Hi,
I just received the dreaded engine light and the code was P0420 catalyst efficieny is low. With the light on the car runs fine, when cleared the car ran a little rough, but not bad. I changed the catalytic convertor last fall and put two new O2 sensors in, Can anyone think of something else that might bring on this light.

I have been told the cat might be plugged, I doubt it based on how well the car runs and the young age of the cat.

The cat is a two piece model that bolts together. It looks just like the OEM (not a catbak) except it is flanged between the manifold and the cat itself. That version was $200 instead of $400 and was not certified for California (I did not care...) I am thinking maybe the gasket is leaking or something else is leaking but the car is not louder at all.

I was told my rings could be leaking oil into the cylinders which will burn through and plug the cat but I would expect to be able to smell it or see smoke. While the oil is a little below full (1/2 to 1/4 quart low, I do not see strong evidence if anything burning or leaking)

I was told to check some vacum line along with the throttle. If the cat is plugged during this test the revved throttle will hesitate before returning to the low level. The details were very unclear and I lost his call so I could not verify it. Anyone heard of this?

Thanks for the help.
KR ('96 Civic DX w/177k miles, runs and looks great)

 

The only thing I can think of is thecat itself. Aftermarket cats don't last as long. I had an aftermarket cat go out in less than 1 year. I had replaced both O2 sensors as well, but it appears as though Hondas are very sensitive about those aftermarket components. The aftermarket O2 sensors were slightly off, and making the car run just a little rich. This in turn burned the cat. The car ran fine, but it failed emission. I ended up putting the OEM stuff back on(the original ones that came on my civic and already had 140K miles) and it passed emissions. So bottom line is, OEM is always better when it comes to smog stuff. Now I live in cali so I need to pass those strick emissions. If you don't and just want to clear the P0420, look into the O2 simulators.

 

ive got a code 420 too----

 

I would check the O2 sensor readings. If Sensor 1 and Sensor two are giving similar readings (both rich i believe), then it is very possible that the cat is bad or clogged. But if the cat was clogged, the engine should stop working after pressure builds in the exhaust and forces exhaust back into the cylinders.

 

O2 sensors oscillate from 0V to 1V at a specific rate. The second one should oscillate much less and stay closer to 450 mV. If they both oscillate at the same rate and to the same extremes, the Cat has burned through(or the catalyst material no longer does its thing). This is kind of like it was not there(inefficient). The other way a cat can fail is it becomes clogged.

A clogged Cat doesn’t necessarily stop the engine, as it could be restrictive, but not completely clogged. Most of the time people will experience a loss in low end power, hard to start, and bad gas mileage, not a completely dead engine. To check for a clogged you can do a vacuum check on the intake manifold pressure as follows: attach a vacuum gauge to an intake manifold source(such as brakes or pcv hose). Allow the engine to reach operating temperatures. Gradually raise the engine speed to about 2000 RPM. Note the vacuum should read about the same as it did at idle. Then quickly release the throttle. The vacuum should momentarily rise then smoothly drop back down to normal. Should the vacuum rise slowly or does not quickly return to normal levels then the exhaust may be restricted.

 

Thanks for the info, I will check all this out. I was wondering, since the cat/manifold is a bolt together component, what if the bolted joint leaked, from a broken bolt or something. Could that trigger the light? The car is running fine and the mileage is typical 33-34MPG. I leak might mess up the reading a bit too eh? I am hoping I do not have to put another cat on this car for $400+. Maybe there is a warranty...

Thanks

 

I have the same problem in my 97. In the spring im going to put on a aftermarket header and a high flow cat to see if it corrects the problem, if not im probably going to run a simulator for that second 02 sensor. PM Chimera, he got a simulator for the o2 sensor that reads off the cat and he doesnt throw a CEL anymore.

 

yeah that could cause it too, but I personally don't think so. If your car is running fine, then just look for an O2 simulator. I have a feeling your car is just sensitive to the aftermarket Cat. Maybe it is a high flow cat and those normally trigger the CEL.

 

Spend 70 bucks on an O2 sim. I had this problem for a while, tried replacing the O2 sensors, the cat itself is fine, got an O2 sim from casperelectronics.com works great, plug n play.