In order to maintain a reasonably low emission level of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) a 3-way catalytic converter (TWC) is used. The catalyst within the converter promotes a chemical reaction that oxidizes the HC and CO present in the exhaust gas. This reaction converts the gases into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx , converting the NOx into nitrogen. The engine control module (ECM) monitors this process using the post catalyst heated oxygen sensor (HO2S) signal. The post-catalyst HO2S located in the exhaust stream after the TWC, produces an output signal that indicates the oxygen storage capacity of the catalyst. The oxygen storage capacity (OSC) determines the ability of the catalyst to convert the exhaust emissions effectively. If the catalyst is functioning correctly, the post-catalyst HO2S signal will be far less active than the signal produced by the pre-catalyst HO2S.
To determine OSC, the ECM commands a rich air/fuel mixture until all oxygen is removed from the catalyst. The ECM then commands a lean air/fuel mixture and monitors the rear heated oxygen sensors to calculate the oxygen storage capacity. The catalyst is operated in this mode until one of the following conditions occur:
• The oxygen stored in the catalyst exceeds a calibrated threshold, which is determined from the rear HO2S signal.
• The rear HO2S indicates the catalyst to be completely saturated with oxygen, which is determined from the rear HO2S signal.
If the ECM detects the average OSC is less than a threshold, this DTC sets. This indicates that the TWC oxygen storage capacity is below a threshold considered acceptable.
So, your bad gasket and missing(?) bolts may have affected the system. Try your repair first and wait at least 4 ignition cycles to see if it is corrected.Inspect for the following conditions:
• The catalytic converter is an original equipment part.
• Inspect the exhaust system for leaks, damage, or loose or missing hardware in the area from the converter to the heated oxygen sensor (HO2S) 2.
• The HO2S 2 is secure and the wiring is not damaged or contacting the exhaust.