These aluminum cased ceramic resistors have multiple uses when converting from standard incandescent or halogen bulbs to LEDs.
First, when upgrading signal bulbs from incandescent bulbs to LUMENS HPL LED bulbs, many factory signal flashers will 'hyperflash' (flashes very quickly) because of the change in resistance from a standard bulb to LEDs. LUMENS HPL LED Resistors are used in conjunction with LUMENS HPL LED bulbs to slow the signalling speed back to normal. If the vehicle has a signal relay, we recommend using LUMENS HPL LED Flasher relays, as it's more cost effective than installing resistors to maintain the flashing signal speed. On top of that, it's much easier to switch a relay than to solder in multiple resistors into the vehicle.
Secondly, on vehicles equipped with CANBUS electrical systems, the computer may sense a drop in resistance with the newly installed LED bulb. When this occurs, some LEDs might not shut off, the LED bulb might shut off after a few seconds, or the LED may flash. By adding the proper LED resistor, the vehicle will be tricked into thinking that the original bulb is still intact, and ignore the LED bulb.
Please note that these resistors get extremely hot and need to be mounted appropriately.