A gas sensor includes: an outer housing having an outer housing gas inlet and an outer housing gas outlet for receiving a flow of gas; and an inner housing disposed within the outer housing such that a gas flow passage is defined through the gas sensor between the inner housing and the outer housing to allow gas to enter the outer housing gas inlet and exit the outer housing gas outlet. The inner housing is provided with an inner housing gas inlet and an inner housing gas outlet each in fluid communication with the gas flow passage. The inner housing gas inlet and inner housing gas outlet are positioned relative to the gas flow passage such that when gas flows through the gas flow passage there is a pressure gradient across the inner housing gas inlet and inner housing gas outlet which causes gas to pass through the inner housing.