During device operation the continuous self test monitors both the sensing element and ASIC performance. Sensing element outputs 4 independent unit acceleration vectors (a1 to a4) while X-Y-Z acceleration is calculated in the ASIC based on the unit vectors. During matrix calculation, the self diagnostic value (d) is also determined and compared to predefined threshold value. The self test is started by a CTRL-register command and it is calculated continuously during operation until it is disabled. Possible errors are indicated in an error status register and in SPI frame. Continuous self test calculation does not have any effect to normal operation.