Are the front and back oxygen sensors in a 1999 Volvo S70 the same?

My 1999 Volvo S70 threw the code P0420, Catalyst Pressure Below Threshold or something along those lines. I just replaced my catalytic converter last july, and replaced the air filter not long after the code was thrown. The guy at Autozone said this probably means I need to replace the 02 sensor. I was wondering if the front and rear sensors are identical, or if they are two different parts.|||Call the guy at Azone, or go see them...have them look it up again, and they'll tell you...could be either way.