Catalyst Monitoring
CATALYST MONITORING
The efficiency of catalyst operation is determined by evaluating the oxygen storage capability of the catalytic converter using the pre and post oxygen sensors signals. A properly operating catalyst consumes or stores most of the 02 (oxygen) that is present in the exhaust gas (input to catalyst). The gases that flow into the catalyst are converted from CO, HC and NOx to CO2, H2O and N2 respectively.
Catalyst Able To Store Oxygen
(Normal)
In order to determine if the catalyst is working correctly, the signal of the post catalytic convertor 02 sensor is evaluated over the course of several pre cat. 02 sensor oscillations. During the evaluation period, the signal of the post cat. sensor must remain within a relatively constant voltage range.
Under normal closed loop operation the changing air/fuel ratio in the exhaust gas results in lambda oscillations at the pre-catalyst sensor. These oscillations are dampened by the oxygen storage activity of the catalyst and are reflected at the post catalyst sensor as a fairly stable signal. Depending on how the vehicle is being operated at the time of evaluation and the type of catalyst or coating being used, the signal may be in the lean or rich voltage range.
Conditions for Catalyst Monitoring:
Catalyst temperature is an internally calculated value that is a function of load/air mass and time.
NOTE: The catalyst efficiency is monitored once per trip while the vehicle is in closed loop operation.
As part of the monitoring process, the pre and post 02 sensor signals are evaluated by the control module to determine the length of time each sensor is operating in the rich and lean range (t Rich and t Lean).
Example: Catalyst Monitoring Process/Calculation (Using Siemens Sensors).
Example: Ratio Calculation (see above figure)
The evaluation period of the pre and post sensor signals is done over a predefined number of oscillation cycles. During each evaluation cycle the control module calculates the ratios for each cycle and the smaller of the two calculated values is stored and added the lowest ratio of the next evaluation cycle.
Each time an evaluation cycle is completed a cycle counter is incremented by 1. Once the counter reaches a predetermined number of cycles and the sum of all the ratio calculations is greater than a fixed value, a fault will be set in the DME.
The catalyst monitoring process is stopped once the predetermined number of cycles are completed, until the engine is shut-off and started again. After completing the next "Customer Driving Cycle" whereby the specific conditions are met and a fault is again set, the "Check Engine" light will be illuminated.
NOTE: In the following illustrations the Siemens and Bosch Post Catalyst Oxygen Sensor signals are both at a high level and are operating in opposite conditions. This is a result of a difference in the internal composition of the catalyst, type of coating being used and operating conditions of the vehicle.
Example: Pre & Post Catalyst Oxygen Sensor signals (Siemens)
Example: Pre & Post Catalyst Oxygen Sensor signals (Bosch)
If the catalyst is defective the post O2 sensor signal will reflect the pre O2 sensor signal (minus a phase shift/time delay), since the catalyst is no longer able to store oxygen.
Catalyst Not Capable Of Storing Oxygen
(Defective)
Example: Both Siemens And Bosch Systems
NOTE: The lean ratio/rich calculations in this case will be approximately equal to 1. If a catalyst efficiency fault sets, this indicates that at the time the signal was checked, the post oxygen sensor signal reflected the pre oxygen sensor signal. If these signals are checked with a scope it may not be possible to duplicate these signals.
Associated Fault Codes: Catalyst Efficiency