You'll return no codes because you'll have a completely independent wiring system. Power goes from battery to lights. The relay tap draws no noticeable current. The entire system is 'off the grid' as they say in Wyoming.

To answer your question as to WHY: 1) so the driver can operate the foglights independently of the headlights. Stock Volvo setup is foglights on when the headlights are on, period. But I like to run them as DRLs, sometimes in conjunction with parking lights....in a Volvo you have to hardwire your own circuit to accomplish this. (I'm doing this with OEM foglights and switch.) 2) Volvo OEM foglights look good, but on the new models they're far too recessed to be of any practical use, they're more of a decoration. 3) Becausue running foglights (or any lights) from a computer is needless, electronic trickery. You know how we're replacing $400 ABS modules in our S70's? Wait till you're replacing $600 modules because the lights won't work!