The P0420 code in your 2018 Toyota Sienna indicates that the catalytic converter efficiency on bank 1 is below the ECM's threshold. A steady (not flashing) check engine light means the issue isn't severe enough to cause immediate misfires, but the emissions system needs attention.
How Catalyst Efficiency Is Measured
The ECM compares readings from oxygen sensors upstream and downstream of the catalytic converter. A healthy catalyst produces very different readings between these sensors—the downstream should be much more stable. When readings become similar, the ECM determines the catalyst isn't converting pollutants efficiently.
Common Causes of P0420
Catalytic converter degradation from age or contamination. Oxygen sensor failure (upstream or downstream) providing inaccurate readings. Exhaust leaks before the catalyst affecting sensor readings. Rich or lean running conditions that damaged the catalyst over time. Misfires that sent unburned fuel to damage the catalyst.
The 2018 Sienna Exhaust System
Your Sienna's 3.5L V6 has two exhaust manifolds with catalytic converters near the engine. Bank 1 (passenger side) has its own converter. The system design helps converters heat up quickly for emissions efficiency.
Diagnostic Approach
Compare upstream and downstream O2 sensor waveforms—downstream should be stable while upstream switches. Check for exhaust leaks before the catalyst. Review fuel trim data for rich or lean conditions. Check for historical misfire codes that might have damaged the catalyst. Consider sensor replacement as a lower-cost diagnostic step if sensors are older.