Today it was relatively warm (high 30s) compared to least few days, I took the opportunity to do some work. Preparing the car for the next round of NJ emission inspection.
Changed the spark plugs (Bosch F8DC4) which were 6 months old looked OK.
Changed the air filters did not look too bad (2 year olds).
Checked the duty cycle (with Fluke automotive meter) when engine cold (open loop) it is rock solid at 50% when warmed up (closed loop) it varies from 50 to 60 % I would guess the average is ~ 54% at idle. The idle is very smooth, still the duty cycle varies quite a bit. Do you think I need to change the O2 sensor? it is 2 years old or ~30 k mi.