What is a likely cause if the post-catalytic converter O2 sensor shows irregular readings?

If the post-catalytic converter O2 sensor shows irregular readings, a misfire in the engine is a likely cause because misfires can lead to incomplete combustion of fuel. When an engine misfires, it can send abnormal amounts of unburned fuel or raw exhaust gases through the exhaust system, including after the catalytic converter. This can cause fluctuations in the readings from the post-catalytic converter O2 sensor, as it senses variations in oxygen content related to the combustion quality and unburned fuel flow.

In contrast, normal operation of the vehicle would typically result in stable readings from the O2 sensor, indicating effective combustion and catalytic activity. A healthy catalytic converter would also display consistent O2 sensor readings, as it would be functioning properly to convert exhaust gases without allowing irregularities. Incorrect fuel quality could potentially impact engine performance, but misfires triggered by other issues would create a more immediate and noticeable effect on the O2 sensor readings. Therefore, the indication of irregular O2 sensor readings aligns most directly with the presence of a misfire in the engine.