Catalyst Monitor

To comply with clean air regulations, vehicles are equipped with catalytic converters. These converters reduce emission of hydrocarbons, oxides of nitrogen and carbon monoxide.

Normal vehicle miles or engine misfire can cause a catalyst to decay. A meltdown of ceramic core can cause a reduction of exhaust passage. This can increase vehicle emissions and deteriorate engine performance, driveability and fuel economy.

The catalyst monitor uses dual oxygen sensors (O2S's) to monitor efficiency of converter. The dual O2S's strategy is based on fact that as a catalyst deteriorates, its oxygen storage capacity and its efficiency are both reduced. By monitoring oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The upstream O2S is used to detect amount of oxygen in exhaust gas before gas enters catalytic converter. The PCM calculates A/F mixture from output of O2S. A low voltage indicates high oxygen content (lean mixture). A high voltage indicates a low content of oxygen (rich mixture).

When upstream O2S detects a lean condition, there is an abundance of oxygen in exhaust gas. A functioning converter would store this oxygen so it can use it for oxidation of HC and CO. As converter absorbs oxygen, there will be a lack of oxygen downstream of converter. The output of downstream O2S will indicate limited activity in this condition.

As converter loses ability to store oxygen, condition can be detected from behavior of downstream O2S. When efficiency drops, no chemical reaction takes place. This means concentration of oxygen will be same downstream as upstream. The output voltage of downstream O2S copies voltage of upstream sensor. The only difference is a time lag (seen by PCM) between switching of O2S's.

To monitor system, number of lean-to-rich switches of upstream and downstream O2S's is counted. The ratio of downstream switches to upstream switches is used to determine whether catalyst is operating properly. An effective catalyst will have fewer downstream switches than it has upstream switches i.e., a ratio closer to zero. For a totally ineffective catalyst, this ratio will be one-to-one, indicating that no oxidation occurs in device.

The system must be monitored so that when catalyst efficiency deteriorates and exhaust emissions increase to over legal limit, MIL (Check Engine lamp) will be illuminated.