My Turn Signal Is Blinking Fast? Causes and Solutions

Troubleshooting Guide: Your turn signal is blinking fast, But all the bulbs are good—what should you Do?

A fast-blinking turn signal is often an indicator of a burned-out turn signal bulb. But what if the turn signal is blinking fast on your car but all the turn signal bulbs work? What do you do then? Here’s a problem I discovered on a Subaru, but it applies to any vehicle that uses a dual filament turn signal bulb.

Well, most late-model vehicles use an electronic flasher relay to switch the power on and off of the turn signal bulbs. The electronic flasher monitors the current flow to the bulbs. If it detects less current draw, like is present when you have a burned-out bulb, it will go into rapid blinking mode.

But the turn signals can blink fast even if the turn signal bulb isn’t burned out

This is what really messes you up. You move the lever and get a turn signal blinking fast condition, but all the bulbs work!! However, if you look closer, you’ll see that one of the fast-blinking bulbs isn’t as bright as the same bulb on the opposite side.

Here’s what’s happening:

Car makers can use either two single filament bulbs or one dual filament bulb for their turn signals. In a two single filament bulb setup, one bulb is the parking light and the other bulb is just for the turn signal. In a dual filament bulb, one filament is for the parking light and the other filament is for the turn signal. Subaru uses a dual filament bulb for many of their models.

The filaments in a dual filament bulb each have a different resistance value, so each filament burns at a different brightness. However, if the filament for the turn signal breaks and lands on the filament for the parking lights, the current will still still flow but the total current will be less than the electronic flasher expects to see. It interprets this as a burned out bulb and cycles the flasher much faster. The bulb burns dimly because the brighter filament is shorted to the dimmer filament.

How to fix the problem with a replacement bulb

Remove the dimly lit but rapidly blinking bulb. Look for any corrosion on the bulb contacts or in the socket and clean where needed. Install a new bulb. Do NOT automatically replace the $40 flasher relay. It’s almost ALWAYS a bad bulb.

©, 2018 Rick Muscoplat