in thermal communication with a wax material in a valve the amount of electrical current adjusted to one of two states the amount of electrical current adjusted in response to a sign of a derivative of an output of a sole device temperature sensor in this way heater current can be adjusted to control device temperature within a designate range in some examples the method includes where a first of the two states is substantially zero current and further comprising adjusting the amount of electrical current to the first state in response to an engine temperature less than a lower threshold of the desired device temperature range the method also includes where a second of the two states is substantially rated heater current and further comprising adjusting the amount of electrical current to the second state in response to a device temperature greater than an upper threshold of the desired device temperature range the method includes where the two states are less than rated heater current in one example the method further comprises adjusting the amount of electrical current in response to engine speed the method further comprises adjusting the amount of electrical current in response to engine load additionally the method includes where the two states are adjusted in response to operating conditions the method of fig5 also provides for adjusting device temperature comprising selecting a desired device temperature range and a desired device sub temperature range the desired device sub temperature range within the desired device temperature range in response to.