2001 Pathfinder LE Error code question

[drick132]

My SES light just turned on and found out it's a P0161 code. My manual says it's "Post-converter oxygen sensor heater or circuit fault." Which O2 sensor is this referring to?

Also, I just had my belts replaced. Now I hear a loud squealing noise when I start my car sometimes. Does anyone know what's causing this noise? Does the tension need to be checked? I'm taking it into the dealer tomorrow, but I'm just wondering what's creating the noise.

TIA for any info!

[amb4081]

After replacement, the belts will stretch a little and need retensioning. The belts have probably just strecthed a little bit.