I have a 2008 Chevy suburban 1500 5.3 four-wheel-drive eighth digit vin 3 had motor rebuilt but guy put in bent thermostat which I thought ruined my brand new 02 sensors because I kept getting p0420 code which is bank one so I changed up and downstream sensors then I got p0430 which is the other side and now I’m getting p0420 again and I’m not sure what to do. Also I had brand new cat put in before engine rebuild and he put in new plugs. I fixed thermostat and put in new temp sensor

 

any 02 sensor codes?
did you put in a weld in cat?

 

Yes I had p0420 so I changed bank 1 up and downstream sensors for bank 1. I drove it the next day after erasing code and it was fine all day. The next morning I got P0430 which is bank two while idling in dunkin drive through so I changed up and downstream on bank 2 side. Erased code and same thing drove for a day no issues but now check engine back on saying p0420. The cat was from rockauto it has welds on it but it bolts on. The cat was put in before I had engine rebuilt so I’m wondering if guy who rebuilt didn’t put a gasket back or something but I can’t tell

 

so it was a bolt on y pipe. cheap cats will reset the low efficiency dtc due to low cerium content.
if you had no gaskets you should hear an ex h uast leak

if the 02 are switching normally... and you have no 02 codes, heater fuses ok..... and no ex haust leaks... you may have a poor q uality cat. post a link to what you bought

 

https://www.rockauto.com/en/catalog/...converter,5808
isn’t P0420 an 02 code? Heater fuse? My heat/ ac has also been intermittently working off and on.

 

 

po420/p0430 are low oxygen storing efficiency codes for the cat. they are not 02 sensor codes. the02 sensor are used to determine if the cat is working. the pcm compares 02 cross counts of the f ront and rear 02s. the front should switch fast...the rear slow is redox reactions are occurring. W hen the o2 swith at the same rate the dt c sets.

the o2 sensors have heater elements to warm them up ...they only send signal when hot. this is nothing to do with the hvac system.

looks like a cheap aluminized ypipe...a good ypipe cost $400+in canada. look at the improved design one on the rockauto site. can you return yours for that one?

 

So if the downstream sensors are switching slower than upstream than my cat is working properly? Possibly I’ll check it out but I had to pay 190$ in shipping

 

Check the underhood label. If it has CA (California) emissions then federal cats will code out low efficiency after a while. My brother went through that when he had a shop replace the cats on his Impala.
Just because you may not be in CA doesn't mean you don't have a CA emissions vehicle. A lot of states require CA emissions on vehicles sold in that state, NJ and NY do just for an example.