Your catalytic converter is a chemical reactor bolted to the exhaust. It doesn't filter particles — it transforms toxic gases into less harmful ones using precious metal catalysts. Understanding what's happening inside helps you diagnose efficiency codes, understand why leaded fuel kills cats, and why cold starts produce the dirtiest exhaust.

A modern three-way catalytic converter (TWC) handles three simultaneous reactions:

• Oxidation of CO: 2CO + O₂ → 2CO₂ (carbon monoxide becomes carbon dioxide)
• Oxidation of HC: Unburned hydrocarbons + O₂ → CO₂ + H₂O
• Reduction of NOx: 2NOx → xO₂ + N₂ (nitrogen oxides broken back into nitrogen and oxygen)

The catalyst substrate is a ceramic or metallic honeycomb with roughly 400–900 cells per square inch, coated in a "washcoat" of alumina that massively increases surface area. Deposited onto that washcoat are the actual catalysts: platinum, palladium, and rhodium. Platinum and palladium handle oxidation; rhodium handles NOx reduction. A typical converter uses 3–7 grams of precious metals total — which is why cat theft is rampant.

Here's the critical constraint: a TWC only works efficiently within a narrow air-fuel ratio window around stoichiometric (14.7:1). This is called the lambda window, roughly ±0.5 AFR from stoich. Too rich and there's not enough oxygen for oxidation. Too lean and NOx reduction fails. This is exactly why your engine runs closed-loop fuel trims using O₂ sensors — the upstream sensor feeds the ECU, and the downstream sensor monitors converter efficiency. When the downstream sensor starts mimicking the upstream signal's switching pattern, the ECU sets a P0420 (catalyst below threshold).

Light-off temperature is another key concept. The converter needs to reach roughly 250–300°C (480–570°F) before it hits 50% conversion efficiency. Below that, it's doing almost nothing. This is why cold-start calibrations run slightly rich with retarded timing — the retarded timing sends hotter exhaust gas downstream to heat the cat faster. Some manufacturers add a close-coupled cat right at the exhaust manifold specifically for fast light-off.

Rule of thumb: A healthy cat operating above light-off should show a temperature rise of about 50–100°F from inlet to outlet — the exothermic reactions generate heat. If inlet and outlet temps are nearly equal, the cat is dead. If the outlet is dramatically hotter, you may have a misfire dumping raw fuel into it.

Real-world example: the Toyota Prius is notorious for P0420 codes at higher mileage. The frequent engine start-stop cycling means more cold-start events, which accelerates catalyst degradation from thermal cycling. The fix is straightforward — replacement — but understanding why it wears explains the pattern.