Smart Fortwo Engine Air Filter — 451 and 453
Service interval: Every 20,000 mi or annually · More often in dusty conditions
Tools you'll need
- Phillips screwdriver or T-handle Torx (varies by year)
- Clean rag for wiping out the airbox
- Nitrile gloves
What this is and why it matters
The engine air filter is the cheapest part you'll touch on a Smart and it lives a hard life. A dirty filter starves the engine of air, drops fuel economy, and on the turbocharged 451 Brabus and 453 H4Bt it makes the turbo work harder than it should. Replacing it is a ten-minute job with a screwdriver.
The other reason to do it yourself is the upsell trap — quick-lube shops love to charge $50-70 for a $20 part and three minutes of labour. Doing it on your own driveway is straightforward.
What you'll need
Listed above. The only real choice you need to make is the filter itself.
- Genuine Mann or Mahle paper filters are the safe pick. They're what the car came with from the factory.
- Avoid oiled cotton-gauze filters. More on this below.
Step by step
The location is similar across variants — paper filter element inside an airbox accessed from the engine bay or under the rear engine cover.
- Open the rear hatch and remove the plastic engine cover. On the 451 the cover lifts after a couple of plastic fasteners are turned. On the 453, the cover comes off the same way.
- Locate the airbox. It's the rectangular plastic housing with the intake hose running into the engine. On both the 451 and the 453 the airbox sits on top of the engine, accessible from above.
- Release the airbox lid clips or screws. Year-dependent — early 451s have spring clips, later ones use screws or T-handle fasteners.
- Lift the lid and pull the old filter out. Note which way it's oriented. Most filters are directional; the rubber sealing edge faces up or out.
- Wipe the inside of the airbox. Leaves, mouse bedding, and grit love to collect in there. Clean it before the new filter goes in.
- Drop the new filter in, oriented the same way as the old one. Make sure the rubber seal sits flat against the airbox edge — a filter that's not seated lets dirty air past.
- Close the lid, refasten the clips or screws. Don't over-tighten — these are plastic threads. Snug is enough.
- Engine cover back on. Done. Start the car briefly to confirm nothing's loose.
Common gotchas
- Avoid oiled cotton-gauze "performance" filters on the 451 and 453. The MAF sensor downstream of the airbox is contaminated by the oil mist these filters release, especially when re-oiled aggressively. A fouled MAF triggers P0102 (MAF Circuit Low) and similar codes, which can take a lot of cleaning to fix. There's a fault code page on this site for exactly this scenario. Stick with paper.
- Filter orientation matters. A backwards filter still passes air, but the seal won't seat properly and unfiltered air gets past the edges.
- Watch for mouse damage. Smarts that sit for a season often pick up rodent residents in the airbox. Anything more than a bit of bedding means inspect the wiring around the engine bay before you put the cover back on.
- Don't blow out a paper filter and reuse it. It's tempting but the paper fibres are damaged when the filter is dirty enough to need cleaning. Replace, don't recycle.
- The CDI diesel's airbox sits in roughly the same place as the petrol 451, but the filter element is sized differently. Buy the right part for your engine code.
When to skip DIY
Frankly, there's not much excuse to skip this one. The only real reason to outsource is if your engine cover fasteners are stripped or the airbox lid clips are broken — fix those once and you're set for life. If you're at a shop for an oil change anyway, having them swap the filter at the same time is a minor convenience charge, not a rip-off.
Parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Paper air filter — 451 NA / Brabus | $15-25 | Search Google |
| Paper air filter — 451 CDI | $15-25 | Search Google |
| Paper air filter — 453 | $15-25 | Search Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread.
Manual references
- Browse Smart workshop manuals on smartcarmanuals.com — model-specific reference manuals on the home page; pick your chassis code section for torque specs and detailed procedures.
How-to videos
Related fault codes
- P0102 Mass Air Flow Circuit Low on Smart Fortwo 451 / 453
- P0171 System Too Lean on Smart Fortwo 451 / 453