Sorry, I posted at the wrong place. My car has both leaking strut problem and SES light on. Will move the strut comment to the right place shortly.
On the SES light, turns out that the engine oil pressure was low, caused by a dangerously low engine oil level. Was told 5 quarts oil should be replaced with each oil change, and for mine, only 2.5 quarts old oil came out. That means it lost 2.5 to 3 quarts of oil over ~6,500 miles!! No leak at oil pan or anywhere else. Mechanic think the oil is seeping through the piston rings. I am still wandering why it could not be a leaking spark plug, or leaking at some rubber seals? If proven, the piston rings will be expensive ($3000 or even higher?) to fix as it requires opening up the engine. But I was told I can keep topping off ever 1,000 miles or so, if it turns out indeed I am burning that much. And this will not affect smog test, I was told.
Now the low oil was not warned by Check Engine Oil light, surprisingly. The mechanic says some Nissan cars have design flaw here - the sensor checks oil pressure at a location that is NOT the weakest in the whole flow. Was told low oil messed up timing, and/or creates frictions in the engine, triggering SES slight and those two emission related error codes. Whoever said on the internet that oil change has nothing to do with SES light is wrong.
I have always taken good care of the car - had service contract till 70K miles, and after that oil change every 3,750 miles, until this last time at 82K miles I changed to 6,500 miles interval. So if my engine does burn oil, I blame Nissan for its low quality.
Going forward -
1. 5,000 miles oil change interval, with synthetic oil.
2. Check oil level every 1,000 miles.