Potential cuases of ej de check engine lights once warned up?

Hey fellas sorry for asking so many questions lately, ive recently pulled my engine and done a manual swap and im almost certain ive installed everything correctly however i have been getting a check engine light usually once the car is warmed up and will start backfiring on deceleration or when i let off throttle only one pop however the volume will vary on 2-5 minute drives its pretty quite backfiring however the longer the drive the louder the single backfire will get on deceleration. I decided to change the Pre Cat o2 sensor as i noticed when i pulled out the old one it was chalky light grey, while i was changing it to a new one i noticed the thread was rusted on the headers but still screws in just spins on the spot once it gets to a certain torque (still too loose for a o2 sensor) so maybe it has a poor seal and is getting bad / false readings causing the engine to run rich? On top of that there has been two occasions of parking the car after a 10 ish minute drive and once returning to the car it will start then die immedietly after 1 second of idle, all i did was wait 5 minutes in the car and it would start up again perfectly fine. i also notice rough idling when started sometimes.

any ideas on potential causes would be appreciated cheers

While I don’t have a definitive answer the 02 sensor not sealing properly could cause incorrect readings. Not sure it it would work but an idea may be to try the metal epoxy stuff you can buy and knead a very small amount and put it around the thread and see if you can then put it in while it is still plyable. If anything it might help make that seal even if it doesnt get as tight or it may not. Just an idea.
Also check you don’t have any other exhaust leaks further down as that may cause the poping you are getting. Check the condition of your muffler’s too.

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used plumbing tape ahahaha and it acctaully worked, not very worried about the dodgy fix as im getting new headers soon anyway.

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