Catalyst Efficiency

📖 YOUCANIC Automotive Glossary

Catalyst efficiency is the measured effectiveness of the catalytic converter at converting harmful exhaust pollutants (hydrocarbons HC, carbon monoxide CO, and nitrogen oxides NOx) into harmless gases (water H2O, carbon dioxide CO2, and nitrogen N2). The ECU monitors catalyst efficiency by comparing the switching patterns of the upstream (pre-catalyst) and downstream (post-catalyst) oxygen sensors. A healthy catalyst stores and releases oxygen, which dampens the downstream O2 sensor’s switching pattern — the downstream sensor should hold relatively steady compared to the rapidly switching upstream sensor. As the catalyst degrades, the downstream sensor’s pattern begins to mirror the upstream sensor’s rapid switching, indicating the catalyst is no longer effectively storing oxygen and converting pollutants.

When catalyst efficiency drops below the OBD2 threshold, the ECU sets P0420 (Bank 1) or P0430 (Bank 2) — Catalyst System Efficiency Below Threshold. However, a P0420/P0430 is not always a failed catalyst — a lazy or contaminated downstream O2 sensor, exhaust leaks near the sensors, or engine misfires sending unburned fuel into the converter can all cause false catalyst efficiency codes. The YOUCANIC UCAN-II Mode $06 data shows the actual catalyst efficiency measurement compared to the threshold, helping determine if the catalyst is truly marginal or if another cause should be investigated first.