Little help please W/an Op Amp

[Dr. Jerryrigger]

So I didn't really know the best place to post this, so I picked here.
Anyway I want to make a DFCO indication light.
I have a leads from my O2 sensor. The voltage to ground on both wires is around 5V across the two wires I get .335 or so. When driving with the DMM in my cup holder it jumps to .5V or so when it is in DFCO.
So with an op amp i was hoping wire up an LED so that it would light when the voltage jumped up. 14V*.5V vs. 14V*.335V should be a sufficiently large difference to have a LED light and not light, or dim/bright would be fine w/me.
The thing that I don't know about, is the signal wires are +5V to ground...

I don't really know what I'm doing here, but is seems possible, and maybe really easy.
Thanks!

If i can get this working I'll write up a little "how-to" or something.