PBT composites with multi-walled carbon nanotubes (MWCNTs) and graphite were obtained for use as sensors in conventional non-destructive testing. Their electrical conductivity, thermal diffusivity, electrothermal properties, and piezoresistive response were investigated. PBT/MWCNT composites showed the highest strain sensitivity, while PBT/MWCNT–graphite hybrid composites showed better electrical stability and better heat transfer, which indicates their potential as sensors in conventional non-destructive testing.