Which condition is a sign of a failing voltage regulator?

Fluctuating battery voltage is indeed a clear indication of a failing voltage regulator. The voltage regulator's primary function is to maintain a constant voltage level for the electrical system of a vehicle, ensuring that the battery charges properly and that electrical components function effectively. When the voltage regulator is not performing as expected, it cannot stabilize the voltage output from the alternator as the engine RPM changes. This inconsistency in voltage can lead to the battery being overcharged or undercharged, which can cause various electrical issues, including problems with lights, the ignition system, and other electronic components within the vehicle.

Consistent battery voltage, stable engine RPM, and normal engine temperature indicate that the electrical system is functioning correctly and that there are no issues with the voltage regulator. Therefore, noticing fluctuations in battery voltage is a critical signal that should prompt further investigation into the voltage regulator's performance.