Code P0420 on the 2024 Toyota Tacoma indicates the catalytic converter on Bank 1 is operating below efficiency threshold. The engine control module monitors catalytic converter function by comparing upstream and downstream oxygen sensor readings, detecting when conversion efficiency drops below acceptable levels.
Catalytic Converter Function
The catalytic converter converts harmful exhaust gases (hydrocarbons, carbon monoxide, nitrogen oxides) into less harmful emissions (water, carbon dioxide, nitrogen). Precious metals inside the converter act as catalysts for these chemical reactions. Over time or due to damage, conversion efficiency decreases.
P0420 Detection Method
The upstream oxygen sensor sees raw exhaust with varying oxygen content as the engine cycles between rich and lean. A functioning catalyst stores and releases oxygen, causing the downstream sensor to see a much more stable signal. When the downstream sensor's signal begins to mirror the upstream sensor's fluctuations, efficiency has degraded.
Common Causes
On a 2024 model, actual catalytic converter failure is unlikely unless the vehicle has high mileage or experienced engine problems. More common causes include oxygen sensor problems (particularly downstream sensor), exhaust leaks near sensors, or engine issues causing incomplete combustion that damages the converter over time.
Diagnostic Strategy
Verify there are no other engine codes that could indicate problems damaging the converter. Compare upstream and downstream oxygen sensor waveforms. Check for exhaust leaks. On a low-mileage 2024, investigate root causes before concluding the converter has failed.