I have a check engine light for my oxygen sensor, does anyone know where the downstream oxygen sensor is located on this car? I have looked in my owners manual and cant seem to find out its location, I have also looked on the forums and haven't been able to find anyone with a similar issue. I have the 2000 Volvo S40

 

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The sensor harnesses are located up on the firewall and then cable down to the exhaust/cat pipe. The specific OBD2 codes will indicate which sensor is bad. Note the front sensor measures o2 for fuel trim and the rear sensor measures cat efficiency. Both have a heating element which can go bad (and can even be subject to a blown fuse)

You may be able to see the front (upstream) sensor and reach from the top.

 

According to the code it's Bank 1 Sensor 2. So the downstream, just not able to find where it is even after looking under for it.

 

trace the wires - its probably on the top side of the exhaust after the catalytic converter. the two sensor systems have one sensor ahead of the cat to adjust fuel trim and use a second sensor after the cat to measure cat efficiency and "long term" trim