Ok, got a 98 ES300 with about 138k.

CEL with code P0135, Bank 1 Sens 1.

Which is Bank 1? Front or rear as facing the engine bay from the front of the car?

Also, I want to go back with OEM, who has the best pricing? Sewel or are there other vendors here?

Thanks

I think the oem denso sensor is around $109. A bosch one is around $36.

Rear, behind the engine.

Bank 1 (B1) refers to the bank that includes cylinder No. 1.

^ Bank 2 (B2) refers to the bank opposite bank 1.

^ Sensor 1 (S1) refers to the sensor that is located before the catalytic converters.

^ Sensor 2 (S2) refers to the sensor that is located after the catalytic converters.

That's an excellent diagram JSM1284.

I will add a photo of my rear sensor (similar 1MZFE engine to that of the ES).