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ok so i keep throwing the catalyst efficiency code. replaced both o2 sensors because they were bad ...  seems about every 200 miles it comes back saying the same code .. but theres no smell , no power restriction , cat doesnt get cherry red like there suppose to when there bad... It had cai , header , exhaust , high flow cat , pulleys , tsx intake maifold , the p2r spacer for nitrous , all i can think of right now... lets hear your guys input

As long as you are sure that the cat itself is not bad then throw a spark plug non-fouler on the secondary O2 sensor.

Im gonna go with Paul on this one since he fixed my code and if your running a high flow cat thats definately the culprit! Defoulers it is!

+1

I'm running a spark plug non-fouler on my Magnaflow high flow cat after getting a P0420 code.

I've got a High flow cat on mine and I've put on approx. 150 miles on the cats since then. The reason I replaced it with a HF is because the car threw a cat inefficiency code. I forget what my OBD reader said but I knew it had something to do with the cats and something inefficiency. This was around mid winter 08'.

When I took off the OEM cats, the catalyst looked fine to me but replaced it anyway. I guess I'll wait to see if I get a code because of the high flow cat.

ok , just figured it kind of odd because it didnt start happening until i did a fuel system clean..

High flow cats have been known to throw codes with our cars.  Even the so-called "OBDII compliant" converters will code.  If you buy one, you're taking that risk.  Defoulers may or may not fix the problem.  They generally work but they cannot be relied upon with 100% certainty.  The only 100% effective solution is to purchase an OEM catalyst or a Hondata Kpro.

I have nearly 550 Miles since I've changed my cat to High Flow and still no CEL's. The bad part is I'm going to jinx myself now. If anybody is interested in the converter I used, its this one. http://cgi.ebay.com/ebaymotors/2-Catalytic-Converter-High-Flow-Stainless-Steel_W0QQitemZ200346105043QQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item2ea58ef4d3&_trksid=p4506.c0.m245&_trkparms=65%3A2|39%3A1|240%3A1318

The code everybody gets is the P0420 for catalyst innefficiency.  This is because a high flow cat has a less dense cell system or is of straight through design.  The primary A/F sensor is located on the header and is what the computer uses for fuel trim calculations.  The secondary sensor is used for emissions and is placed half way through the catalyst.  If the ECU sees little to no change in the voltage between the two sensors, it thinks there is a problem with the catalyst.  Spark plug defoulers are used to combat the problem since they block the sensor from a portion of the exhaust flow which causes less gas to hit the sensor, which leads to a lower voltage output to the ECU.  Sometimes this works, and sometimes it does not.  Every car is tempermental with this kind of modification.

Reviving this old thread.  I just got a test pipe made for my car and I have the cat efficiency code even with a o2 simulator (defouler).  Does anyone, by any chance, know which of the 4 wires on the o2 sensor is the signal wire?  I'm just going to solder in a resistor to eliminate the cel on my car. 

Might be easier to double up the defouler

Get a longer defouler like Mike said.  I wouldn't mess around with resistors Brian.

I'm messing around with resistors.  :D  In fact, I'm putting in a diode on the sig+ wire to the o2 sensor.  I'm going to do this today and if it eliminates my problem, I'll write up a diy.  It will cost $2 at most.  But it will require soldering.  I'll keep you guys posted.

What is a diode going to do?  I think this is a very bad idea man. 

My assumption is that your using a zener diode to attempt to produce a constant voltage level?

Just went through emissions and got `catalyst not ready`. I had the negative off my battery for about 30 minutes last week when installing my k&n short ram. Before that I was getting P0420 because of my DC header. What's strange is that I have driven more than 2 hours worth since I disconnected the battery, which should have set everything back to ready. Should I just keep the car running for 30 minutes in the garage? Or should I disconnect the negative from the battery again for 15 mins and see if it resets? I just need something to indicate to me weather or not I need to put my stock header back on to pass emissions.

Found this:

If you take your vehicle for a state
emissions test shortly after the
battery has been disconnected or
gone dead, it may not pass the test.
This is because of certain ‘‘readiness
codes’’ that must be set in the onboard
diagnostics for the emissions
systems. These codes are erased
when the battery is disconnected,
and set again only after several days
of driving under a variety of conditions.

If the testing facility determines that
the readiness codes are not set, you
will be requested to return at a later
date to complete the test. If you must
get the vehicle retested within the
next two or three days, you can
condition the vehicle for retesting by
doing the following.

Make sure the gas tank is nearly,
but not completely full (around
3/4).

Make sure the vehicle has been
parked with the engine off for 8
hours or more.

Make sure the ambient
temperature is between 20° and
95°F.
Without touching the accelerator
pedal, start the engine, and let it
idle for 20 seconds.
Keep the vehicle in Park
(automatic transmission) or
Neutral (manual transmission).
Increase the engine speed to 2,000
rpm, and hold it there until the
temperature gauge rises to at least
1/4 of the scale (about 3minutes).

Select a nearby lightly traveled
major highway where you can
maintain a speed of 50 to 60 mph
(80 to 97 km/h) for at least 20
minutes.
Drive on the highway in D (A/T)
or 5th/6th (M/T). Do not use the
cruise control. When traffic allows,
drive for 90 seconds without
moving the accelerator pedal.
(Vehicle speed may vary slightly;
this is okay.) If you cannot do this
for a continuous 90 seconds
because of traffic conditions, drive
for at least 30 seconds, then repeat
it two more times (for a total of 90
seconds).

Then drive in city/suburban
traffic for at least 10 minutes.
When traffic conditions allow, let
the vehicle coast for several
seconds without using the
accelerator pedal or the brake pedal.

If the testing facility determines the
readiness codes are still not set, see
your dealer.

Are you getting the P0420 code still? Did you disconnect the battery to erase it so you could pass your inspection?

I was able to pass inspection after doing that procedure.

I didn't disconnect the battery to pass, I was doing some electrical work so I had disconnected my battery.