Coolant low...top up or replace? (bought recently so dont know what was used before)
 

I've not long had my latest impreza and noticed that the coolant is between Low and High in the header tank so reckon it needs topping up. There's also an airlock I believe as you can hear water noise on startup from behind the dash which may account for the drop in the level?

As I don't know what coolant has been used already and don't want to mix it, is it best to replace it all rather than risk topping it up (hoping the airlock will work its way round) ?

Drain it all and start again is the best way to go about it. When refilling do it very slowly to avoid air locking again.

When you say between High and Low do you mean the header tank or the plastic expansion tank by the nearside headlight?

Level sounds fine to me.

Sorry, yes I do mean the expansion tank.

It doesnt matter what level the expansion tank is at. just make sure the header tank is full when the car is cold and the expansion tank level will go up and down as the car heats up and cools down.

My understanding is that the level in the expansion tank should be halfway between low and high, after a run if you pop the bonnet with the engine still idling the level will sit at the high mark.
As above the header tank should be brim full when cold.

The header tank is full to the brim when cold, so it sounds fine.

Thanks everyone!