PIC Input question

As part of my project, I want to monitor the engine coolant temp in a vehicle. The coolant sensor uses a 5v input, and the output (varied based on temperature) goes back to the ecu. If I connect a wire to the sensor return, say near the ecu, and have it as an analog input into a pic, is it going to alter the temp reading going to the ecu?

Sorry for the newb question, and thanks for any answers.

It depends on the output impedance of the sensor.

How could I determine what the output impedance of the sensor is?

Probably not very easily?, perhaps the best solution is to use a high input impedance buffer from the sensor to the PIC.

I have no idea what that means...I've got a lot to learn.