Mazda RX-4 Back Up Light Bulb Size

The Mazda RX-4 back up light requires bulb size 1156 or P21W depending on the specific vehicle configuration. Data for the lighting components covers the range from 1974 to 1978.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1974-1978	1156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156

Other Locations

Additional Notes

The Mazda RX-4 uses the 1156 bulb across all listed model years, spanning 1974-1978, for the back up light position.
The 1156 is an ANSI-standardized bulb type, meaning its physical and electrical specifications conform to a recognized North American standard used across the US and Canada.
The 1156 operates on a 12-volt electrical system and draws 21 watts, which is consistent with standard automotive incandescent back up light applications.
The 1156 is a single-filament bulb, meaning it serves only one lighting function, as opposed to dual-filament types that handle two functions such as braking and running light within a single housing.
The 1156 uses a BA15s bayonet base, where "BA15s" denotes a bayonet fitting with a 15mm base width and a single contact pin, which locks into the socket with a push-and-twist motion.
The BA15s single-contact base of the 1156 is not interchangeable with the BA15d base found on dual-filament bulbs such as the 1157, despite the similar bayonet sizing, as the pin configuration differs.