A Tiny Culprit Might Cause a Big Issue

Dear Car Talk

Dear Car Talk | Sep 21, 2018

Dear Car Talk:

"Dad, my car won't start" were the first words of a phone call from my daughter who is 1,500 miles away. Her typically reliable 2015 Madza 3 with less than 25,000 miles had stopped working. Specifically, the push-button start didn't start the engine. Pushing the button repeatedly brought no response at all.

I first suspected the key fob (weak or dead battery). That was ruled out when the key fob was able to lock and unlock the doors from 50 feet away. After attempting the starting process several times and asking the typical "dad" questions ("Is the car in park?" "Is your foot on the brake?"), she exclaimed: "It started!"

I had her drive immediately to the dealer since it was still under warranty. Of course, the car started perfectly then. A subsequent examination revealed a defective brake light switch. In today's cars, this switch doesn't only activate the brake lights when the pedal is depressed; it also sends a signal to the computer that the brake has been engaged and it's OK to start the car. The faulty switch resulted in an intermittent signal.

My question is, given the critical role that this once-simple part now plays in modern cars, why hasn't it been re-engineered to be much more robust and reliable? -- Pete

It probably hasn't been re-engineered because not enough of them fail, Pete. As you say, the brake must be depressed in order to start the car. That's a carryover from the 1980s, when Audis were thought to be possessed with "unintended acceleration" demons.

Upon investigation, they found that at least some of the cases of cars accelerating through the back walls of garages were caused by people stepping on the gas when they thought they were stepping on the brake. So carmakers started engineering "interlocks." They made it so you couldn't shift out of park unless your foot was on the brake. And then, when keyless ignition came into use, they required your foot to be on the brake before the car would start.

And the simplest way to engineer that stuff was to use the existing brake light switch -- which was already signaling when the driver's foot was on the brake. While we know that most carmakers won't use a 5-cent part when a 4-cent part will do, to be fair to them, I can't remember the last time I replaced a brake light switch. So in general, they seem to be pretty darned reliable.

You were right to have her try stepping on the brake a few times, because a faulty brake light switch sometimes will work intermittently, or just be out of adjustment and respond to a harder press of the pedal. It's also worth noting that there's an ignition-shift interlock, which prevents the car from starting unless the transmission is in park or neutral. That also can cause a non-starting condition.

So if you find yourself in a similar situation in the future, aside from monkeying with the brake pedal, you should move the shifter around in park, and then try it in neutral, too. With your foot on the brake, of course.

But to be honest, Pete, we really haven't seen an epidemic of faulty brake light switches. If we do, we'll report the outbreak to the CDC. (That's the Center for Defective Car-parts, right?)

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One