Catalytic converters are simple, but getting them to work is not

Quick Overview

The catalytic converter in the Nissan Cube failed its efficiency test, triggering a P0420 code, which was ultimately confirmed to be a failed converter after ruling out significant exhaust leaks that were causing cabin smell and sensor confusion near the upstream sensor.

Key Points: The Nissan Cube displayed a P0420 fault code, indicating the "catalyst system efficiency below threshold," meaning the catalytic converter was not working right and would fail emissions testing. The catalytic converter is a section of exhaust pipe containing a ceramic grid coated with precious metals (platinum, rhodium, palladium) designed to convert nitrogen oxides into nitrogen and CO2, and unburnt hydrocarbons into CO2 and water vapor. The engine computer orchestrates complex sensor feedback loops to maintain a near-stoichiometric air/fuel ratio (14.7:1 for gasoline) because the three-way catalyst requires hovering around this point to reduce NOx (when slightly rich) and oxidize fuel/CO (when slightly lean) by oscillating and storing oxygen. An initial investigation revealed significant exhaust leaks near the upstream oxygen sensor, which caused cabin smell and could have confused the computer into running chronically rich, but fixing the leak did not resolve the P0420 code, confirming the catalyst itself was inefficient. The downstream oxygen sensor monitors catalyst efficiency; if the catalyst works, the downstream sensor should not see the rich/lean oscillations the upstream sensor registers, but in this case, the downstream sensor constantly mirrored the upstream readings. After replacing the catalytic converter, the downstream oxygen sensor stayed consistently rich once warmed up, eliminating the oscillations, and the permanent P0420 code eventually cleared, confirming the new converter resolved the issue. The speaker noted that a flashing check engine light indicates a severe misfire that can poison the catalyst, and people often incorrectly replace oxygen sensors for catalyst codes, which the computer usually detects and reports separately.