Oxygen Sensor: Description and Operation

On Board Monitoring

The vehicle drive train is continually monitored throughout its life to maintain its proper function and ensure that emission levels do not exceed accepted limits.

Catalyst Efficiency Monitor
Catalytic converters oxidize unburned Hydrocarbons (HC) and Carbon Monoxide (CO) by combining them with oxygen to produce water vapor, and reduce nitrogen oxides to nitrogen and oxygen. When the engine air fuel ratio is lean, the oxygen content of the catalytic converter reaches its maximum value. When the air fuel ratio is rich, the oxygen content is depleted. If the air fuel ratio remains rich for an extended period, the converter may fail to convert the harmful gases.

The Catalyst monitor operates once per trip, and is not a continuous monitor. The monitor waits until all entry conditions are met, including the modeled catalyst temperature reaching its threshold. Once all entry conditions are met, the monitor starts to run. The fuelling is cycled rich and lean (called dither) by approximately 3% to get a reaction at the downstream Oxygen Sensor (O2S). At the start of the monitor, delay counters operate so that the fuelling is stable when the diagnosis takes place. If the entry conditions then drop out, the monitor result and execution timer are held at the values that they were when the entry conditions dropped out. The next time entry conditions are met the monitor carries on from where it stopped previously. This will happen for a maximum of four attempts, after this, the monitor will reset and the diagnosis restarts.

The monitor runs for a calibratable period of time, after which the monitor results are made. The monitor results are decided by accumulating the locus of the downstream O2S signal versus the accumulation of the upstream O2S. The more active the downstream sensor, the less oxygen storage capacity the catalyst has, so the higher the locus value.

With a 100,000-mile catalyst, the downstream O2S is not so active, so lower locus values are obtained.

A judgment is made when the monitor has finished. The judgment made can either be "normal" or "fail". The normal judgment is made if the accumulated count
is lower than a calibratable threshold at the judgment point. The failure judgment is made if the accumulated count equals or exceeds the calibratable threshold at the judgment point. If a failure judgment is made, then the relevant DTCs are stored within the engine management system.