What does it mean?
Nowadays vehicle with emissions control systems use heated oxygen sensors. The purpose of the heating element build into the oxygen sensor, is to get the sensor up to operating temperature (600 degrees) quicker and start the exhaust gases monitor before .The Engine Control Module (ECM) monitors the temperature of heated oxygen sensors 1 (Pre-Catalyst O2 sensor) and heated oxygen sensors 2 (Post-Catalyst 02 sensor).
A three way catalyst converter like the one shown in the image has a high oxygen storage capacity. The oxygen sensor compares the oxygen inside the catalyst against the oxygen outside the catalyst, before entering the catalyst and after it exit. This oxygen measure is read by the ECM as a voltage. The ECM simply compares one sensor to another (frequency ratio) if this ratio approaches a specific limit value, the three way catalyst malfunction is diagnosed.