You may have noticed that your 3157 or 7443 replacement bulb has a CK, SCK, SACK, SRCK after the bulb number. That means that your vehicle uses a non-standard 3157 or 7443 bulb. Though they may look the same externally, the difference is in the way the socket is wired. Only a handful of vehicles made in certain years, from a few manufacturers use a CK wired socket.
Before we get into how the sockets differ, let's look at the effects of installing the wrong bulb. Installing a CK or non-CK bulb into the opposing socket, the LEDs will not work properly or worse, they will blow your fuses that protect that specific lighting circuit. If you've already encountered one of these issues, chances are pretty good you have the incorrect bulb for the base type.
One question you may ask is 'why doesn't this affect incandescent bulbs?' Incandescent bulbs illuminate using heated coil wires that only require one positive and one negative contact. In electricity terms, they are not polarity sensitive. Polarity sensitive simply means that a bulb can only be inserted and make contact in one way to work properly (positive to positive, negative to negative). Most LED bulbs are polarity sensitive, which is why making sure you get the right bulb for the socket is important.
Though both sockets may look identical externally, the difference between the two is the position of the ground contacts in the socket. The best way to determine if your socket is wired for CK or non-CK is by using a test light or multimeter to determine which metal contacts on the socket are positive and which are negative. See the image below to compare which socket type you have.
NOTE: Remove all tail light bulbs before testing, otherwise the filament in the brake circuit will read as a "false ground".
One simple way to protect yourself from installing the wrong bulb, is to use our new 3030 Series bulbs. The new bulbs are NOT polarity sensitive, and will work in both standard, and CK wired sockets. At 950 lumens per bulb in an all aluminum housing, the 3030 Series is a win win for you and your vehicle!