There's a Weird Way Certain Audio Tracks Can Brick a Car Stereo

A 2016 Mazda 6
Photo: Mazda

A 2016 Mazda 6 refused to play one podcast in particular, and the car’s owner was vexed. Everything else was streaming over Bluetooth just fine. But it turns out that the titling of an audio track can be more important to car audio functionality than you might think, in specific and odd situations.

On the tech podcast Reply All, in a recent installment of the segment called Super Tech Support, the show’s hosts Alex Goldman and PJ Vogt dove in to try and figure out why the 2016 Mazda 6, and apparently some Nissans as well, had their infotainment systems go haywire when they tried to play the podcast 99% Invisible.

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If you’re a fan of Reply All or podcasts in general, I don’t want to ruin the entertainingly meandering path of discovery that Goldman, Vogt, 99% Invisible’s host Roman Mars and a couple other celebrities take you on while troubleshooting the issue.

So you can follow the link to listen when you’ve got about 40 minutes of free time, or just scroll down and I’ll spoil the ending for you.

If you guessed that the percent sign in that podcast’s title was causing the problem, you’re correct. Partially.

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Certain symbols can’t be used in certain slots for text because they inadvertently screw up the programming behind whatever software you’re using. The percent sign followed by I, as in “%I,” as in 99% Invisible, has a meaning in the programming language C, for example. But Brandon Goozman, one of the authors of the Mazda’s audio coding, explained on Reply All that the 2016 Mazda 6 doesn’t use C.

Instead, Goozman posited that %I was interrupting another signal in the Mazda stereo’s operation.

ALEX: And so then I said, “Okay so why is the stereo restarting?” And Brandon told me that when they designed the car, they made it so that the radio was regularly sending a little message to the rest of the car saying ”I’m working, I’m working, I’m working.” Brandon called it a heartbeat.

PJ: Uh-huh.

BRANDON: And the hardware layer is listening for that. And as long as it gets it, it’s happy. If it doesn’t get it, it will assume that the UI has frozen, and it will restart it to a known good state.

ALEX: It’s, it’s saying,“It’s not communicating with me.”

BRANDON: Yeah–

ALEX: “Something is wrong.” And, and it just start–starts over.

So Reply All figured out that while %I didn’t mean anything to the Mazda’s stereo, it is enough to confuse the system into continually restarting.

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As for a fix, the shows hosts confirmed with Mazda that a firmware update solves the problem. But if you don’t want to pay to get that done, 99% Invisible’s publisher plans to re-upload the podcast with a differently worded title (“percent” spelled out, presumably) for the few via-Bluetooth listeners who might have encountered this problem.

Who knew a car stereo could be so picky?

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About the author

Andrew P. Collins

Reviews Editor, Jalopnik | 1975 International Scout, 1984 Nissan 300ZX, 1991 Suzuki GSXR, 1998 Mitsubishi Montero, 2005 Acura TL