Code BMS_a122 was reported by the “Battery management (BMS)” controller in the car. The letter “a” in the middle places it among entries of the alert type.
The Battery Management System — the brain of the high-voltage battery. It watches individual cell voltages, temperature, isolation resistance and the permitted power output. On a used car this is the most closely watched group of codes, because battery health shows up here. Codes about isolation or cell instability are among those worth having checked promptly.
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.
Indicate that a thermistor reading is irrational
BMS cannot measure the temperature on any modules
BMS has faulted while charging or attempting to charge due to a fault
BMS does not receive the UI charge request message
BMS receives the UI charge request message
BMS does not receive charging-related requests from the UI
Indicates a BMB reference voltage was out of bounds
BMS has detected that one or more bricks' voltage is above the warning threshold
Wall connector contactor output shows no voltage when expected to be closed on one or more phases
No voltage is seen on Wall Connector output when contactors are expected to be closed
Voltage is seen at the wall connector output or contactors switch to open state
Wall connector will not operate
Wall connector charge handle temperature too high
info from manualCC_A013
Wall connector charge handle temperature exceeds 78 C
Wall connector charge handle temperature falls below 58 C or user interaction if charging was stopped
Wall connector will reduce reported current capability in 25 percent increments until condition clears or current reaches zero and charging is stopped
Master unit current limit set too low
info from manualCC_A024
Master unit current limit divided by number of units in the sharing network is less than 6 A
Master unit current limit set high enough to allow 6 A or greater per unit in the network
Input wiring terminals are too hot as detected by non-contact IR sensor
info from manualCC_A041
Input wiring terminal IR emission exceeds threshold
Input wiring terminal IR emission falls below threshold
Charging rate will be reduced by 50% and one red blink on LEDs
Wall connector PCBA temperature too high
info from manualCC_A011
Wall connector PCBA temperature exceeds 100 C
Wall connector PCBA temperature falls below 80 C or user interaction if charging was stopped
Wall connector input wiring connections are incorrect
info from manualCC_A009
Wall connector line and neutral conductors are swapped
Wall connector input wiring is corrected
Wall connector will not operate while input wiring is swapped
Wall connector microcontroller internal self test fault
info from manualCC_A014
Wall connector microcontroller failed internal self test
Wall connector microcontroller passes internal self test
Wall connector will not operate until self test passes
AC line PLL failed to lock. Check grid frequency and quality.
PLL was unable to identify valid grid frequency and phase
AC line PLL successfully locks
GFCI cannot function without locked PLL, Wall Connector will not operate
Wall connector input terminal temperature sensor shorted, open, or too cold
info from manualCC_A026
Input terminal temperature sensor measured as open or short circuit
Wall connector input terminal temperature reading passes rationality check
Vehicle side protective earth is disconnected from wall connector
info from manualCC_A002
Vehicle side protective earth is disconnected or high impedance
Vehicle side protective earth is connected
Wall connector will not allow charging until vehicle side protective earth is connected
More than three units configured as slaves were detected on the sharing network
info from manualCC_A023
More than three units are configured as slaves in the sharing network
Three or fewer slave units are detected on the sharing network
info from manualCC_A028
Wall connector current limit selection dial is set to an invalid position
Rotary dial changed to valid setting and unit is power cycled
Microcontroller supply rail is incorrect
info from manualCC_A020
Microcontroller supply voltage is outside of valid range
Microcontroller supply voltage enters valid range
Wall Connector WiFi module hasn't checked in with the real-time processor
WiFi module has failed to check in due to a long running process, update, or software bug
WiFi module checks in with the real-time processor
Charging will continue normally, but connectivity and billing may be delayed or unavailable
Wall connector charge handle temperature sensor shorted, open, or too cold
info from manualCC_A027
Charge handle temperature sensor measured as open or short circuit
Wall connector charge handle temperature reading passes rationality check
AC RMS input voltage is too low
info from manualCC_A008
AC RMS input voltage fell below 160 V RMS for more than 2 seconds
AC RMS input voltage rises above 170 V RMS for more than 2 seconds
Wall connector will not allow charging while the input voltage is too low
Ground fault detection circuit self test failed
info from manualCC_A004
Ground fault detection circuit self test passed after being re-run. Test re-runs after 15 minutes (non-EU), cable cycle, or reset button.
Wall connector will not allow charging until the ground fault circuit self test passes
info from manualCC_A029
Pilot level transitions to invalid voltage level
Wall connector will not operate while pilot level is invalid
Wall connector PCBA temperature sensor shorted, open, or too cold
info from manualCC_A025
PCBA temperature sensor measured as open or short circuit
Wall connector PCBA temperature reading passes rationality check
info from manualCC_A015
Neutral conductor is grounded on vehicle side
info from manualCC_A005
Neutral conductor conductor impedance to ground on vehicle side is less than 2 Ohms with contactors open
Neutral conductor impedance to ground on vehicle is greater than 2 Ohms with contactors open
Wall connector will not allow charging if there is a short between neutral and protective earth when a vehicle is connected before closing contactors
Wall connector proximity line disconnected or vehicle is not providing proximity voltage
info from manualCC_A019
Proximity line in charge cable has no voltage present but a vehicle is connected
Proximity line in charge cable rises to a valid voltage level
Wall connector will not allow charging while the proximity line is broken because it cannot determine if the cable is latched
Wall connector charge handle is too hot
Wall Connector handle temperature exceeds 65C
Wall connector handle temperature falls below 60C
Ground fault current exceeded threshold
info from manualCC_A003
Ground fault current has been corrected and the test re-run. Test re-runs after 15 minutes (non-EU), cable cycle, or reset button.
Wall connector will not allow charging until ground fault current path is corrected and the test re-run.
info from manualCC_A018
Master and slave units in sharing network have different nameplate current ratings
info from manualCC_A030
Wall connector nameplate current rating differs from other wall connector in sharing network
Wall connector no longer detects units with different nameplate current rating in the sharing network and the unit is power cycled
Wall connector protective earth is disconnected or grounding scheme DIP switch set incorrectly
info from manualCC_A001
Wall connector AC input side ground impedance is above threshold
Wall connector AC input side ground impedance has fallen below threshold for adequate time
Wall connector will not allow charging until the protective earth connection is corrected
Output voltage, current, and energy monitoring failure
Metering hardware fails to initialize
Metering hardware initializes successfully
Wall Connector cannot detect welds, over-current, or meter energy. Wall Connector will not operate.
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.