Replace Back Up Fog Light Bulb Mercedes-Benz 2014 to 2020 X156
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Is the rear fog light and reverse light not working on your Mercedes-Benz GLA250? Learn how to fix the problem by replacing your vehicle’s light bulbs. This guide provides a step-by-step procedure for replacing the rear fog light and reverse (backup) light on the Mercedes-Benz GLA-Class. This tutorial is based on the 2017 GLA250 X156 chassis.
At a Glance
- Fits: Mercedes GLA-Class X156 (2014 to 2020), based on a 2017 GLA250
- Access: remove the inner liftgate trim panel
- Bulbs: 921 reverse and the rear fog bulb listed in this guide; confirm from the old bulbs
- Tools: plastic pry tool
- Warning message: clears once the new bulb works
Bulb Out Warning or Light Not Working?
- Cluster warning such as Check reversing lamp or Check rear fog lamp: the car has detected a failed bulb. Replace it and the message clears once the light works.
- New bulb, still not working: check the socket contacts for corrosion and the fuse.
- LED replacement bulbs: can trigger bulb out warnings or flicker on Mercedes models unless they are designed to be CAN bus compatible.
- Rear fog light only on one side: many cars have a single rear fog lamp by design, usually on the driver side.
Confirm the bulb numbers from the old bulbs before ordering, since they can vary by model year and market.
What you will need
- Prying tool
- 1235 light bulb
- 921 light bulb
Procedure
- Remove the trunk door panel. Click on this link to see the step-by-step procedure for removing and installing the trunk panel.
- Locate the rear and reverse fog light socket behind the inner tail light assembly.
- Remove the sockets by turning and pulling them out from the tail light assembly.
Removing the rear fog light socket. 
- Remove the rear fog light bulb from the socket by twisting it counterclockwise and pulling it out.
- Line up the new rear fog light bulb to the socket and twist it clockwise to lock it in place.
- Remove the reverse light from the socket by pulling it out. Then, line up the new reverse light to the socket and push it in to lock it into place.
- Reinstall the sockets back to the tail light assembly by pushing them in their original position.
- Reinstall the trunk panel. For more help with this step, see this guide.
Removing the rear fog light socket. 
Frequently Asked Questions
Why is only one rear fog light working on my Mercedes?
Many cars have only one rear fog lamp, typically on the driver side. If the car has two and one is out, check that bulb and socket.
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Can I use LED bulbs for the reverse lights on my GLA?
Yes, but choose CAN bus compatible LEDs. Others can trigger bulb out warnings or flicker.
Do I need to remove the tail light to change these bulbs?
No. On the GLA they are reached from inside the liftgate after removing the trim panel.
Check these troubleshooting and repair guides for more help on your Mercedes-Benz.
