What usually causes U0401?

A fault in that module, mismatched software or coding.

U0401Invalid data received from ECM/PCM A (engine)

U0401 means “Invalid data received from ECM/PCM A (engine)”. It's a generic network code (invalid data received from a module), so it means the same on every make, including VW, Škoda, Audi, SEAT and Cupra.

Fix soon
SystemNetwork
TypeGeneric (same on all makes)

How urgent is it?

Worth fixing soon; usually drivable meanwhile, but watch for symptoms.

Which system and module

Scanline files it under Network. The first letter, U, means network (communication between modules).

Network codes can be stored by any module on the car's data network; on VW Group cars the Gateway (VCDS address 19) often stores them too.

What usually causes it

A fault in that module
mismatched software or coding

Typical causes are a starting point, not a diagnosis of your car.

Related codes

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Scanline explains fault codes; it isn't a repair authority. This page explains what the code means; it can't tell you what is wrong with your car. Ross-Tech's wiki has their own notes on VW Group faults.