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Checking Brake Lights

I’m a commercial driver and I’ve developed a tool that I use to check my brake lights unassisted during my daily DOT required vehicle inspection. I thought I might as well try to develop a market for it since I could never find anything like it currently available. In talking to a buyer at one of the auto parts stores here in Oregon, I was told that all you need to do is turn on your hazards to check your brake lights. If the hazards are working then the brake lights would also be working. I disagree. I believe there is some kind of sending unit that is activated when you apply the brakes and that’s what sends the current to the brake lights. If that sending unit goes out the hazards will work but not the brake lights. Can you please tell me who’s correct? On some vehicles, like school buses, the hazards are independent of the brake lights so there’s no question there. Thank you.

The brake pedal has a brake light switch. So the hazards will work, but if the switch fails you have no brake lights.


Commercial trucks and some older consumer cars and trucks used the same lights for the turn signals and the brakes, so turning on the hazard flashers will test the light bulbs them selves, but not the brake light switch or the turn signal switch.

Most cars and small trucks today have amber turn signal lights separate from the brake lights so the hazard signal will not help, but many cars and small trucks today have a brake light monitor that will tell you if the brake lights are not working. The indicator is the little door ajar icon, there is a red bar under it that will light up if any bulb fails to light. I don’t think that it will tell you if the brake light switch isn’t working though.

I have a class B air brake commercial license and work on them, as well.
Tester, you are correct. Functioning hazards are no guarantee that the brake lights work.
Some of them do indeed use the same filament.
I’ve seen plenty of trucks that had a bad brake light switch, but the hazards were working fine.
There’s no substitute to using the stick on the pedal to check your brake lights.
By the way, Lisle makes a nice brake pedal depressor. The part number is 21520. It’s heavy duty.
I use it frequently during my preventative maintenance services.

Personally I don’t think anything except a physical inspection will satisfy DOT. For myself, I just push the brake pedal when the garage door is door is down to check them. I also have a mirror that I use sometimes.

I doubt that the guy’s truck even fits in the driveway.
And a large mirror isn’t always available.
Chock the tires.
Stick on the pedal, I say.
Bing, you’re right that there’s no substitute for a physical inspection.