A heat exchanger unit, especially for a fuel-operated vehicle heater, with a heat exchanger housing (16) with an outer wall (24) and with an inner wall (30), wherein the outer wall (24) and the inner wall (30) define a heat carrier medium flow space (36), through which heat carrier medium (M) can flow, wherein at least one sensor mounting opening (42) is formed in the outer wall (24) and a sensor (40) is received in the at least one sensor mounting opening (42) in a fluid-tight manner and displaceably and protruding into the heat carrier medium flow space (36) with an area (50) protruding into the flow space and supported on the inner wall, wherein a prestressing element (52) acting on the sensor (40) for supporting it at the inner wall (30) is associated with the sensor (40), wherein the prestressing element (52) is fixed in a fixing area (54) in relation to an outer housing (58) enclosing the heat exchanger unit (22) in at least some areas and it acts on the sensor (40) in an action area (56) for support at the inner wall (30).