I've been taking the T/A out of hibernation lately and one of the things I'm noticing is that my bank 2 O2 sensor will lean out intermittently. If I'm at idle it will cause idle to drop slightly and run rough. Every time it happens, the fans kick on, but I've only gotten a SES light once with a code 64 (bank 2 lean). So far that is the only code I have. My ideas are the O2 sensor is tweaking on me but it only has 20K miles on it. Maybe a problem with fuel delivery? Last summer I was having major problems with the fuel pump simply stopping in high heat and I haven't replaced it yet. What do you guys think?

 

Could be the O2 sensor, but check the fuel pressure just for the heck of it. DTC 64 sets in closed loop when the PCM sees a very lean reading - less than 200mV for at least 50 seconds. If it was leaning out in power enrichment (PE) mode, you would get DTC 55.

Could also be an injector shutting down or any other extended cause of a misfire. All that air from a misfiring cylinder is going to cause the O2 sensor to show lean. The PCM should react by adding a lot of extra fuel. So its hard to see a 200mV lean condition existing for 50 seconds.

 

Could also be something simple like a bad plug wire. That will cause a cylinder to mis-fire which allows a lot of air to hit the o2 causing it to spike lean.