What Is a Catalytic Converter?
The catalytic converter (commonly called "the cat") is an emissions control device fitted to the exhaust system of virtually all modern petrol and diesel vehicles. It converts toxic combustion byproducts into less harmful compounds before they exit the tailpipe.
Inside the cat is a ceramic or metallic honeycomb substrate coated with precious metals — primarily platinum, palladium, and rhodium. These metals act as catalysts, triggering chemical reactions without being consumed themselves.
What Does the Catalytic Converter Do?
Three main reactions occur inside the cat:
- Oxidation of carbon monoxide (CO) → CO₂
- Oxidation of unburnt hydrocarbons (HC) → CO₂ + H₂O
- Reduction of nitrogen oxides (NOx) → N₂ + O₂
The converter needs to reach an operating temperature of around 400–800°C to function efficiently — this is why cold-start emissions are higher. Modern vehicles use a small close-coupled catalyst near the engine manifold to heat up quickly, alongside a larger cat further downstream.
Why Are Catalytic Converters Stolen So Often in NZ?
The precious metals inside a catalytic converter have significant scrap value — particularly rhodium, which has traded above $20,000 USD per troy ounce in recent years. Thieves can remove a cat with an angle grinder in under two minutes, and scrap dealers historically paid well for them.
New Zealand has seen a significant rise in cat theft, particularly targeting:
- Toyota Prius and other hybrids (higher precious metal loading)
- Toyota Hilux and Land Cruiser
- Subaru Impreza and Forester
- Older Honda and Suzuki models
See our dedicated guide: Catalytic Converter Theft in NZ
Signs of a Failing or Damaged Catalytic Converter
| Symptom | Notes |
|---|---|
| Rattling from under the car | Internal substrate broken — often after a kerb strike |
| Sulphur (rotten egg) smell from exhaust | Rich running condition, overloading the cat |
| Reduced power and acceleration | Blocked cat restricting exhaust flow |
| Check Engine light (O2 sensor codes) | Cat efficiency below threshold |
| Failed emissions test | Cat not converting gases effectively |
| Sudden loud exhaust noise | Cat has been stolen |
WoF Implications
A missing or damaged catalytic converter is a WoF failure in New Zealand. NZTA requires that exhaust systems meet emission standards and are structurally complete. If your cat has been stolen or has disintegrated internally, you will not pass a WoF until it is replaced.
Excessive exhaust smoke or a strong sulphur smell at the tailpipe can also trigger a failure if the inspector deems emissions excessive.
NZ Repair Cost Estimates
| Repair | Typical NZ Cost |
|---|---|
| OEM catalytic converter replacement | $800–$2,000 fitted |
| Aftermarket cat replacement | $400–$900 fitted |
| Universal cat (welded in) | $300–$600 at a muffler shop |
| Anti-theft cat protection device | $200–$600 installed |
Note: Fitting a non-compliant cat that doesn't meet NZ emission standards is illegal and will cause a WoF failure. Always ensure replacements are certified for your vehicle and region.
When to Book a Mechanic
- You hear a new rattling from under the car (especially after going over a kerb)
- There's a sulphur smell from the exhaust that wasn't there before
- Your car lacks power and the Check Engine light is on
- You suspect theft — look under the car with a torch if safe to do so
Don't delay on a rattling cat — broken internal substrate can be sucked into the engine or block exhaust flow entirely, causing significant further damage.