1. A system comprising: a gas turbine engine comprising a combustor and igniter configured to produce hot gas to propel an axial shaft; a sensor mounted to detect signals emitted by the igniter in the combustor; and a processor in electrical communication with the sensor, the processor configured to perform at least the steps of: receiving from the sensor first data that indicates signal changes in each of a plurality of time bins during operation of the igniter in the combustor; determining information characteristics of the first data in each of the plurality of time bins; determining a value of a similarity measure that indicates similarity of the information characteristics to target information characteristics; determining a condition of the igniter based on the value of the similarity measure; and operating a device based on the condition of the igniter.