99 Integra
 

I happen to purchase a 99 integra its an LS automatic. It is bone stock as all hell. After driving for a few months. I had a check engine light pop on me. I had it scanned. And I have 2 codes. P0420 catalyst system efficiency below threshold bank 1 probable cause:
A/F ratio concern
Air leak near HO2S (heated oxygen sensor) Bank 1
Ignition system concern
Faulty catalytic converter
And code P0430 which is the same as above but bank 2
It has started to waste gas now.

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tune up time. cap. rotor. plugs. wires. make sure to use oem honda or ngk

change the o2 sensors. reset the ecu and drive it

if you have access to reading the live data on the vehicle i would start there first before replacing things aimlessly, fuel trim readings specifically if the lt/st fuel trim are close to where they should be i would suspect a bad catalytic converter. on the other hand if they are not i would start looking into tune type repairs as mentioned above by cbaracing. but a car that age wouldnt be out of the ordinary to see a converter go bad. specially if original. also if you do have access to a scanner test 02 sensors for correct operation.