I have a 1998 Honda Civic with a check engine light that is continuously on. The code reads 420 for catalyst efficiency. I have had the front and rear O2 sensors replaced and the light remains on. Every time the system gets reset and the light gets turned off, it comes back on within 2-8 miles or so, or before the car can complete a "cycle". It is a nightmare to inspect because in Pennsylvania cars will not pass with a check engine light on, and the process of inspecting it involves, the light being turned off so I can drive the car a little hoping to get it through the cycle so it will pass on the machine. If you turn the light off and put it right on the machine it will fail, and if I'm lucky enough to be able to drive the car for a couple of minutes without the light coming back on, I bring it back to the garage to have it tested and it will still usually fail four or five times before it finally passes. I've dreaded this process, and the amount of money I've spent in addition to replacing O2 sensors that may have been fine. I have been putting my inspection off for months past its expiration for the past two years and have been ticketed for expired inspection both years. I would like to solve this problem once and for all and have the situation permanently resolved.

Have you considered replacing the catalytic converter?

Hi, this is Macksmom's husband... I have been a mechanic for many years... and got bored and signed in under my wife's account :)

Most manufacturers will cover catalysts up to 100K miles. If you've had the O2's replaced, it is either the catalytic converter, an exhaust leak, or a PCM (which may also be covered under warranty).

Have you had any driveability issues? If so, it is most certainly that your catalytic converter needs replaced. If you are not have any driveability issues it would lead me to believe it is a PCM or wiring problem.

Unfortunately, with codes of this nature you need to see what the sensors are actually doing which would require a scan tool.