Systems and methods for performing a NOx self-diagnostic test

A method comprising:

• operating an exhaust nitrogen oxide (NOx) sensor during engine running operation and adjusting engine operation in response thereto;
• determining a duration to delay a self-diagnostic (SD) test of the exhaust NOx sensor after a vehicle-off event based on engine operating conditions at the vehicle-off event:
• performing the SD test after waiting the duration, wherein performing the SD test comprises determining whether the NOx sensor is degraded based on outputs received from the NOx sensor after a first completed SD test and only when the outputs are generated under conditions where a temperature at the NOx sensor is less than a threshold, an oxygen concentration is within a threshold range, and a change in oxygen concentration during the SD test is less than a threshold;
• and
• generating an alert in response to determination of NOx sensor degradation from the SD test.

A method comprising:

• operating an exhaust nitrogen oxide (NOx) sensor in a first mode during engine running operation, where an oxygen concentration in a chamber of the exhaust NOx sensor is controlled to a lower first threshold;
• determining a duration to delay a self-diagnostic (SD) test of the exhaust NOx sensor after a vehicle-off event based on engine operating conditions at the vehicle-off event:
• performing the SD test after waiting the duration, wherein the determining the duration to delay the SD test is further based on whether the NOx sensor reached a dew point during a most recent drive cycle, prior to the key-off event, where the duration is reduced responsive to a determination that the NOx sensor reached the dew point during the most recent drive cycle;
• and
• generate an alert in response to determination of NOx sensor degradation from the SD test.

A method for an engine comprising:

• waiting for outputs from a NOx sensor positioned in an exhaust system of the engine to stabilize after a key-off event before performing an SD test;
• and
• after waiting for the outputs from the NOx sensor to stabilize, performing the SD test only when one or more of an ambient pressure is within a threshold pressure range, an ambient temperature is within a threshold temperature range, and a voltage of a vehicle battery is greater than a threshold, where the performing the SD test comprises determining whether the NOx sensor is degraded,
• wherein the performing the SD test comprises running the NOx sensor in both a first mode and a second mode, the second mode before the first mode, wherein the first mode comprises adjusting an oxygen concentration in a cavity of the NOx sensor to a first level and measuring a concentration of NOx in the cavity, and wherein the second mode comprises adjusting the oxygen concentration in the cavity of the NOx sensor to a second level and not measuring the concentration of NOx in the cavity.