About a month ago I was returning from vacation in the mountains. About half way home while driving on the Interstate, I heard a fairly loud bang that came from the passenger side of the engine compartment. At first I thought I had run over something and didn't see anything. I thought that if it were something serious I would see a message on the dash. I drove the rest of the way home (about 200 miles) and no message. About 3 days after the trip I started the car and a message appeared stating the coolant level was low and not to start engine. I checked the reservoir and it was empty. I added enough water to it to bring it up to the filled mark. I have driven it now for about 3 weeks and today the message has come back on. I looked at the reservoir again and it was about an inch below the fill line. I don't see any signs of a leak and don't smell any antifreeze? Any suggestions??

The noise may or may not be related to the low coolant. You may have 'popped' a rock. The fact that the coolant was empty bothers me, but you are in a hot climate. Being an inch low after filling sounds normal, some air was purged from the radiator and used your antifreeze. If after refilling, it goes down again, then there will be a leak and you will have to check all of the hoses with a paper towel - it will turn green if you wipe any old/new antifreeze. It's too soon to expect the hoses to break.
Watch and wait,
Klaus
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98 V70Rawd(101Kmi), 95 854T(85K mi), 88 245(165K mi)

Look at the hose connection at the bottom of the Expansion tank (coolant sensor). You can lift the tank up and away (slides into a slot from the back) to inspect it. I had the same problem and had to replace the coolant sensor. If the leak is not at that fitting, keep looking.

Gary

First of all, on my 2001 the plastic tank is part of the pressurized system. It is not an overflow like on other cars. The cap is a pressurized cap. When mine started using coolant, the seam on the tank had failed. Ihad it fixed under warranty.