According to an example embodiment, there is a magneto-resistive (MR) sensor having an operation mode and a test mode. The MR sensors comprise a first plurality of first MR elements and a second plurality of second MR elements. A first signal processing unit is connected to the first plurality of first MR elements for generating a first output signal; the first output signal has a first phase. As second signal processing unit is connected to the second plurality of second MR elements for generating a second output signal; the second output signal has a second phase. There is a third signal processing unit or processing the first and second output signals; the third signal processing unit is adapted to activate one of the operation mode and the test mode on the basis of the first and second output signals. When the first and second plurality of first and second MR elements are exposed to a homogeneous magnetic field, the third signal processing unit is adapted to activate the test mode.