I have 2 error codes P0420 and P0430 which are catalyst or O2 sensor problems.
Some threads have said that replacing all 4 sensors will solve the problem or at least start with upstream one.
But my mechanic said that usually it means broken catalyst anyway like no point in changing just sensors.
So what are your thoughts on that? As I see many have resolved the issue with just changing the sensor.

Also, when I clear the code, it reapear after approx 5-6 days.
So any suggestions?

 

Before running out and replacing the cats at presumably no small cost…. I’d pull the cover off of your exhaust manifolds and check for cracked manifolds. I’ve effectively replaced all 4 O2 sensors on my car and both of my manifolds were cracked which is a common issue. As I didn’t want to make a huge project out of removing them at the time, I used some cast iron repair goop (kind of like JB Weld) to try and seal them. It’s not 100%…. But I get the same codes about every month or so and clear them and just deal with it.

From what I’ve gathered on this site… the early OBDII diagnostics on these cars are a bit temperamental. But at this point my car runs well and pulls pretty good so I’m living with it.

 

There are some free things you can do to improve your O2 sensor readings before the big bucks of cats or sensor change

The cracks are a good point as well as the muffler seal for exhaust manifolds

Interesting you have 4 O2 sensors on a European model , do you see 2 sensors at the very front of the downpipes as you look from above in the engine compartment , swaping option as the forward or aft 2 can be acting up

Gotta go to dyalisis now

Starting with fuse # 10 , 12 , 14 , or 16 right engine bay fuse box , one of these fuses are the heaters for all O2 sensors inside the sensors to heat up so they read properly

 

Other than cracks in the exhaust manifold the donut gasket between the downpipe and casting can get loose , 15 mm medium depth with long extension with universal from under the car , there is one that has low clearance against the pipes and by slightly inserted socket square end you can get your 15 degree clearance

There are 2 small ground studs on the rear engine firewall that protect the sensitive O2 sensor's signal

The O2 sensor connectors can get corrosion in them , there is a mounting position map as the connectors can get swapped and not know it

The O2 sensors should have a crush washer as this effects signal from reading someone else , mine were missing

Pins cleaning of the red ECU connector