if I unplug the boost sensor the engine runs and idles the same which leads me to believe it's shot (causing my terrible 14 mpg). If I pull the vacuum line on the sensor (which runs to a vacuum nipple on the motor right where the ACV and airpump used to be) the car stalls. Should the car run differently if this sensor is unplugged? If it's toast how much does a new one go for (almost afraid to ask) mazdatrix didn't have one listed on the site...btw, the car is an S4 NA

Thanks guys,

Jon

 

14 mpg is typical, rotaries suck down gas like crazy

 

If you disconnect the electrical connector to the boost sensor,the ecu assumes the sensor or circuit is nfg and assumes a failsafe value of -96mm Hg for vacuum in the manifold.If you diconnect the vacuum hose,you introduce a vacuum leak in the manifold which affects idle and also assuming the boost sensor is working properly,the ecu thinks the manifold pressure is at atmospheric pressure.
The proper way to check your boost sensor is given in the FSM.
I doubt that is the reason for your crappy gas mileage--it could be a host of things including a lead right foot.