Check engine light on code comes up po420 what is the problem

catalyst efficiency below threshold bank 1 is a PO420. But i would check the oxygen sensor out first that also will cause a check engine light .A cheap one can fail but to check yours take it out and heat the end with a torch with and check it with a voltmeter. I would google that first if your not familiar with that method.Some vehicals take about 30-60 miles before the code will pop back up but with newer vehicals it will pop the code right away.

Go to google & type in ---OBD2 Codes
1998 honda accord PO420 etc

Use silicone grease on the boots.

P0420=Catalyst System Efficiency Below Threshold Bank 1,
The air filter is not the cause here,this is just a code saying your converter is not doing its best,

In almost all cases, a P0420 code indicates a problem with 1 or more O2 sensors. They send incorrect emissions information back to the ECM and trigger the Check Engine Light. If the Catalytic Converter is fine (no problems with the engine hesitating or getting a lot of heat from under the car) then it's most likely the O2's.

If the code has cleared on its own then you need to disconnect the battery and re-connect it. This will clear the codes out of the memory. If you don't want to do this you can go to an auto parts strore and have them hook the little computer up to your car and they can clear out the code as well.

P0302-cylinder #2 misfire detected
P0420- catalyst efficiency below threshold bank 1
P0340-No cam signal at PCM.
The Cam sensor most likely needs to be replaced first. Clear the codes after that and drive and see what comes back on.

P0420 is catalyst system efficiency below threshold (bank 1)

P0440 is evaporative emission control system malfunction

P0441 is evaporative emission control system incorrect purge flow

Replace the catalytic converter.

most of the time its the sensor that cause this,id try replacing the sensor on it,that usually fixes that code,but you have to get the code cleared off with a scanner ,and then re-scan it to make sure it doesn't come up again,good luck with it.