I am looking to replace the rear lights on my 2021 Hyundai Elantra SEL with LED lights, since the base incandescants are not bright enough for my needs.

Looking through the owner's manual, (Section 2 page 8) I have become quite confused at which type of bulbs I need. I am looking to replace (most of) the rear lights, and since I have the SEL trim I am under the impression my car falls under "Type A." For the stop lamp, it lists a "P21W/5W" bulb as replacement, a W5W bulb for the inner tail lamp, and a P21/5W bulb for the outer lamp. What is the difference between the P21W/5W light and the P21/5W light? Is there a difference?

I am sorry if this is a stupid question, I have never touched the lights on a car before and my expertise is almost solely in tires/wheels since that is what I do at my job. Thank you for your assistance.

(An additional little bonus question, I am unclear on what the difference is exactly betwen the tail lamp and the stop lamp. What is the difference? How many stop lamp/s are there that I would need to replace?)

  • Welcome to Motor Vehicle Maintenance & Repair! Aug 18, 2021 at 0:55

2 Answers 2


There are 3 bulbs needed (not including headlamps):

  1. 21W for indicators

  2. 5W for sidelights

  3. 21/5W or 21W/5W for stop and tail lights when they are in the same lamp. They have 2 filaments in them and need fitting correctly as the pins control their position so the stop and side filaments get the correct supply.

All down to how the lamps are designed, as 21W can be used for stop lights when they are in separate lamps.


The tail lamp is the running lamp (max 6w), the stop lamp is the brake lamp (max 21w), and there is also the fog lamp (max 21w), or the equivalent in lumens for LED lighting.

My previous Skoda (with incandescent bulbs) had different bulbs at each side at the rear. On the nearside was a 5 watt bulb for the tail lamp, on the offside was a single-filament 21 watt bulb for the tail lamp and the fog lamp, with two pins. As the tail lamp it would run off reduced voltage, for the fog lamp it would run off full voltage.

Note that in UK, there is a maximum permitted luminance for rear lights. A vehicle is manufactured in accordance with the Construction & Use regulations and you are supposed to run the vehicle unmodified. Modern vehicles also have circuitry to detect if a lamp has blown, and you might affect that by using the wrong bulb.

I suggest you don't tamper with the lighting.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .