Ok a few months ago I had OBX headers installed on my 2002 4runner. A few weeks later I noticed low rpm ping under slight load. I did full tuneup with with NGK plugs and wires. I seafoamed the intake and gas and even tried to put high octane gas. The ping was still there. Someone here said replace the upstream O2 sensor because even if there is no code it be bad . So I installed genuine Toyota sensor today
I removed the old one and noticed both nuts fairly loose and one was only hand tight. Not only was the sensor and gasket black but my installer cut a corner of sensor mount off along with trimming gasket. So I figured this was definitely my problem. I put in the new sensor but had to reuse old gasket because i thought sensor came with one. If the sensor gasket is leaking will it cause the pinging or effect the downstream sensor? That was replaced 4 years ago when it was throwing a code. Any info is appreciated. Thanks

Worth replacing the gasket. If it is sucking air by it I would think it would affect the way it runs.

I already ordered this afternoon from Toyota.

Air leak on the oxygen sensor = fuel issues.

I just ordered two Denso 234-4162's for my 2000 T4R from Amazon and they both came with a gasket. Is your new sensor from the dealer or another parts store?

New denso out of box from ebay for a fraction of price.

Ok so looking around engine i found a vacuum hose disconnected from air intake tube. Could the leak be causing my problems?

I would say definitely. Not the same vehicle but on my Dad's Ford Taurus he had a vacuum line off and it ran like crap and overfueled to the point the cat/con overheated and broke the exhaust union.
He was not the type to heed a warning light so this took some time.

Ok . Thanks. Before I found this i filled tank with 93 octane and Lucas octane boost. Knocking was gone and engine ran great.