99 Civic- check engine light on - OBD 2 code > PO420 "Catalyst efficiency below threshold bank 1 question

[Pawl]

139,000 miles currently on car. At 100,529 mi I changed the exhaust manifold because it was cracked where O2 sensor was threaded in. I believe the nu exhaust manifold I purchased came with a new catalytic convertor attached to it when I bought it. I think thats the only way they come. I paid $256 for it 08/19/09.
On 09/22/09 with 101,136 mi I installed a new Primary O2 sensor right there where you can see it when you open the hood. So I am going to take a wild guess and say the rear or secondary oxygen sensor might need replacing. I have no record in my log for this car of ever replacing it so this original secondary sensor currently has 139,000 mi on it.
In the Haynes Manual it says the PCM uses the secondary(rear)oxygen sensor signal to monitor the efficiency of the catalytic converter. The OBD 2 says the "Catalyst efficiency below threshold (Bank 1)". Doesn't that sound like the secondary sensor could be bad? Any thoughts?

[Pawl]

139,000 miles currently on car. At 100,529 mi I changed the exhaust manifold because it was cracked where O2 sensor was threaded in. I believe the nu exhaust manifold I purchased came with a new catalytic convertor attached to it when I bought it. I think thats the only way they come. I paid $256 for it 08/19/09.
On 09/22/09 with 101,136 mi I installed a new Primary O2 sensor right there where you can see it when you open the hood. So I am going to take a wild guess and say the rear or secondary oxygen sensor might need replacing. I have no record in my log for this car of ever replacing it so this original secondary sensor currently has 139,000 mi on it.
In the Haynes Manual it says the PCM uses the secondary(rear)oxygen sensor signal to monitor the efficiency of the catalytic converter. The OBD 2 says the "Catalyst efficiency below threshold (Bank 1)". Doesn't that sound like the secondary sensor could be bad? Any thoughts?

[Greenblurr93]

It sounds like the catalytic convertor isn't preforming efficiently like it should...

the rule of thumb is 'a cat doesn't dies on its own, its murdered'

did you ever find out why the first cat failed? chances are the same thing happened to this cat, and you'll keep putting cats in it until its fixed.

[Pawl]

The reason the 1st catalytic convertor was replaced is because the original Honda exhaust manifold had a crack in it where the Primary sensor was threading into the exhaust manifold. I believe the catalytic convertor on the original Honda exhaust manifold was permanently attached to the exhaust manifold so I could not reuse it on my new exhaust manifold.
The new exhaust manifold had a catalytic convertor that came together as a pair. So I dont believe the catalytic converter on the original exhaust manifold stopped working. Through my original inspection I found a visible crack in the exhaust manifold from the threaded opening for the primary 02 sensor stretching across the manifold. It almost seemed like it was a poor design.
So I am going to roll the dice and replace the original secondary sensor and see what happens. I will post later. Thanks for your reply.

[CivicSpoon]

What kind of manifold did you buy? Is it a header with an aftermarket cat bolted to it, past the oil pan? Like this:


Or is it one that shares a similar design to the original one, where the cat is bolted similarly to it? Like this:


If it's like the 1st pic, is it a high flow cat?

Could you find tell us what the exact code it was throwing, it might help identify the issue.
And yes...the stock manifolds are junk.

[Pawl]

The code is P0420 and the OBD2 screen said the following: "Catalyst system efficiency below threshold - bank 1 -". So 39,000 miles ago I replaced the exhaust manifold with an aftermarket that looks like the original one. At the same time I also replaced the primary 02 sensor with a new one by Bosch. There is a total of 139,000 miles on this car. The secondary 02 sensor at the bottom of the catalytic convertor is the original meaning it has 139,000 miles on it. So I rolled the dice and replaced the original secondary 02 sensor. I cleared the trouble code with the OBD 2 and have driven it about 20 miles. So I am just keeping an eye on things to hopefully have fixed the problem and not see the check engine light come back on. I will write back after I give it some miles and report back in. I want to thank everyone for their input and help on this.Happy Holidays