A vehicle's red brake warning light stays on whenever the ignition is on and the engine is running. Any of the following could be the cause except:

The red brake warning light being illuminated indicates an issue that affects the braking system as a whole rather than a specific part of the Anti-lock Braking System (ABS). When the ABS has a faulty wheel sensor, it typically does not trigger the red brake warning light; instead, it may generate a different warning, such as an ABS light, to indicate that there is a problem with the ABS system alone.

On the other hand, a low brake fluid level in the reservoir, an engaged parking brake, or an out-of-adjustment parking brake switch can all create situations that affect the primary braking system, thereby triggering the red brake warning light to indicate that the driver should check the brakes immediately. Ensuring that the primary brake functions correctly is critical for safety, which is why those conditions cause the warning light to engage.