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P0240 & the 3rd Cat

New 1/7/05

Here's your "primer". There are three cats on our cars, but only two of them are monitored by the ECU. As you know, there are exhaust manifolds on each bank of the engine (front and rear). The exhaust from the front goes through one cat, and the exhaust from the rear goes through another cat before the two exhaust streams join together at the Y-pipe to go through the third cat (which is not absolutely necessary for emissions, but is really just a "safety net" to keep emissions clean when the car gets really old and the two primary cats begin to lose their effectiveness.) Before each of the primary cats, there is an oxygen sensor which measures how much oxygen is in the exhaust to help the ECU determine if there is too much or too little fuel being added to the engine. After each of these two cats, there is another oxygen sensor to determine whether or not the cats are doing their job in reducing emissions. If the oxygen level after the cat is too close to the oxygen level before the cat, then obviously the cat isn't burning up (or "converting" to be technical) the extra crap in the exhaust.

Now, just because you are getting a cat-related code doesn't necessarily mean the cat itself is to blame. An engine misfire allows fuel to pass through the cylinder unburnt, so it just flows out the exhaust, where it can leave trace deposits on oxygen sensors and cats. Over time, this can cause an oxygen sensor to report false or skewed readings, so it may look like the cat is to blame when it is actually a faulty oxygen sensor. That is why we are curious as to whether or not you have known of any misfires. Of course, a bad coil on the SHO will not always be noticeable, but it might sporadically misfire and contribute to oxygen sensor problems. Most of the SHO owners who have had coil failures have put up with them for many many miles without a misfire code being thrown. Coolant and other contaminants in the exhaust stream can do the same thing.

**ASIDE** I have heard of persistent misfires contributing to cats getting clogged up with soot, but I think that is an illusion/coincidence. Soot is created when oil burns at low temperatures. An engine which is burning oil at a high rate will often have fouled plugs, contributing to a misfire. So really the clogged cat AND the misfire are both EFFECTS, caused by the burning oil.

I hope I got all that right. :-)


Ben wrote:

 nope, just the cat code. Ok, but its saying that the cat is not  working properly because of the rear bank of the engine? Its kinda  confusing. IF I DO have a bad cat (which I hope I don't) why is it  telling me the rear bank of the engine?   I am still learing, so I am assuming that since this is dual exhaust,  one side of the dual exhaust is for the rear bank of the engine, and 1  side is front?????? you may all think what an idiot I am but also be  aware that I have little automotive experience. I have always wanted  to learn and you all have helped me severely and I really do  appreciate it.   Someone told me yesterday that they had bad plugs and they also had  the same stalling issue. Since the code is telling me a malfunction  with the cat on the rear side, and since I need to change the rear  plugs, my best bet would to be to change those back plugs and take it  from there. hehe, Im learnin! I'ma learnin! I feel like Sherlock  Holmes!   Ben  

*/Bob Gervais <BobGervais@cox.net /* wrote:   Bank one is the rear bank.   This is basically telling you that the catalytic converter is not  performing as expected. It may be the cat, the sensor, or  something completely unrelated.   In the case of your misfiring, it could very well be causing the  420 code, due to raw fuel being dumped into the exhaust.   Are there any other codes, besides the misfire?   -Bob

*Subject:* RE: P0420  hmm yeah I noticed someone else has the same code I do. I guess im lucky we don't have emissions testing in Tennessee, well at least not yet :) Now I'm stumped with the bank 1 and 2. Is this telling me  that the problem is on a certain side of the engine??? I was reading that some symptoms for this code is misfiring,  and since I am having a stalling issue, it looks like maybe  this is triggering the catalyst code. I am going to reset the PCM and see what happens. I will drive between the desired MPH so the computer can run the test and see what happens. It looks like I am ripping into the engine again this weekend,  those back plugs need to be changed. Does anyone recommend testing anything else? Has anyone else  ever had experience with this code?


Chris Archer <carcher@mynra.com/  wrote: Did you try the Google search on the website? I'm pretty sure that code is covered there. A catalyst efficiency  problem would be an unlikely cause of stalling unless the  cat is clogged.

ok well i finally went to Advance today to pull the  code that came on about a week a go. P0420 Catalyst Efficiency Below Threshold (Bank 1)  It came on while i was in Pennsylvania visiting family.   I have no idea what this code is, could anyone enlighten me? Could this be reason for my occasional  stall?  Any help would be great.


Emissions testing is required in most of TN, lucky for you it just isn't in your area yet. In Nashville and surrounding counties they plug into the computer and if it throws a code it fails. It will be where you are soon enough, be ready for it.

Rick Glass


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