What's this piece? it's under my car.

The undercover looks like a piece in front of the tires. Regardless of where it came from, I would put it back on. These cars have lots of expensive modules, sensors and cables that are protected by these covers.

 

It appears that the oil change door has broken away under the car. The plastic push in rivets that held it in place have broken away.

Many owners replace the plastic hinge area where the door tore off with an aluminum piano hinge and the plastic rivets with M6 bolts and rivet nuts.

 

If its the oil door you don't need to put it back on. Many leave it off and have no problems.

SM-G960U ?

 

It is the oil change door. The plastic "hinge" eventually wears out. As @Pdaddy said, many leave it off. I'd prefer to have it in place. Not har to reinstall is as @Georgina Rudkus described. It's up to you.

 

That sounds very elaborate. I riveted a couple pieces of scrap plastic to the formerly-hinged edge, drilled a hole in each one and used two more plastic push pins. Done.

What I like is that I don't have to raise the car as high to change the oil as when that panel is hinged and has to swing down. Now it just comes out.

 

I don’t use that hinged flap, instead remove the whole under panel, when doing DIY oil changes (or whatever). Wash out the associated fasteners each time they’re off. It’s a bit more work, but the cover will last forever.

I’ve had it off maybe 25 times now (6 monthly oil changes plus misc). A few of the plastic fasteners were getting messed up, missing a tooth or two, so I got a set of replacements (2 kinds, parts 3 and 5 in post #2), and sub the new ones in as needed.

Still, if you’ve got rushed dealership or 3rd party mechanics doing oil changes, it will be abused. It’s a poor design, the flap portion; 4th gen remedied this, revised the fasteners at the “flap” and made it totally removable.

 

In fact, the Gen 4 is secured with screws rather than the pushpins that seem to wear out or disappear.

 

This is the hinged oil change cover I added after the factory setup came off. I used a piece of stainless piano hinge and rivets backed by washers on the reverse. Offset the rivets so they don't restrict full opening.

I have a bag of the pushpins ready if needed.

 

Another close up view.

 

Our previous civic hybrid also had a hinged under panel, but the plastic was thicker and less brittle, and purposely thinner at the hinge. You could flex it till the cows come home; it bent easily, and showed no signs of cracking. In short, it was well designed.

 

The undercover was re-designed in 2012 on account that it was exceptionally fragile and broke off when it was introduced with the 2010 Prius. Toyota actually issued a bulletin and replaced those on the 2010 and 2011 models for FREE.

Toyota, however, did not address the underlying issue of the weak plastic push-in rivets that hold the trailing edge of the undercover. That is easily remedied by replacing the push in rivets with rivet nuts and bolts, as they did with the removable oil change cover for the 2016 + Generation 4 models.

 

They’re fine, will last donkey’s years, IF you finesse them out, and ALWAYS rinse the grit out before reinstalling.

trust me, I practically live under there...