Catalytic Converter
Also known as: cat · catalyst
Quick answer
The catalytic converter is a chemical reactor in your exhaust that converts the engine's three worst pollutants into carbon dioxide, water, and nitrogen, using precious-metal coatings at high temperature. It has no moving parts and can last the vehicle's life — unless engine problems poison or overheat it.
Inside the stainless shell is a ceramic honeycomb washcoated with platinum, palladium, and rhodium. As exhaust flows through at 400–800°C, those metals catalyze reactions that finish what combustion left undone: unburned hydrocarbons and carbon monoxide get oxidized, and nitrogen oxides get reduced back to plain nitrogen. The precious-metal content is why converters are expensive — and why thieves target them.
A healthy converter is a victim-in-waiting: it has no wear mechanism of its own, but it inherits every upstream sin. Misfires send raw fuel into it, which burns inside the converter and melts the honeycomb. Oil or coolant consumption coats the catalyst and deactivates it. A chronically rich mixture overheats it slowly. This is why our P0420 page insists on ruling out engine causes before buying a converter — a new one installed under the same conditions dies the same death.
The computer grades converter health by comparing the oxygen sensors before and after it: the downstream signal should be calm while the upstream one switches rapidly. When the rear starts mirroring the front, efficiency has dropped, and the famous P0420/P0430 codes appear.
Signs it’s failing
- ⚠ P0420 or P0430 (efficiency below threshold) — the formal diagnosis
- ⚠ Rotten-egg (sulfur) smell, especially under load — the converter struggling with a rich mixture
- ⚠ Loss of power, like the engine is breathing through a straw — internal melting/clogging restricting exhaust flow
- ⚠ Rattling from underneath — the honeycomb broken into pieces
- ⚠ Glowing-red converter shell after driving — severe overheating from raw fuel, an emergency
- ⚠ Failed emissions test on tailpipe readings
Trouble codes this part can trigger
Frequently asked questions
- Why are catalytic converters stolen?
- For the platinum, palladium, and rhodium inside — a few grams each, but worth real money at recyclers. Trucks and hybrids are favorite targets (ground clearance and cleaner, richer catalysts respectively). Etching your VIN on the shell and anti-theft shields are the common deterrents.
- Do 'cat cleaner' additives work?
- On a converter that's merely contaminated and borderline, an additive plus a long hot highway drive occasionally buys time. On a worn or melted converter, nothing in a bottle rebuilds precious-metal surfaces. They're a $20 lottery ticket, not a repair.
- Can I replace it with a cheap universal converter?
- The bargain universals often can't satisfy the efficiency monitor — P0420 returns and the money is spent. Use an OEM unit or a quality direct-fit converter that's emissions-certified for your vehicle (California has stricter requirements than other states).
- Is it illegal to remove it?
- Yes — federal law in the US, and similar laws nearly everywhere, prohibit removing or gutting converters on road vehicles. Beyond the law, modern computers detect the missing converter and the vehicle fails any inspection.