Hello Volvo Forums,
I am looking for some help with the air pump on my 850 Non-Turbo. Or should i say "what i think is the air pump".

There's a constant whining from the engine compartment, and it's coming from a small pump plainly visible under the hood. I think it pumps air from the intake into the exhaust.

Tonight I'm replacing the front O2 sensor for my check engine code (2-1-2). Wondering if this will stop the whining, or if they are separate issues.

Would be helpful to understand the purpose of the air-pump in order to figure out why it's running non-stop. What type of sensor controls it?

(if it stops with the new O2 sensor that would be great! but still curious about the air pump's functionality, etc and gathering info "in-case")

thanks,
Ben