P0420 please help!

I just got my oil changed about 2 weeks ago and it was overdue. My light just came on this week. Someone told me that it could be my 02 sensor, they said that the oil residue from the overdue change could have messed up one of my 02 sensors. Is this feasible, or was he just shooting me a line? How much could this cost me if it is not covered?

Thank you for the information

Hi guys I thought you might be able to help me out. I'm getting a p0420 on a 2007 chevy malibu 2.2 liter I replaced the cat with a Dorman and erased the code after about a few miles it went back on so I put another one in reset it and same issue replaced upstream sensor still the same downstream seems fine. Maf sensor seems fine. I checked the data and I noticed that the code was set in open loop? So I reset it again and watched the data and I noticed that when it switched to open loop at idle speed the downstream o2 sensor started jumping up and down and it set the code but in closed loop its steady at around.800a bit high but steady also the code was always set in idle I also have an evap leak but that also shouldn't matter heres the freeze data what's going on!
Abslt tps  22.7%
Eng speed 789
Baro prs hg 29.8
Calc load. 38.4
Maf lb/m 0.64
Map hg 13.0
Out temp 77°
Coolant 198°
IAT 88°
Ign adv 12.0°
St ftrm 0.0%
Lt ftrm 5.5%
Veh speed 0
Abs load 28.2%
Fuel system open
Rel tps 8.6%
Thot cmd 12.9%
Abs tps 22.7%
Acc pos D 17.6%
Acc pos E 8.6%
Cat temp 1258°
Evap purge 0.0%
Cmd eq rat.869

I would greatly appreciate your help thanks!

Thanks for the response. The downstream sensor seems to be steady it's high but steady regarding misfire there's no misfire count as for an exhaust leak all the gaskets are new. You mentioned a leaky fuel injector, can that set a p0420 code? If it can that would make sense I think with the downstream sensor reading high from a rich condition but what I still don't get is how the code was set in open loop and why the downstream sensor fluctuated in open loop at idoe when it set the code? Thanks guys I really appreciate your help!
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I keep getting this code I have replaced my catalytic converter and both o2 sensors. My coolant bottle is leaking around the seals so my mechanic is going to replace it and check the pcm to see if it needs to be replaced so what else could it be?

I have already replaced the catalytic converter and I still have the same code coming up with the check engine light. What could the problem be?

The cat was replaced by meineke. I'm not sure if they use OEM, I don't think so though.

So it is possible that everything is working fine and the code is just coming up because it's not a GM part? So to be safe I would have to check everything else anyway just to make sure?

The light was off for about a week after the cat was replaced. Then it came on and I scanned it with my Actron PocketScan Plus OBD II Code Reader. It came up with P0420, P0420pd. I brought the car back to meinek and they scanned it with their Code reader and said it was probably the O2 Sensor. He erased the code to see if it would come back and it has.

Well, when I went to meineke and told them the code I was getting, they lifted the car up, looked at the cat and said the cat needs to be changed. They plugged their code reader and got the same code and still said to change the cat. They didn't test or thoroughly check anything.