📖 What It Does
The "cat" cleans exhaust before it leaves the tailpipe. It is the single biggest reason modern cars have far lower emissions than 1970s vehicles. It also generates the back-pressure the engine expects.
⚙ How It Works
Exhaust gases pass through a ceramic honeycomb plated with precious metals. At operating temperature (around 800°F), the catalyst speeds up reactions that strip oxygen from NOx and add it to CO and unburned fuel. Two oxygen sensors (upstream and downstream) report to the ECU whether the cat is doing its job.
⚠ Symptoms When It Fails
Symptoms include the P0420 code, sulfur (rotten egg) smell, sluggish acceleration, rattling from underneath, and reduced fuel economy. Most cats fail because of upstream problems (misfires, rich fuel mix, oil burning) that overheat and contaminate the substrate. See full symptom guide.
💰 Replacement Cost
Replacement: $300–$2,500. Aftermarket direct-fit cats start around $300 installed. OEM units on luxury or hybrid vehicles can exceed $2,500. Theft has driven prices up sharply. See full cost breakdown.