Local Controller did not issue a go around after receiving an AMASS Alert; noting the vehicle previously on the runway had cleared but he/she had failed to clear the AMASS equipment.

Narrative

I assumed the Local Center position. During the position relief briefing I was advised that an airport vehicle was on Runway 18C inspecting lights. The 'Vehicle on Runway' light was illuminated and the 'Runway Closed' placard was in place in the strip bay. I didn't register that the 'X' were in place on the AMASS to show the runway was closed. I coordinated with the RADAR Controller that there was a vehicle on the runway and that it wasn't available. The vehicle reported clear of Runway 18C. I removed the placard and turned off the 'Vehicle on Runway' light. I then advised the RADAR Controller the runway was available again. A SW4 was vectored to Runway 18C. I scanned the runway and issued landing clearance when the SW4 called on the Tower frequency. When the aircraft was short final the AMASS alarmed indicating the Runway was closed. I tried to clear the 'X's from the display but was unsuccessful as the selection box for the AMASS was in the 'Configuration' selection window and not the 'Closed Runway' window. The instruction of the AMASS was to send the aircraft around. I failed to send the SW4 around. The AMASS alarmed a second time. I finally cleared the X's from the display. Recommendation; In the future I will be more diligent in scanning the AMASS display as part of my runway scanning prior to giving landing clearance. I will review the AMASS each time I assume a Local Control position to verify its status.