I own a 1999 Honda Civic Ex. My check engine light came on a couple of months ago. I have cleared it a few times and it keeps coming back after about a day. I took the car to the shop and they hooked it up to the computer. The fault is Catalyst system Efficiency below Threshold Malfunctioning or defective catalyst
system. I have noticed that when i clear the fault my car idles really low around 100-200. In fact it has died a couple of times on me. Also the car has about 92,000 miles on it. Where should I start looking?

 

Two things that will cause that code:
#1 a failing catalytic converter.
#2-a failing secondary oxygen sensor.
And after clearing codes you should let the car idle for about 10 minutes to allow the ECU to "learn" the idle.

 

Definitely check your O2 sensor... that will certainly cause a poor idle if it's bad and it's also responsible for destroying most cat convertors when they go as well. Change it soon before it's to late and you end up having to replace the cat!

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 00Red_SiR &raquo;</TD></TR><TR><TD CLASS="quote">Definitely check your O2 sensor... that will certainly cause a poor idle if it's bad and it's also responsible for destroying most cat convertors when they go as well. Change it soon before it's to late and you end up having to replace the cat!</TD></TR></TABLE>

This is false. The O2 sensor that would be causing this code is the secondary O2, not the primary. A faulty secondary O2 will not cause a poor idle, nor will it in any way "destroy" a cat. The information in b19coupe's above post was completely correct.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Padawan &raquo;</TD></TR><TR><TD CLASS="quote">

This is false. The O2 sensor that would be causing this code is the secondary O2, not the primary. A faulty secondary O2 will not cause a poor idle, nor will it in any way "destroy" a cat. The information in b19coupe's above post was completely correct. </TD></TR></TABLE>

This is correct, I missed in the post where it said it was the secondary O2 and not the primary O2 causing the problem. What I said would be more in ref to the primary O2 being faulty.

 

So what would you replace first? I know that the cat comes off really easy. Should I just replace both or one at a time? The secondary O2 sensor is the one contected directly to the cat right? Thanks for all the help.

 

Yes that's correct it's the one connected to the cat but I've heard that one is pretty expensive to replace. Generally speaking I would say that if you haven't really had a problem with your primary O2 before and your car's exhaust sounds normal and not like an air hose with a leak (indicating your cat is clogged) while accelerating. I would replace the secondary O2 as it is probably failing from the sounds of it.

 

Ok I will try that. I was talking to a couple of guys at work and they said that if it was the sensor that the fault code would list the sensor and not the catalyst sytem. I will noticie today what it sounds like. Thanks again.

 

You can also try switching the primary and secondary sensors. If it still throws the same code, your problem is probably the cat. If it throws a different code (related to the primary O2), then it's most likely your sensor.

 

i also had the same code on 2nd o2 sensor it was because there was an exhaust leak at the cat.