While pulling the fuse for my ABS module, I noticed that there was nothing in relay spot #16 (air pump relay). Is this normal? I cant even find out what exactly the "air pump" is for? I have a 2000 V-70 XC AWD. Thank you all.

I suspect the secondary air pump. If you bought it used, there's a good chance that it was acting up and the previous owner removed the fuse. Also, some cars didn't have the air pump at all which would also explain the "missing" fuse. The secondary airpump circulates warm air at start-up into the throttle body to help with condensation and to help the front 02 sensor have a truer reading. If you look under your battery and you don't see a black housing for an air pump you don't have one. You can also see if its there from under the car, looking up towards the battery. These often go bad due to water getting in them...they rust out and make a noise and then seize up. Water is let in through a defective SAS valve that leaks. The SAS valve is the mushroom shaped brass colored device up near the windshield (on the driver's side) under the hood and windshield cowl.
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1998 S70 T5 Emarald Green Metallic (125K), 2004 V70 2.5T Ruby Red (45K), Previous Owner of Black '94 850 Turbo Wagon (Sold at 140K and miss her). My cars have been running so well lately they've got me worried.