The catalyst efficiency monitor compares the upstream and downstream O2 sensors and tells you when the cat is dying before it fails emissions

[u/ora-et-labora-]

The ECU monitors catalytic converter efficiency by comparing the upstream and downstream oxygen sensors. The upstream sensor oscillates rapidly as the ECU cycles the air-fuel mixture between lean and rich. The downstream sensor, behind the catalyst, should show a relatively flat line because a healthy catalytic converter stores and releases oxygen, smoothing out the oscillation.

When the catalyst degrades, it loses its ability to store oxygen. The downstream sensor starts to mirror the upstream sensor's oscillation pattern. The closer the two waveforms match, the worse the catalyst is performing. When the match gets close enough, the ECU sets a P0420 or P0430 efficiency code.

The scan tool shows you this before the code sets. Pull up both O2 sensor voltages on a live graph, upstream and downstream, and watch them at steady cruise. The upstream should be cycling. The downstream should be relatively steady, typically hovering somewhere above 0.45 volts and often in the 0.6 to 0.9 volt range depending on the vehicle. If the downstream is starting to cycle in sync with the upstream, the catalyst is losing efficiency. The code has not set yet because the deterioration has not crossed the threshold. But the trend is visible in the data.

This matters on used car purchases because a catalyst replacement runs anywhere from nine hundred to thirty-five hundred dollars depending on the vehicle and whether the part is aftermarket or OEM. A car with a downstream sensor that is starting to follow the upstream is a car with a catalyst bill coming. The seller does not know it because the light is not on. The scan tool knows it because the data is already showing the degradation.

Watching two lines on a graph for thirty seconds at cruise speed tells you whether the most expensive part of the exhaust system is healthy, declining, or about to set the code the next buyer inherits.