On CA vehicles equipped with sub-O2 sensors, a quick check of TWC operation can be made by comparing the signal activity of the main oxygen sensor with the sub-oxygen sensor. Since the main O2S in located upstream of the converter and the sub-O2S is located downstream, a signal comparison would indicate whether a catalytic reaction is taking place inside the converter. If the catalyst is operating, the main O2S signal should normally toggle rich/lean, while the sub-O2 sensor should react very slowly (similar to a bad main O2S signal.) Main and sub O2S signals can be observed using the graphing display of the Diagnostic Tester (OBD-II) or V-BoB on other models.
NOTE: Before any catalyst efficiency tests are performed, it is important that both the engine and converter are properly preconditioned. Remember, proper feed gas conversion cannot take place until the closed loop control system is actively maintaining ideal mixture and the catalyst has reached operating temperature. To ensure these conditions are met, particularly during cold ambient conditions, operate the engine off-idle until the TWC is sufficiently heated. This will ensure optimal catalyst conversion efficiency.
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