The Lexus GS400 front turn signal uses a 1156 bulb size while the rear turn signal requires a 7440 bulb size. The replacement chart covers 1998 to 2000 model years with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Lexus GS400 uses two distinct bulb types across its turn signal positions for the 1998-2000 model years: the 1156 for the front and the 7440 for the rear.
Both the 1156 and 7440 are ANSI-standardized bulbs rated for use in the US and Canadian markets, operating at 12 volts with a 21-watt output.
The 1156 is a single-filament bulb mounted on a BA15s bayonet base, a base type that uses a pin-and-twist locking mechanism to secure the bulb in its socket.
The 7440 is a single-filament wedge-style bulb in the T20 form factor, which relies on the wedge shape of its base rather than a bayonet mechanism for retention.
Because the 1156 uses a BA15s bayonet base and the 7440 uses a T20 wedge base, the two bulb types are not interchangeable despite sharing the same voltage and wattage ratings.
The front turn signal position on the Lexus GS400 (1998-2000) requires the 1156 bulb exclusively, as the socket is designed for the BA15s bayonet base.
The rear turn signal position on the Lexus GS400 (1998-2000) requires the 7440 bulb exclusively, as the socket is designed for the T20 wedge base.