An apparatus and a method can be used to pre-process an output signal from an ambient condition sensor. The preprocessing removes noise pulses which are not correlated with an ambient condition being sensed. Preprocessing can be carried out by comparing the present output value to a prior output value and selecting a minimum value there between. The apparatus and method can incorporate storage for two prior values and the present output value can be compared to two prior values and a minimum or a maximum of the three values selected. Additional processing can be carried out by comparing the present output value to a nominal expected clear air output value. If the present value exceeds the nominal expected output value, a minimum is selected among the present output value and one or more prior values. If the present output value is less than the nominally expected value, a maximum is selected from among the present output value and one or more prior output values.