T501 can also be the pressure sensor for accumulator.
There was a service bulletin about these, in certain temparature cnlan cause this error.

Try to swap with the brakepressure sensor, they ar the same.
Brake sensor is located right above the hydr.pump on the axle.
The other lives down on the brakevalve rightside under pressurefilter.
For this one remove batterij, there is a hole for easy acces.
Error should change to T502

Check the accumulator, remove the fan on the rightside, there it lives.
Use an allenkey to open the backside, do this SLOWLY.
If it hisses, there is pressure. If oil comes out, replace the accumulator