Code EPAS3P_a120 was reported by the “Power steering — 3rd gen, primary” controller in the car. The letter “a” in the middle places it among entries of the alert type.
The primary channel of third-generation power steering. Newer cars have a redundant steering system — this is its main half.
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.
The electric power assist steering (EPAS) internal private bus is receiving an unexpected status counter value, resulting in the chassis bus electric power assist steering (EPAS3S) Electronic Control Unit (ECU) to lose functionality.
The status counter does not increment as expected.
The status counter increments as expected.
The power steering function is operational, but steering assist will likely be reduced. A customer facing alert describing a reduced level of assist for power steering may be present. Autopilot may be disabled in some cases.
DAS_steeringControl message not received
DAS_steeringControl message not received by EPAS
EPAS receives DAS_steeringControl message again
Target torque correlation error
Invalid RCM_yawRateQF value received
Invalid DAS_steeringControlCounter value received
DAS_steeringControlCounter is not incrementing correctly
EPAS observes DAS_steeringControlCounter incrementing correctly
Torque sensor zero point not calibrated
Invalid motor cogging compensation values
Cogging torque service is not performed or incorrect value is stored
Invalid SCCM_steeringAngleSensorCrc value received
Unexpected SCCM_steeringAngleSensorCrc value received by EPAS
EPAS receives expected SCCM_steeringAngleSensorCrc value
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.