Check engine light turns on for a few days and then turns off for weeks/months? Code P0420 on a 2004 Honda Civic Hybrid?

Over the last year my check engine light has come on about 3 or 4 times for a few days at a time and then turns off again for a very long period of time. I know the code means my catalytic converter is below threshold but why would it turn off? Could it be something else? Should I look into replacing it or wait it out? Any advice is appreciated!

Look at this site - shows you what caused the code and what to do for repairs

Cat converter is failing. See a mechanic and check what the O2 sensors are doing.

GEEZ. Take it in to a mechanic.

If that's really all it is, you won't have to worry about it until emissions testing in required. Then it could be really expensive to fix. So you might as well take it to a mechanic now.

A P0420 code means you need to replace the converter for Bank 1.
Better have a mechanic read the engine data to make sure all the sensors are working with in spec and check the Short and Long Trim fuel readings.to make sure the motor isn't running too rich or it will ruin the new converter.

P0420 - Catalyst System Efficiency Below Threshold [bank 1] - is produced by the ECU when the output of the downstream O2 sensor reports a high and fluctuating level of free oxygen in the exhaust after the catalytic converter. Generally, it can be caused by poisoning or erosion of the cerium in the catalytic converter… But I have never seen that happen in my (admittedly limited) experience. Much more often it is the result of an exhaust leak ahead of the converter. It can also be the result of mixture control problems.

Since yours is clear much more often than it is in, I feel safe in saying it is not the converter doing it. There isn't even any sane explanation for the mechanism for that behavior in a converter. In the same way, being clear more than on, my attention focuses on mixture control rather than exhaust. Exhaust leaks tend to get worse, fairly rapidly.

Start with conservative treatment. If you have been using random gasoline try using only top tier brands (see the source). That was all it took for my daughter's Sienna. Top tier gas cleans up the injectors about as quickly as injector cleaner does. If the code keeps coming back, the next stop is a muffler shop to have the exhaust checked for leaks. The last resort is converter replacement, which typically helps until the underlying condition gets worse.

That condition will NOT leave you stranded by the side of the road.

The computer will set code P0420 when the difference between the oxygen sensor readings before and after the catalytic converter falls outside a predetermined range. The cause might be a failure in the catalytic converter, or a failure in one or both of the oxygen sensors.

I've sometimes seen this code set, and clear on it's own like that. (Once, it was after driving in a hard rainstorm: I wondered if one of the oxygen sensors had gotten wet.)

You might want to find out if there are any service bulletins outstanding on your car. Sometimes, the manufacturer will issue a software patch that changes the threshold of sensor readings before they set a code. Otherwise, as long as it keeps going away after a bit, you should be able to ignore this.

But maybe save up some money so you'll be ready if it fails completely.

Lookit

The Catalyst is weak. It failed when the computer ran it's programmed test of it. Then the next three times it ran the test it passed so the light went back off, it's designed that way. This will continue until it finally doesn't pass the test anymore and light just stays on. I don't work on the hybrid but most Civics around that year are the 1.7 engine. They ALL crack their exhaust manifold. The crack lets in oxygen and that makes the reading from the oxygen sensor wrong which can set false catalyst codes but the repair is to replace the manifold which has the catalyst inside of it anyways so it gets renewed. If you hear exhaust leak tick noise that may be related. With this condition it could effect mileage and performance. Otherwise this code is not urgent, you won't break down because of it. The car's emissions will be higher that designed however.