A method of diagnosing vehicle NOx sensor faults that includes sensing that an exhaust gas recirculating (EGR) valve is closed and that fuel flow to a vehicle engine is above a predetermined rate using an electronic control unit (ECU) located on a vehicle; recording the output received from an NOx sensor relative to an amount of fuel consumption over a period of time using the ECU; calculating the mean of the NOx sensor output relative to fuel consumption recorded over the period of time using the ECU; performing a least-squares estimation (LSE) using the ECU for more than one calculated mean based on an expected NOx sensor output; generating a NOx sensor gain for the NOx sensor using the ECU based on the LSE; and determining whether the NOx sensor gain is above or below a predetermined threshold.