Oxygen Sensor Location

B1S1 is on the exhaust pipe upstream from the catalytic converter. The other is on the C/Converter. Here is a guide t help walk you through the repair and the location of the oxygens sensors in the diagrams below.

https://www.2carpros.com/articles/how-to-replace-an-oxygen-sensor

Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.

Hi davegoya, It is the primary sensor.

I have replaced the original oxygen sensors, Denso, with non-OE sensors, Bosch. I cannot clear the CE nor the VTM-4 lights. I also cannot engage the VTM-4 system. On computer analysis, the DTC still indicate the O2 sensors. As an electrical engineer, I am somewhat confident I installed them correctly. The MDX runs fine with no apparent problems. I am confused. Please help.

check to make sure that the correct sensors were installed in the right order. the primary is an air/fuel sensor and the secondary is the heated oxygen sensor.two different sensors that are not interchangeable.

They both are 4-wire sensors. Two wires in each are listed as a heater circuit. I have rechecked the color codes and signal wires between the OEM and bosch wire color coding. I have cleared the VTM light but not the CEL. I'm now getting P0135, P0141 and P0133, indicating oxygen sensor heater circuit. The MDX still runs fine. I am perplexed.