Two catalytic converters.
In standard factory configuration, the 2003 IS300 employs a dual-cat exhaust system: a close-coupled catalytic converter near the engine and a second catalytic converter further downstream. This two-cat setup helps the vehicle meet emissions requirements across the engine's operating range.
What makes up the IS300's dual-cat exhaust
Understanding the layout helps when diagnosing issues or planning maintenance. The IS300's exhaust path typically includes two catalytic converters placed at different points along the undercarriage.
- Close-coupled catalytic converter (pre-cat): located near the exhaust manifold, heats quickly to start emissions reduction as soon as the engine runs.
- Main catalytic converter: located downstream along the exhaust line, further toward the center or rear of the vehicle, continuing the emissions cleanup before the exhaust reaches the rear resonator and muffler.
Together, these two catalysts form a two-stage system that was common for the IS300's era and helps meet federal emissions standards.
Locations and practical notes
Close-coupled catalytic converter
The close-coupled cat sits near the engine and is designed to begin reducing pollutants once the exhaust reaches sufficient temperature. It is typically mounted close to the exhaust manifold on the cylinder head area and can be more exposed to heat and road debris.
Main catalytic converter
The downstream or main catalytic converter sits farther down the exhaust path, usually along the left or right undercarriage rail, before the exhaust resonator or muffler. It handles the bulk of the catalytic conversion once the exhaust is hot.
Summary
Bottom line: The 2003 Lexus IS300 features two catalytic converters—one close to the engine and a second downstream. This dual-cat arrangement is standard for the model and era, contributing to emissions performance and compliance.