Apparatus and method to diagnose a nox sensor

A method, comprising:

• raising a temperature of an SCR catalyst for a predetermined time period while dosing urea;
• maintaining the temperature of the SCR catalyst without dosing urea for a second predetermined time period;
• comparing data from a first NOx sensor and a second NOx sensor without dosing urea over a third predetermined time period;
• and
• providing a NOx sensor condition index for at least one of the first NOx sensor and the second NOx sensor in response to the comparing.

An apparatus, comprising:

• a diagnostic staging module structured to maintain a diagnostic temperature target in an SCR catalyst for a predetermined time period;
• a reductant dosing module structured to provide a reductant dosing command;
• a reductant doser responsive to the reductant dosing command;
• the diagnostic staging module further structured to, at the end of the predetermined time period, command a reductant dosing module to stop dosing reductant for a second predetermined time period and to continue to maintain the diagnostic temperature target;
• the diagnostic staging module further structured to, at the end of the second predetermined time period, continue to command the reductant dosing module to stop dosing reductant and to maintain the diagnostic temperature target for a third predetermined time period;
• and
• an SCR diagnostic module structured to, during the third predetermined period of time, provide a sensor condition index in response to a comparison of data from a first NOx sensor upstream of the SCR catalyst and a second NOx sensor downstream of the SCR catalyst.