Can I use Advanced Sensors even when I am stressed? I know that you can use it to do an action when on an asteroid, but because it is a "Free Action" does stress matter?

Advanced Sensors:

Immediately before you reveal your maneuver, you may perform 1 free action. If you use this ability, skip your "perform Action" step during this round.

Stress: (core rules page 17)
There are several factors that can cause pilots stress, such as executing difficult (red) maneuvers (see Step 4 on page 7). While a ship has at least one stress token, it cannot execute red maneuvers or perform any actions (even free actions).

I think you can work it out from there

Can I use Advanced Sensors even when I am stressed? I know that you can use it to do an action when on an asteroid, but because it is a "Free Action" does stress matter?

You can, however, use the Advanced Sensors before executing the stressful maneuver

Stress always matters when it comes to taking actions unless your Tycho. Stress will prevent you from taking action either as part of the perform action step or given a 'free' action. Stress does NOT prevent you from acquiring token from non-Actions.

I'd say you could activate Advanced Sensors while Stressed BUT you will not be able to perform that free action due to the Stress. All this would mean is that you're giving up the "Perform Action" step after your maneuver so even if you cleared the stress you still couldn't perform a normal action. To put it another way I can't see any benefit to attempting to use Advanced Sensors while stressed so why try.

Can I use Advanced Sensors even when I am stressed? I know that you can use it to do an action when on an asteroid, but because it is a "Free Action" does stress matter?

You can, however, use the Advanced Sensors before executing the stressful maneuver

Or, to take the concept further, use the Advanced Sensors for an action, use Push The Limit off of it, gain a stress and clear it immediately through a green maneuver.