Code PCS_a027 was reported by the “Power conversion system (PCS)” controller in the car. The letter “a” in the middle places it among entries of the alert type.
The Power Conversion System — the combined onboard charger and DC/DC converter. It handles charging from the socket and generating the 12 V supply. These codes mostly show up as slower or interrupted charging.
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.
The text below comes from Tesla's technical documentation. We leave it in the original English so that translation cannot shift its meaning.
PCS does not detect coolant flowing
Charger detects that coolant flow is insufficient
Charge cable is removed or inserted or battery management system issues clear fault request
Charging derated due to too large of a voltage drop between starting charge and during charge
info from manualPCS_A053
Charger detects greater than 12% input voltage drop after ramping power
Charger input voltage exceeds alert clear threshold
Reduced AC charge rate, possibly due to bad wiring external to vehicle
PCS detects the HV bus impedance is lower than the expected value
DCDC converter detects low impedance on HV bus when precharging (short circuit or capacitance too high)
Charging derated due to high measured grid impedance (impedance detection algorithmn)
PCS experienced a watchdog reset
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.