Drivers side low beam fuse blows on a 2002 Mercury sable I can replace the fuse and it works ok as long as the engine isn't running.

If fuse 31 is good, there may be a problem with the headlight control circuit board in the smart junction box.
http://www.revbase.com/BBBMotor/Wd/DownloadPdf?id=110641

Check voltage circuits. The ten amp fuses are for low beam. Turn on the headlamps, lo beam setting, use a test lite to check voltage on those two fuses. If no voltage may be problem with dimmer switch?

Probably Fuse, Relay, column switch. I'd start wth the fuse, then check the relay. (Assuming the driving beams work.) There should be a relay box under the hood, likely drivers side. If you're mechanical, you can swap the relay for headlights with another one of the same model/type. Swap temporarily, and if lights then work, just replace th relay for the light.

Mercury Sable - YouTube

? 6:06
www.youtube.com/watch?v=Mlb1V5vNgFM
Jan 3, 2012 - Uploaded by 1A Auto Parts http://www.1aauto.com/1A/HeadLights/Mercury/Sable/1ALHP00223 ... Find out why. Close. How to Install Replace Headlight and Bulb Mercury Sable 00-05 ... How To Install Replace Fog Lightand Bulb 00-03 Mercury Sable

No, the compressor will NOT cause these fuses to blow. You have a SHORT in your electrical system. you have a HOT wire getting into the Ground for that system

This is usually caused by a failed front control module as this unit controls the lighting on the vehicle!
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your flasher is bad its a round silver plug near or in fuse box

Check for power on both sides of the mega fuse located fuse block in front of engine along the side.

For Audi A4 the fuse box is located on the left side of dashboard behind a plastic cover (for left hand steering wheel vehicle). On inner face of the plastic cover you will find the fuse diagram.
First fuse on first row from left upper corner is Fuse Nr.20 = F20 (15A) - RH headlamp low beam, headlamp adjustment.
First fuse on second row from left upper corner is Fuse Nr.21 = F21 (15A) - LH headlamp low beam, headlamp adjustment.

ITS WILL BE A BAD EARTH ,, CLEAN THE EARTHS UP ON BOTH LIGHTS NISSAN A BAD FOR THIS PROBLEM ,, SAME OVER HERE IN THE UK ,, POLICE GIVE TICKETS FOR ANY THING