Where the air filter sits and why it matters

The Freightliner Cascadia has two air filters: the primary filter and the secondary filter. Both sit in the air intake system on top of the engine, inside a housing unit called the air cleaner assembly. The primary filter catches large particles, and the secondary filter catches smaller dust and debris that slip through. When either one gets clogged, your engine has to work harder to pull air, which burns more fuel and reduces power.

On a Cascadia, the air cleaner assembly is mounted vertically on the driver's side of the engine compartment, usually near the top. You'll see a large cylindrical or rectangular plastic housing — that's where both filters live. Replacing them is a straightforward job that takes about 15 to 20 minutes once you know where everything is.

Key Takeaways

  • The Cascadia's primary and secondary air filters sit in a single assembly on top of the engine, and both should be replaced at the same time.
  • You need only basic tools — a wrench or socket to remove the housing bolts, and your hands to pull out the old filters and drop in the new ones.
  • The primary filter is the larger outer element; the secondary is the smaller inner element that sits in the center of the housing.
  • Freightliner recommends checking the filters every 50,000 miles and replacing them when they show visible dirt or at 200,000 miles, whichever comes first.
  • Ordering the correct part number for your model year matters — Cascadia filters changed between generations, so verify your year before buying.

What you need before you start

Gather a wrench or socket set (usually 10mm or 13mm, depending on your model year), a clean rag, and the replacement filters. You'll also want a flashlight or headlamp so you can see inside the housing once you open it. Have a small container or trash bag nearby — old filter material sheds dust, and you don't want it falling into the engine.

Order the correct filters before you begin. Freightliner Cascadia air filters are sold by model year and engine type. The primary filter part number and secondary filter part number are different, so you need both. Check your truck's build sheet or contact a Freightliner dealer with your VIN to confirm the exact part numbers. Buying the wrong size wastes time and money.

How to remove the old filters

Park on level ground, shut off the engine, and let it cool for a few minutes. Open the hood and locate the air cleaner assembly — it's the large plastic housing on top of the engine. You'll see bolts or clips holding the top cover on. Use your wrench or socket to remove these fasteners (usually three to five bolts). Set them aside in a safe place so you don't lose them.

Lift the cover straight up and set it aside. Inside, you'll see the primary filter — a large cylindrical or accordion-shaped element that fills most of the housing. Grab it by the top and pull it straight out. Don't twist or force it; it should come free with steady upward pressure. Once the primary is out, you'll see the secondary filter in the center of the housing — it's smaller and looks like a cartridge. Pull that out the same way.

Before you install the new filters, wipe the inside of the housing with a clean rag to remove any loose dust or debris. Use your flashlight to check that the housing is clean and that the rubber seal around the bottom is intact and not cracked. If the seal is damaged, the housing itself may need to be replaced, so check that before moving forward.

Installing the new filters

Take the new secondary filter and insert it into the center post of the housing. It should slide down and seat firmly at the bottom. The secondary filter is smaller and goes in first. Make sure it's centered and fully seated — it should not rock or shift when you press gently on it.

Now take the new primary filter and lower it over the secondary filter. The primary filter is larger and surrounds the secondary. Lower it straight down until it rests on the bottom of the housing. Again, it should seat firmly and not move when you press on it. Both filters need to be fully seated so air doesn't bypass them and pull unfiltered air into the engine.

Place the cover back on top of the housing and hand-tighten the bolts first. Once all bolts are snug, use your wrench or socket to tighten them in a crisscross pattern (like tightening a wheel) so the cover seats evenly. Don't over-tighten — snug is enough. Over-tightening can crack the plastic housing.

After you've finished

Start the engine and let it idle for a minute. Listen for any unusual sounds or air leaks around the housing. If everything sounds normal, you're done. The engine should feel slightly more responsive because it's pulling cleaner air with less restriction.

Write down the mileage and date on a piece of tape and stick it to the air cleaner housing. This helps you track when the filters were last replaced and reminds you when to check them again. Freightliner recommends inspecting the filters every 50,000 miles — if they look dirty, replace them. If they still look clean, you can usually go to 200,000 miles before a replacement is necessary, though heavy dust or off-road driving shortens that interval.

Part numbers and where to order

Freightliner Cascadia air filter part numbers vary by model year and engine. For example, a 2008–2017 Cascadia with a Detroit or Cummins engine uses different filters than a 2018 and newer model. The primary filter and secondary filter also have separate part numbers.

You can order filters from a Freightliner dealer, a truck parts supplier like Stertil-Koni or Truck Hero, or online retailers that stock OEM Freightliner parts. Prices typically range from $40 to $120 per filter depending on the supplier and whether you're buying OEM or an aftermarket equivalent. Always verify the part number matches your truck's year and engine before checkout.

Frequently Asked Questions

How often should I replace the air filters?

Freightliner recommends checking them every 50,000 miles and replacing them when they show visible dirt accumulation. If they still look clean, you can go up to 200,000 miles. Heavy dust environments, construction sites, or unpaved roads reduce that interval significantly — check them more often if you drive in those conditions.

Can I replace just the primary filter and leave the secondary?

You can, but it's not recommended. Both filters work together, and if the secondary is already dirty, it will clog faster once the primary is new. Replacing both at the same time ensures even airflow and extends the life of the new primary filter.

What happens if I drive with a clogged air filter?

A clogged filter restricts airflow to the engine, which increases fuel consumption, reduces power, and can trigger a check engine light. Over time, it can also allow unfiltered dust to reach the engine, causing wear on internal parts. Replace it as soon as you notice reduced performance or visible dirt on the filter.

Do I need a dealer to replace these, or can I do it myself?

You can do it yourself if you're comfortable with basic hand tools. The job takes 15 to 20 minutes and requires only a wrench or socket set. A dealer will charge labor time on top of parts, so doing it yourself saves money. If you're unsure, a dealer can handle it quickly.

What's the difference between OEM and aftermarket air filters?

OEM filters are made by Freightliner or their approved suppliers and are may provide to fit and perform to spec. Aftermarket filters are made by third-party manufacturers and are usually cheaper but may vary in quality and fit. Both work, but OEM filters carry a warranty and are a safer choice if you're unsure.