I have a 98 civic dx. Over the last 2 years iv been having a war with the p0420 code. The first time i got the code i replaced the cat with an oem. i did so because i noticed a crack on the header. A few monthes later it came back i replaced the lower co2 sensor. the code came back a short time later. i replaced the upper sensor. once again the code came back so i took the car to a tech he said it was my cat because he checked the o2 sensors. so i buy a SECOND cat. the code is back and before i waist anymore more money on parts or techs i want to do a lil research. also i told my dreadful story to a couple guys i got all sorts weird advice. such as a clugged muffler, bad aftermarket air intake, bad ecu.
please help thanks

 

Your cat is killed because the engine runs too rich. Unburnt fuel burns in the cat and destroys it.

Figure out what is causing the engine to run rich, fix it, and THEN replace the cat.

Any other CEL codes?

Is the thermostat stuck open?

is the ECT sensor bad?

Is the ignition timing off?

Is the fuel pressure too high?

Are the plugs, wires, cap, and rotor old?

 

also check for vacuum leaks on the intake. This can actually cause a few different o2 sensor codes depending on the severity and location of the leak.

 

A vacuum leak could cause a rich or lean code, but I doubt it'd cause a P0420. A P0420 is "Catalyst Efficieny below threshold"; basically, the computer looks at the primary O2 voltage and compares it to the secondary O2's voltage, using the primary sensor as a reference. A vacuum leak would only affect the primary 02.

 

A vacuum leak would not generally cause the engine to run rich, though an exhaust leak in the header or exhaust pipe near the primary O2 sensor would. I forgot to add that to my original list.