PLEASE HELP!!! COULD A FAULTY GAS/FUEL CAP COULD BE THE PROBLEM OR GO AHEAD AND REPLACE THE REAR O2 SENSOR DUE TO P0420 CODE AND HIGH MILEAGE?

2006 S40 MANUAL 5 speed with 176,000 miles

I am getting p0420 code (at Autozone) with my CEL going on and off (mostly on now) but have noticed a couple of times that it comes on after having filled up and closed the gas/FUEL cap. The light HAS come on after about a 3 mile distance after fill up. Could it be a bad gas cap? Is the cap supposed to be under pressure with a slight "woosh" sound when you open the gas cap? Mine does not and I remember in my lifetime cars gas caps hissed a bit when opened? WITH THIS CODE COULD IT BE A BAD FUEL/ GAS CAP?

I'm guessing downstream 02 sensor? If so ... several parts houses want to know which engine it has.. S7 or S4?

2006 Volvo S40 2.4I 2:2.4L 2435CC L5 FI VIN: B5244S7
2006 Volvo S40 2.4I 2:2.4L 2435CC L5 FI VIN: B5244S4

I also read you need a special tool to change the 02 sensors? Are there tools that will work in an average tool set? Am I chasing "rabbits" or am I on a logical track? I would like to fix this myself if it is a relatively easy fix. THANKS in advance!!

 

It definitely could be. Do you smell gas at all?