Systems and methods are described for controlling an engine in a vehicle. One example method includes, after a threshold duration of engine idling, continuing engine idle operation when an exhaust NOx sensor indicates a NOx level below a NOx threshold, and stopping engine idle and shutting down the engine when the exhaust NOx sensor indicates the NOx level is above the NOx threshold. In this way, when the exhaust emissions are below the NOx threshold, it is possible to prolong engine idling operation using the exhaust NOx sensor.