P0420 Code Nissan Primera

I have a Nissan Primera (2001). The Malfunction Indicator (Check engine) Light is on from some time. I got it diagnosed and got the following code. P0420 - Catalyst System Efficiency Below Threshold (Bank 1) .

The mechanic suggested to replace the O2 sensor first, before going to change the Catalytic Convertor. But even after changing the sensor the problem didn't go. Now he is suggesting to change the Cat. The problem is there are 2 Cats in Nissan Primera. He is not sure which one to be changed, he is suggesting to change both. Is the above OBD2 code suggests that which of the CAT could be faulty. Changing 2 Cats is costing hell so I just want to change one of the Cat that is causing the issue.

Is there anything else which I should look into particular?

Thanks

1st thing you should change is your mechanic then do the following:

If car was not giving you any trouble just light was bothering you then
you should have Erase ECU DTC code you can do by using diagnostic tool, Your light would have gone

2nd Check your MAF sensor some time if MAF sensor is disturb for any reason its upsets O2 Sensor

Things to check on your MAF sensor there should be small diode is it present if it is then try to clean your MAF sensor with CARB cleaner or Brake cleaner, is there any leek on molding which is holding sensor inside MAF body, if there is a leek then I will suggest change your MAF sensor


you may find MAF sensor for between £40 to £60 from any Salvage yard

Please check my other posts and let me know if you could suggest any of the issues I am facing with my Nissan Primera SE Plus 2000

Thanks for the reply. My mechanic did erase the code but the light came back again very next day.
Actually I am not having any other issues with the car.
I did like the idea of checking the MAF sensor, but the trouble is I am a novice and don't much about DIY on cars. Can you suggest me something on how and where to find the MAF sensor and then I can do something about it. Is there a site that tells about the internals of the electrics and mechanics of nissan primera.
Any help is much appreciated as even my car is due for MOT and I don't want it to fail for this reason.

I did check your other post but I am afraid I don't know much about it.