Code RCM2_a171 was reported by the “Airbags — newer module” controller in the car. The letter “a” in the middle places it among entries of the alert type.
The newer generation of the airbag and restraints module. The codes mean the same as the RCM group; it is by far the largest code group of all, because the module monitors every sensor and every squib separately.
The most common type. The car logged the event and may have shown it on the screen. The range is wide — from something trivial that clears itself to something that needs attention.
This is how the fault is named inside the car's firmware. For codes without a description it is the most precise clue there is — the name itself says what the event is about.
Tesla only publishes descriptions for some codes. What we do know for certain is its classification and the internal name above — and from those you can reliably tell which part of the car it concerns.
Copy it exactly, letter and number included. Photographing the screen is safest — codes often differ by a single digit, and mixing them up leads somewhere else entirely.
Some codes clear after a screen restart or after the car sits locked for a few hours. If the code does not come back, it was usually a one-off glitch.
A one-off entry and a code that appears after every drive are two different things. It helps a service centre enormously if you know exactly when it shows up.
With airbags, brakes, steering and high voltage there is no point waiting it out. With the other groups you can watch and see whether the code returns at all.
Error codes are only part of the picture. For cars on TesList you also see battery health pulled straight from the vehicle, plus a price calculator that works out from current listings what a given car is actually worth today.