Nitrogen oxide sensor stuck-in-range diagnostic tool

A method for diagnosing a stuck-in-range fault condition of a downstream NOx sensor in a vehicle during a predetermined low exhaust flow condition, wherein the vehicle includes an exhaust system having a controller with memory, a selective reduction catalyst (SCR) device, the downstream NOx sensor which is configured to measure a downstream NOx level on an outlet side of the SCR device, and an upstream NOx sensor which is configured to measure an upstream NOx level on an inlet side of the SCR device, the method comprising:

• detecting, via the controller, the low exhaust flow condition;
• measuring the upstream and the downstream NOx levels using the respective upstream and downstream NOx sensors;
• receiving, by the controller, the upstream NOx level from the upstream NOx sensor;
• receiving, by the controller, the downstream NOx level from the downstream NOx sensor;
• comparing each of the upstream and downstream NOx levels to a zero or a near-zero threshold when the predetermined low exhaust flow condition is active;
• executing, via the controller, a first control action when the upstream NOx level is below the threshold and the downstream NOx level exceeds the threshold, including indicating a stuck-in-range fault condition for the downstream NOx sensor;
• and
• executing, via the controller, a second control action when neither of the upstream or downstream NOx levels exceeds the threshold, including recording a diagnostic code in memory of the controller indicating that the downstream NOx sensor is properly functioning.

A method for diagnosing a stuck-in-range fault condition of a downstream NOx sensor in a vehicle during first and second low exhaust flow conditions, wherein the vehicle includes an exhaust system having a controller with memory, a selective reduction catalyst (SCR) device, the downstream NOx sensor which is configured to measure a downstream NOx level on an outlet side of the SCR device, and an upstream NOx sensor which is configured to measure an upstream NOx level on an inlet side of the SCR device, the method comprising:

• detecting a fuel shutoff event to thereby determine the presence of a low exhaust flow condition;
• detecting an active regenerative event of a particulate filter;
• measuring the upstream and downstream NOx levels using the respective upstream and downstream NOx sensors;
• recording in a first memory location, by the controller, the upstream and downstream NOx levels from the respective upstream and downstream NOx sensors during the low exhaust flow condition when the regenerative event is active;
• recording in a second memory location, by the controller, the upstream and downstream NOx levels from the respective upstream and downstream NOx sensors during the low exhaust flow condition when the regenerative event is not active;
• comparing, by the controller, each of the recorded upstream and downstream NOx levels in the first and the second memory locations to a zero or near-zero threshold;
• executing a first control action when the upstream NOx level is below the threshold and the downstream NOx level exceeds the threshold, including indicating, via the controller, a stuck-in-range fault condition for the downstream NOx sensor;
• and
• executing a second control action when neither of the upstream or downstream NOx levels exceeds the threshold, including recording a diagnostic code in the memory of the controller indicating that the downstream NOx sensor is properly functioning.