The short answer
Get down and look at the underside of the battery. Tesla's packs have flood ports that pop open when liquid gets into the part of the pack that holds its electrical gear, and on current cars nothing tells the driver when that happens. An open port, or one that has been replaced, is the question to ask about. Then read the title history. A flood car that an insurer wrote off is one Tesla treats as unsupported: fast charging stays off until it passes Tesla's inspections, and the warranty is void for good.
Where Tesla draws the line
Tesla's service documentation counts a car as flooded if it sat in, or was driven through, water above the top of its rocker panels, the sills under the doors, for any length of time. Wet carpets after a storm with a window open don't, and nor does a puddle on the highway. Tesla's own routine for a flooded car starts with parking it outside, cordoned off, while the owner calls the insurer, and then disconnecting its low-voltage power.
The flood ports
A Model 3 built from 2017 to 2023 and a Model Y built from 2020 to 2024 have two flood ports on the underside at the back of the battery. A Model S or X built from 2021 has five. They're drains: when liquid collects in the pack's electrical bay, they open to let it out. In 2025 Tesla began replacing all five on many 2021 and 2022 Model S and X under a service campaign, with no inspection first, so new ones on those cars are expected.
When Tesla finds one open it checks that bay for liquid, which its documentation says is most often coolant from inside the pack, and if there is any, the documentation says the battery may need replacing. A port that opened with no liquid behind it just gets replaced.
Why no light comes on
The first flood ports on early Model 3 and Model Y packs had a switch that logged an alert for Tesla service, not a light for the driver, and it could report a port open when it wasn't. Tesla's fix is a second-generation port. Later packs use ports with no sensor at all, and Tesla's documentation for the 2021-on Model S and X puts it plainly: there is no alert when the flood port opens. A car can have had water in its battery with nothing on the screen to say so.
The battery test won't catch it
Tesla's battery health test measures stored energy and nothing more, and Tesla lists moisture inside the pack and corrosion among the things it doesn't look at. A flood car can show a healthy battery. The flood ports are a physical check, so no app, no remote reading and no VIN report can do it for you, ours included.
What to look at
Underneath, the flood ports on the underside of the pack, and fresh parts or tool marks around them. Inside, silt or tide marks under the carpets and along the seat rails, rust on the seat bolts, and a smell that stays with the doors open. On paper, a flood, salvage or rebuilt brand, a total-loss record, or a car that has changed states since a storm. None of these proves a flood on its own. Together they tell you whether to walk away.