Test for a change after servicing a machine

The lengths of metal bars produced by a particular machine are normally distributed with mean length 420 cm and standard deviation 12 cm.

The machine is serviced, after which a sample of 100 bars gives a mean length of 423 cm.

Is there evidence, at the 5% level of significance, of a change in the mean length of the bars produced by the machine, assuming that the standard deviation remains the same?