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I heard a buzzing noise coming from under my 2013 4Runner that had been idle in my garage, keys removed, for a couple of hours. It seemed to be coming from behind the spare tire and as I felt around (blindly!) and followed a wire to it's harness, the buzzing stopped. Dropped tire and found a small module (about the size of a pack of cards) mounted on the driver's side, forward of the spare tire center and somewhat close to the shock / spring (left, in picture). Any idea what it's for? Could it potentially vibrate if the wiring harness was loose? There's a much larger module in a plastic case (center, in picture); could be that one, too. For perspective, you can see the chain used to lower the spare tire, to the right / rear in the picture and the spare tire at the bottom.
 

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It’s a system vacuum test. On other models, it runs somewhere between 3 and 5 hours after you shut the engine off. Can’t remember offhand the exact number of hours after. It’s also noted somewhere in the owners manual.
 

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Excellent, thanks 05Moose! I did a little reading with what you said in mind and found:

1. Key-Off Monitor

This monitor checks for EVAP (Evaporative Emissions) system leaks and canister pump module malfunctions. This monitor starts 5 hours after the ignition switch is turned off. At least 5 hours are required for the fuel to cool which stabilizes the EVAP pressure, thus making the EVAP system monitor more accurate.

The leak detection pump creates negative pressure (VAC) in the EVAP system and the pressure is measured. Finally, the ECM monitors for leaks from the EVAP system, and malfunctions in both the canister pump module purge VSV based on the EVAP pressure.
 
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