A downstream O2 sensor fluctuating like the upstream sensor usually indicates what?

When a downstream O2 sensor fluctuates similarly to the upstream sensor, it typically indicates that the catalytic converter is not functioning properly. The role of the downstream O2 sensor is to monitor the effectiveness of the catalytic converter in reducing emissions. In normal circumstances, the downstream sensor should show a less variable signal in comparison to the upstream sensor if the catalytic converter is working effectively.

If both sensors are fluctuating in a similar manner, it suggests that the catalytic converter is failing to perform its function of transforming harmful emissions. This situation often leads to a consistent reading from both sensors, indicating that the exhaust gases are not being properly converted. The performance degradation of the catalytic converter can result from factors such as damage, contamination, or excessive wear, which would account for the observed sensor behavior.

While issues like a clogged catalytic converter or an incorrect fuel mixture could potentially lead to fluctuations in O2 sensor readings, they typically present differently and may not always correlate both upstream and downstream. The direct cause of similar fluctuations between the two sensors is generally tied back to the catalytic converter's performance.