near fuel tank
  • DEF filler cap will be blue and labeled “DEF”

Knowing exactly where the DEF tank is located makes it easier to keep an eye on fluid levels.

Checking the DEF Gauge

Many newer Ford F350 trucks have a dedicated DEF gauge on the instrument cluster that displays the fluid level.

If your F350 has this gauge, start the engine and check it for the DEF level. The gauge will show levels ranging from full to empty.

Ideally the gauge should read at least 1/4 full. Anything below this level, especially the “low” mark, means it's time to refill the DEF tank soon.

Monitoring the DEF gauge regularly when driving is the simplest way to stay on top of fluid levels in modern Ford trucks.

Using the DEF Menu

Not all Ford F350 models have a dedicated DEF gauge. In trucks without one, the fluid level can be checked using the menu screen:

  1. Turn ignition on, but do not start the engine
  2. Using steering wheel controls, scroll to the “Information” menu
  3. Go to “Truck Info” or “Vehicle Info” submenu
  4. Select “Diesel Exhaust Fluid” status

Checking the DEF Cap

Some Ford F350 Super Duty trucks have a DEF tank cap with an integrated dipstick:

  • Unscrew the cap which will reveal a yellow dipstick
  • Remove the cap and check the fluid level on the dipstick
  • Level should be somewhere between the “MIN” and “MAX” lines
  • Replace the cap tightly when finished

When to Refill the DEF Tank

Refilling the DEF tank is required once levels start dropping low:

  • If the gauge, menu, or dipstick reads below 25%, refill DEF
  • Tank should be refilled well before the “empty” warning
  • Ford recommends keeping tank over 10% full at all times

Running out of DEF can quickly lead to power loss and engine problems. Topping off the tank once levels drop prevents any issues.

Ideally refill the tank when DEF is between 10-25% full for optimal operation.

Adding DEF Fluid

Refilling the DEF tank is simple in a Ford F350:

  1. Locate the blue DEF filler cap
  2. Unscrew the cap slowly to release any pressure
  3. Fill the tank using a DEF dispensing nozzle
  4. Fill until the tank is full and fluid reaches the bottom of the filler neck
  5. Replace and tighten cap when finished

Only use DEF that meets ISO 22241 standards. Many gas stations and parts stores sell DEF. Avoid overfilling the tank past the filler neck.

Long-Term DEF Storage

DEF has a limited shelf life when stored for long periods:

  • Over time, DEF naturally breaks down into ammonia and becomes ineffective
  • DEF has a maximum shelf life of approximately 18 months
  • Always buy DEF in small quantities if truck is used infrequently
  • Discard any old or expired DEF

Only keep enough DEF on hand for short-term use. Replace stale DEF that has been stored for over 18 months.

Proper storage keeps DEF fresh and effective over the long run.

Cooling System Problems

In rare cases, very low DEF levels can lead to emissions system overheating and coolant leaks:

  • SCR systems rely on DEF to properly cool exhaust gasses
  • Running out of DEF can cause excessive exhaust temperatures
  • This added heat can potentially damage hoses and seals
  • Coolant leaks near DEF tank may indicate this issue

Maintaining adequate DEF and avoiding complete depletion reduces the chances of overheating and coolant leaks related to boosted exhaust temps.

DEF Warning Lights

Ford F350 dashboard warning lights related to DEF:

  • DEF Low: comes on when fluid level is low
  • DEF Empty: indicates tank is empty and needs immediate refilling
  • SCR Warning: general fault with SCR emissions system

Additionally, a yellow “check engine” light may indicate a DEF system malfunction.

Do not ignore DEF warning lights. Promptly refill the tank if the low or empty indicator comes on. Severe power loss can occur if driving with critically low levels.

The Bottom Line

Checking and filling DEF is essential for keeping your Ford F350 diesel running smoothly. Monitor the fluid levels regularly via the gauge, menu, or dipstick based on your truck's setup. Refill the tank promptly when the DEF level drops below 25%.

Proper DEF maintenance prevents power issues, elevated emissions, and excessive engine wear. By closely following Ford's fill recommendations, your DEF system will stay in top shape for the long run.


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