Disclosed is an auto injector for administering a medicament. The auto injector comprising: a housing; a cartridge receiver configured to receive a cartridge assembly comprising a cartridge (700) and a cartridge code (1000) feature, the cartridge containing the medicament; a code sensor configured to read the cartridge code feature; a drive module coupled to move a plunger rod; and a processing unit coupled to the code sensor and the drive module. The processing unit being configured to: receive from the code sensor a code signal indicative of the cartridge code feature; control the drive module to move the plunger rod to a first plunger rod position, the first plunger rod position being based on the code signal; receive a trigger event; control the drive module to move the plunger rod to a second plunger rod position following reception of the trigger event.