I need to replace the oxygen sensor that is against the firewall?

Which one? I think there are three of them

Cosmo. Mazda tech

p.S. Are you sure its a 2.5 and not a 2.3 miller cycle?

Hi,

There are a total of three sensors on this vehicle. If the sensors are under the hood and near where the exhaust manifold and downpipe attach, those two will be upstream. There is a third one further back on the exhaust.

I attached two pics below. In the first one, I highlighted the catalytic converter as well as the sensors. In pic 2 I identified the three as front, middle, and rear. That is how they are identified. The front and middle are upstream.

I hope this helps. Let me know if you have other questions.

Take care,

joe

See pics below.