Need help with Jazz CEL light - P0420 error(SOLVED)

[cavosavosk]

Hello!
Im new to this forum and looking for help.

I got my Jazz (2003 1.2i-dsi) for some time now and Tech. Inspection is coming up, so i need to get it ready for it. And yea, check engine light is on and on OBD it shows error p0420

to understand better, i need to write out what i've done so far:
I bought the car at the end of June and I was a bit of an idiot and missed that someone taped out CEL and ABS light (from behind the dash, yeah...). After getting it scanned there was EGR valve flow error and some ABS short error. Not a big deal, knew about the EGR shizzle and went to clean it.
Well some jackass thought something else and massacred the original EGR valve by angle grinder (yes you hear correctly) and then installed a blocking plate. It took me 2 tries and a LOT of effort to clean all canals for the egr valve to function properly, not to mention buying a new used valve. God damn, even the canal going thru the head was absolutely clogged.

EGR error vanished and car runs great. Even fuel consumption improved a bit. But since then p0420 error popped up. I tried cleaning it with scanner, didnt help. We poured some catalytic cleaner additive into the tank (right after cleaning the EGR), didnt help either. I dont drive it like a granny, i rev the Jazz up a lot. But still the error pops up after 15-20km and writes itself into the memory.

I got a recommendation that the first O2 sensor may be a bit off its range, causing the fault code. God knows how long was the EGR blocked off and what damage it could have done.

Im a student on a not so great budget, so i need to solve the issue in a smart manner and without brainlessly throwing parts at it. Any help is welcomed.
Thanks

[cavosavosk]

Some info i forgot:
Oil, filters and sparkplugs (All 8 ) were changed right after the purchase. I checked the plugs regularly, they are holding up great.
Valves were adjusted aswell after purchase.
That was done at around 244000km.

Now it has 252000km, so around 8000km difference. The oil level is not dropping, at some point the engine had piston rings or valve seals done.

[jasonevans]

Sometimes this error is caused by an aftermarket Cat.
Otherwise it's normally the 02 Sensor.

[cavosavosk]

as far as I saw, the CAT is still original. The car comes from Italy, and was brought to my country (Slovakia) at  approximately 130-140k km. Could it be really on its way out? We had a Jazz with similar mileage and same specs and never had such issues.

Regarding O2 Sensors, I can run some measurments through simple OBD diagnostics, but really don't know what to look for. Either way, the upper O2 sensor is going to get replaced (we cannot get it off easily to inspect, so its gonna get removed "destructively") since I got recommendations that it might cause issues. Sent my dad to order a replacement one at local car parts dealer coz I'm not home and will definetly update about it at the end of the week.

Also mentioned the fuel consumption was improved, but didn't mention any figures. Before EGR cleanup I was getting 5.8-6.2L/100km (45-48 British MPG) and after the EGR tackled I dropped to 5.2-5.5 (51-54). These values are taken from trip meter (not calculating between fill-ups) and generally 70-80 percent of the time the Jazz is occupied with 4 ppl with luggage with almost no highways (max 90-105kph)

[guest334]

Avoid internet cheap copy parts, find a local car parts shop which can supply Denso or Febi 02 sensors.

[cavosavosk]

Further update from my side.
After more diagnostics and swapped out O2 sensor, we witnessed improvement on the values coming from the sensor.

Buuuut the catalytic convertor was dead. It was not able to hold the fumes and from what we seen on the sensors, it was like there was no action there in higher revs. Sourced a replacement and swapped it out with my dad and now its smooth sailing. (If anyone wants to know our steps in diagnostics, drop a message below and i can write out what i did)