Mercedes-Benz GL550 front fog light bulbs use H11 size, while rear fog light bulbs use H21W and P21W sizes. The replacement chart covers 2008 to 2016 production 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 Mercedes-Benz GL550 front fog light uses the H11 bulb across the 2008-2011 model years. The H11 is a single-filament halogen bulb operating at 12V and 55W, fitted with a PGJ19-2 base and rated under ECE standards with additional approval for use in the United States.
The rear fog light position on the GL550 transitions between two bulb types depending on model year. The P21W was used in 2008-2009, while the H21W replaced it from 2010 onward through 2016.
Although the H21W and P21W share identical voltage and wattage ratings at 12V and 21W, they are not interchangeable. The H21W uses a BAY9s base, whereas the P21W uses a BA15s bayonet base, making the two physically incompatible despite their matching electrical output.
The H11 bulb is classified as a halogen type, which means it contains a tungsten filament surrounded by a halogen gas mixture inside the bulb envelope. This construction allows it to operate at higher temperatures and maintain greater luminous efficiency compared to standard incandescent bulbs.
Both the H21W and P21W are classified as signalling bulbs rather than illumination bulbs. At 21W, they are intended for visibility and indication functions rather than road lighting, which is consistent with their rear fog light application.
All three bulb types specified for the GL550 fog lights conform to ECE regulations, a standard recognized across Europe and many international markets. The H11 carries the additional distinction of US approval, which is relevant given the GL550's presence in the North American market.
The BAY9s base of the H21W features an offset pin arrangement. This asymmetric design prevents incorrect installation by ensuring the bulb can only be seated in one orientation within the socket.
The BA15s base used by the P21W is a single-contact bayonet fitting with two pins positioned symmetrically. It is one of the most widely used bases in automotive signalling applications globally.