Car Air Filter: What It Does, When to Replace It, and What It Costs

What Is an Engine Air Filter?

The engine air filter (also called the air cleaner element) is a replaceable filter element that removes dust, pollen, insects, and other airborne contaminants from the air before it enters the engine's intake system.

Combustion engines require large volumes of clean air to function. A typical four-cylinder engine at motorway speed consumes around 30–40 litres of air per second. Any contaminants in that airflow will enter the cylinders, abrading the cylinder walls, pistons, and rings over time — causing premature engine wear.

Where Is the Air Filter Located?

The air filter sits inside the air filter housing (also called the airbox), connected to the intake hose that leads to the throttle body and intake manifold. On most NZ passenger cars — Mazda Demio, Honda Jazz, Toyota Corolla — the airbox is a black plastic box visible in the engine bay, usually on the left or right side.

Replacement is straightforward on most vehicles and can be done without special tools.

What Happens When an Air Filter Clogs?

A blocked air filter restricts airflow into the engine. The engine compensates by drawing more vacuum, but this:

• Increases fuel consumption — the engine must work harder
• Reduces power — less air means less combustion energy
• Increases emissions — the engine runs richer (too much fuel for the air available)
• Can damage the MAF sensor — restricted airflow can cause turbulent, inaccurate readings
• Damages turbos — on turbocharged engines, a clogged filter makes the turbo work harder and can cause oil to be sucked back through the turbo's compressor seals

In extreme cases of neglect, a severely clogged filter can collapse, allowing unfiltered air (or filter material itself) to reach the engine — causing significant damage.

How Often Should You Replace the Air Filter?

Most manufacturers recommend replacing the engine air filter every 15,000–25,000 km or every 12–24 months, but this varies considerably:

• Urban driving in dusty or high-pollution conditions shortens the interval
• Rural NZ driving on gravel roads dramatically shortens it — the Waikato, Canterbury plains, and Otago rural roads are particularly dusty
• Clean motorway driving extends it

A quick visual inspection tells you a lot: a new filter is white or light cream. A filter that is grey, brown, or visibly clogged needs replacing.

Air Filter Inspection at WoF

While a clogged air filter won't directly fail a WoF, it contributes to the rich running condition that can cause excessive emissions — which will fail a WoF. A good WoF mechanic may also note a compromised air filter as a recommendation.

Types of Air Filters

Performance drop-in filters (like K&N or Pipercross) are legal in NZ and will pass a WoF as long as the factory airbox is retained. Pod/cone filter setups replacing the entire airbox are a grey area — they can cause issues with the MAF sensor and emissions compliance.

NZ Cost to Replace an Air Filter

This is one of the cheapest preventative maintenance items on your car. Skipping it to save $30 is a false economy when the consequences include a blocked turbocharger or damaged MAF sensor.

When to Replace Your Air Filter

• Visually grey/brown when inspected
• More than 15,000 km since the last replacement
• You've been driving on gravel roads regularly
• Fuel economy has noticeably dropped
• The engine seems sluggish, especially at higher RPM

Further Reading

• Find a mechanic near you
• Turbocharger Explained
• MAF Sensor Explained
• Cabin Air Filter (Pollen Filter) Explained