CANbus LED bulbs and hyperflash
Why LED turn signals blink fast and dashboards report failed bulbs
Hyperflash is the rapid blinking of a turn signal when its bulbs draw less current than the car expects. Flashers and body control modules read low current as a failed bulb and blink faster to warn the driver, so fitting low-power LED bulbs triggers the same warning.
A 21 W turn-signal bulb draws about 1.75 A at 12 V; an LED replacement may draw a small fraction of that. The car cannot tell an efficient bulb from a broken one, so it does what the rules require when a turn signal fails: it blinks fast or lights a warning. Every fix either restores the current the car expects or changes what it expects.
Why do LED turn signals blink fast?
Turn-signal flashers and body control modules watch the current a lamp draws. A failed filament bulb draws none, and safety rules require the driver to be warned, so the car blinks faster. An LED bulb draws a small fraction of a filament bulb's current, looks like a failed bulb, and triggers the same hyperflash.
In the United States, FMVSS No. 108 requires a turn-signal failure to show to the driver as a steady-on or steady-off indicator, or as a significant change in the flash rate; UN Regulation 48 asks the same of direction indicators. Older cars do this with a plug-in flasher relay whose rhythm depends on the load: electronic flashers respond to low current with hyperflash, while old thermal flashers may blink slowly or not at all. Newer cars do it in the body control module, which also watches the brake, tail and other lamps.
What does CANbus-ready mean?
CANbus-ready, or error-free, LED bulbs are built to avoid bulb-out warnings, usually with a resistor inside that raises the current, or with circuitry that copes with the car's test pulses. They work in many cars but not all, because each maker monitors lamps differently, and the built-in resistor makes the bulb run hotter.
CAN bus is the data network that links a car's control modules. LED bulbs borrowed the name because the module that monitors the lamps reports faults over it. Many modules also check lamps that are switched off, with short test pulses or a small standby current. A filament ignores them, but an LED can glimmer, flicker or flash briefly, which CANbus-ready bulbs are also designed to suppress.
How to fix hyperflash
Four fixes work. Fit an LED-compatible flasher relay, if the car has a plug-in one. Use CANbus-ready bulbs. Wire a load resistor in parallel with each LED bulb. Or, on cars that allow it, have the body control module coded for LED lamps. Each restores the current the car expects or changes what it expects.
| Fix | How it works | Best for | Drawbacks |
|---|---|---|---|
| LED flasher relay | Flashes at the normal rate whatever the load | Older cars with a plug-in flasher | A failed bulb no longer changes the flash rate; not possible where the body module does the flashing |
| CANbus-ready bulbs | A built-in load or smart circuitry | Cars with simple current monitoring | Results vary by car; built-in resistors add heat |
| Load resistors | An extra current path beside each LED | Any car, any monitored lamp | Get very hot, must be bolted to metal, and waste the energy the LED saves |
| Coding | The body module is set for LED lamps | Cars whose makers offer an LED setting | Needs a diagnostic tool and the right software |
Load resistors: the math and the heat
A load resistor imitates the filament's current draw. A 6 Ω resistor at 13.5 V passes about 2.25 A and turns about 30 W into heat, so it must be bolted to bare metal, away from wiring and plastic. While the lamp is lit, the LED and resistor together use more power than the original bulb.
Ohm's law sizes it. A 21 W bulb at 12 V draws 1.75 A, which corresponds to about 6.9 Ω, so the common 6 Ω, 50 W part draws slightly more than the bulb did. It is wired in parallel, across the bulb's feed and ground, never in series, where it would starve the LED. On turn signals it is on only half the time; on brake or tail circuits it can stay hot for minutes, which is when CANbus-ready bulbs or coding make more sense.
Where a load resistor goes
Frequently asked questions
Do I need load resistors for LED brake lights?
Only if the car reports a fault or the lights misbehave. Brake circuits can stay on for minutes, so if a fix is needed, CANbus-ready bulbs or coding avoid a resistor that stays hot.
Why do my LED bulbs glow faintly when switched off?
Some cars send a small current or test pulses through lamps that are off, to check them. A filament does not light from it, but an LED can. CANbus-ready bulbs, or a resistor across the bulb, absorb that current.
What does error-free mean on LED bulbs?
It is another name for CANbus-ready: the bulb is designed not to trigger bulb-out warnings, usually with a built-in resistor. Whether it works depends on how your car monitors its lamps.
Why does hyperflash happen on one side only?
The car monitors each side separately, so only the side with LED bulbs, or with a failed bulb, blinks fast.
Are LED turn-signal bulbs approved anywhere?
In UNECE countries, approved LED substitutes exist for a few signal bulbs, such as the amber PY21W and WY5W, valid in the vehicles each bulb's maker lists.
Sources and further reading
- FMVSS No. 108, 49 CFR 571.108: turn-signal operating unit and flasher requirements, including failure indication.
- UN Regulation No. 48, installation of lighting and light-signalling devices: the direction-indicator tell-tale.
- UN Regulation No. 128, LED light sources, and R.E.5 Group 4, LED substitute light source categories (PY21W/LED, WY5W/LED).
Image credits
- Wiring diagram: LEDlights.org.