It is shown how chemically sensitive metal-oxide semiconductor field-effect transistors with a thin discontinuous platinum gate can be modified for the detection of NOx. After a pulse of ammonia the sensors show an increased sensitivity to NOx. The threshold voltage shift induced by NOx is opposite to the direction before the ammonia pulse. The threshold voltage now increases due to NOx exposure, while hydrogen, ammonia and hydrocarbons cause a decrease of the threshold voltage. The temperature dependence of the NOx sensitivity suggests that after the ammonia pulse there are two competing polarisation phenomena caused by the interaction between NOx and the sensing surface. The results are of general interest since they indicate how thin sensing layers can be modified after fabrication to promote sensitivity towards specific molecules. Furthermore they shed some new light on the detection mechanisms of thin discontinuous metal gates.