My friend’s RSX was getting OBD codes P0137 , P0138 and P0139. All three codes are for the O2 sensor (Bank 1, Sensor 2) but there are two sensors for the RSX. It is a bit confusing to determine which sensor needs to be replaced and which sensor is upstream or downstream.

This webpage from Denso explains the banks and sensors. Click the Performance tab. With a straight four engine, there is only one bank on the RSX. Sensor 1 is upstream and located before the catalytic converter and also called the Air Fuel Ratio Sensor. Sensor 2 is downstream and after the catalytic converter.

Honda OEM sensors are made by Denso so it would be cheaper to get a Denso sensor. Searching the part number on Denso yields the following part numbers.

234-9004 upstream sensor (non Type S, manual) – Acura part 36531-PND-A01
234-4122 downstream sensor (non Type S) – Acura part 36532-PND-A01

You can find these parts on Amazon both US and Canada.

The service manual describes sensor 1 as primary and sensor 2 as secondary.