In this paper we report results for an intrinsic evanescent field sensor based on nonregular plastic optical fiber with smart polymer film containing Malachite Green MG+([PhC(C6H4NMe2)3]+) as an absorption reagent, which coats the fiber's thermally imperfected area. Thermal imperfections were created by touching the fiber with a heated metal rounded tip. The changes in color and absorption characteristics of the polymer film depend on the acidic and basic environmental properties of the sensing area. An imperfected plastic optical fiber with Malachite Green coating has been presented for the detection of ammonia vapor. We discuss some experimental results that confirm our theoretical predictions. The combination of different thermal imperfection topologies and film’s thickness enabled the design of a bio-chemical sensor with high sensitivity and linearity over a wide measurement range. Additional increase of the evanescent field interaction can be achieved by implementing the imperfection area on a bent fiber.