good read about o2 sensor troubleshooting
 

Using fuel trims and downstream O2s to analyze intermittent misfire concerns | Search Autoparts


Keep this tool in your memory toolbox for the next intermittent misfire you’re faced with:
• A 15 percent or less change in trim and a downstream O2 above 650mV points to an ignition concern

• A 15 percent or more change in trim and a downstream O2 under 350mV points to a lack of fuel concern