I just pulled the lower O2 sensor off to take a look at it. When I put the HF DP on, only thirty miles ago, this sensor was dusty looking. Now, such a short time later, it looks as though it has creosote deposits built up on it. It was very nasty and pitch black. Should it be this way with the HF DP installed, especially after so few miles? That just doesn't seem normal to me.