A shift light is a warning light on a vehicle’s dashboard that illuminates to indicate when the engine speed is approaching the redline. The purpose of the shift light is to prompt the driver to upshift before the engine reaches its maximum RPM, at which point it would be damaged.
The shift light typically comes on at around 500-600 RPM before the redline, giving the driver plenty of time to react. However, some drivers choose to ignore the shift light and drive right up to the redline in order to squeeze out every last bit of power from their engine. This practice can shorten the lifespan of an engine considerably, so it’s generally not recommended.