Throttle cable?!?!?!

There's no throttle cable and the throttle the car has is not for the purpose of regulating engine load. There is a throttle butterfly but it's there to have a smooth shutdown of the motor; it is also used during the DPF regeneration.

It does get shorted by soot & oil getting into the circuitry. So perhaps that's what is going on.

Once it totally fails the code will stick and you'll notice the engine shuts off like an old diesel tractor.

Until then you're fine.

Take a look here at what happens to the throttle: P2621 Throttle position , output signal - circuit low - Bimmerfest - BMW Forums

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