A Pirani vacuum sensor based on mutual heating between a heater and a distinct temperature probe, separated by a 5 μm air gap, is proposed. The sensor is fabricated by applying a simple post-processing procedure to chips designed and fabricated using the BCD6s process (Bipolar-CMOS-DMOS) of STMicroelectronics. The sensor layout has been optimized to exploit the layers of the original process in order to enhance the sensor performance. The sensors exhibit a resolution better than 0.4 Pa from nearly 0.3 Pa to 1 kPa and better than 50 Pa from 1 kPa to 100 kPa. The sensor response at the lower extreme of the pressure interval is marked by an offset voltage, which is three orders of magnitude smaller than the full-scale value. Finite Element Method simulations suggest that the offset is due to pressure-independent heat transfer due to radiation and conduction through the substrate. The simulated equivalent offset drift is 50 MPa/K.