Which symptom is a sign of low oil level in an automatic transmission?

A symptom indicating a low oil level in an automatic transmission is delayed engagement into reverse. When the transmission fluid is low, it can lead to insufficient hydraulic pressure. This pressure is crucial for the engagement of gears, including reverse. As a result, if there is a delay in the transmission engaging when shifting into reverse, it can be a clear indication that the fluid level is lower than necessary to facilitate proper function.

In contrast, while decreased fuel economy, delayed shifting from third to second gear, and rough idling can all be influenced by various issues in a vehicle, they are not specific indicators of low oil levels in the transmission. Delayed shifting and fuel economy issues could be attributed to other mechanical faults or engine performance problems, whereas rough idling typically relates to engine issues rather than transmission-specific concerns. Therefore, the delayed engagement into reverse is the most direct symptom associated with low automatic transmission fluid levels.