BMW 340i Back Up Light Bulb Size

The BMW 340i back up light requires a bulb with the specific H21W size for proper installation and function. Available data for the bulb covers the range from 2016 to 2018.

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
2016-2018	H21W

Bulb Specifications

Description	Bulb Size
ECE standard (Europe & worldwide). Single-filament signalling bulb with BAY9s base. 12V, 21W.	H21W

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Additional Notes

The H21W is the sole bulb type listed across all three model years of the BMW 340i covered by these specifications, spanning 2016-2018.
The H21W is an ECE-standard bulb, meaning it conforms to the regulatory framework used in Europe and numerous other markets worldwide rather than the SAE standards common in North America.
The H21W operates on a 12-volt electrical system and draws 21 watts, which is typical for single-filament incandescent signalling applications such as reverse lights.
The H21W uses a BAY9s base, which is an offset bayonet fitting. The offset pin arrangement is a deliberate anti-misfit design, preventing the bulb from being seated incorrectly or substituted with electrically incompatible bayonet types.
Because the BAY9s base is not interchangeable with more common bayonet bases such as BA15s or BAY15d, direct substitution with standard single-filament or dual-filament bulbs is not possible without a base mismatch.