Catalyst Monitor

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

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

The catalyst monitor uses four oxygen sensors (O2 sensors) to monitor the efficiency of the catalytic converters. The four O2 sensor strategy is based on the fact that as catalyst elements deteriorate, their oxygen storage capacity and their oxidizing (burning) efficiency are both reduced. By monitoring the oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The upstream O2 sensors are used to detect the amount of oxygen in the exhaust gas before the gas enters the catalytic converters. The PCM calculates the air/fuel mixture from the output of the O2 sensors. A low voltage indicates high oxygen content (lean mixture). A high voltage indicates a low content of oxygen (rich mixture).

When the upstream O2 sensors detect a high oxygen condition, there is an abundance of oxygen in the exhaust gas. An efficiently functioning catalytic converter would store this oxygen so it can be used for the oxidation of HC and CO gasses in the exhaust. As the converters absorb the oxygen, there will be a lack of oxygen downstream of the converters. The output of the downstream O2S will indicate limited activity in this condition.

As the converters lose the ability to store oxygen, and become less efficient, the condition can be detected from the behavior of the downstream O2 sensors. When the efficiency drops, no chemical reaction takes place. This means the concentration of oxygen will be the same downstream as upstream. The output voltage of the downstream O2 sensors will be virtually the same as the voltage of the upstream sensors. The only difference is a time lag (seen by the PCM) between the switching of the O2 sensors.

To monitor the system, the number of lean-to-rich switches of upstream and downstream O2 sensors are counted. The ratio of downstream switches to upstream switches is used to determine whether the 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 is occurring in the catalytic converter.

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

OPERATION  To monitor catalyst efficiency, the PCM expands the rich and lean switch points of the heated oxygen sensor. With extended switch points, the air/fuel mixture runs richer and leaner to overburden the catalytic converter. Once the test is started, the air/fuel mixture runs rich and lean and the O2S switches are counted. A switch is counted when an oxygen sensor signal goes from below the lean threshold to above the rich threshold. The number of Rear O2S switches is divided by the number of Front O2S switches to determine the switching ratio.

The test runs for 20 seconds. As catalyst efficiency deteriorates over the life of the vehicle, the switch rate at the downstream sensor approaches that of the upstream sensor. If at any point during the test period the switch ratio reaches a predetermined value, a counter is incremented by one. The monitor is enabled to run another test during that trip. When the test fails three times, the counter increments to three, a malfunction is entered, and a Freeze Frame record of the engine operating conditions is stored in the PCM's memory. When the counter increments to three during the next trip, the code is matured and the MIL is illuminated. If the test passes the first, no further testing is conducted during that trip.

The MIL is turned off after three consecutive good trips.