Which O2 sensor to replace?

I have a P0420 code on my 2002 C4S and want to try replacing the faulty O2 sensor on bank 1-3 (USA driver side) myself before taking to a shop. Question is how do I tell which one (before or after cat convertor) do I replace. Is there a way to measure resistance, appearance or other criteria to use after I remove both? Thanks.

I believe this code is thrown when the values of the upstream and downstream o2 sensors are too similar. While this could mean an o2 sensor is bad, there are many o2 sensor specific codes that describe possible sensor failures. If both sensors are working normally, the problem would be that the catalytic converter is no longer functioning well. Since o2 sensors are $$$, you might want to try your hand at testing both sensors on that bank - see http://www.autozone.com/repairinfo/c...mpid=PS:1:1:21

Thanks - only code was P0420. How do I get to the sensor specific codes if they did not show up during diagnostic (done at a shop)?

Its not sensor specific. The code is telling you that the catalyst is not working well enough. That could be because the front 02s are reading too much oxygen due to any number of things or it could be that your cats are shot, or your rear 02 sensors are bad. you need to datalog what the sensors are reading (both sensors) to give you an idea of the problem.

Look at the data from both banks. The "good" bank will give you an idea of the values you should be seeing. Then look at the corresponding sensors - bank 1 upstream to bank 2 upstream, and bank 1 downstream to bank 2 downstream. If the upstream sensors are reading the same, then the issue is likely with the downstream bank 1-3 sensor or the catalyst. To determine if it's the catalyst or the sensor, swap the sides of the 2 downstream sensors. If the code follows the moved sensor, you know the sensor is probably bad. If the issue still shows up for the bank 1-3 sensor, the catalyst may be shot.

Looks like the O2 sensors are about $150 each from warehouse33auto.com, so times 4 equals about $600 to replace them all. So, you definitely want to determine if a sensor is bad before you start throwing parts (sensors) at it. As mentioned above, there's no guarantee that you've even got a bad sensor. The sensors might be working just as advertised and telling you that you've got a bad converter. Just FYI, 50 state compliant catalytic converters from the same source are $658 each.

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The steps for dealing with the P0420/P0430 error codes is to if any O2 sensor related error codes are present (or possibly pending) to replace the indicated sensors and clear the codes and road test the vehicle. If the P0420/P0430 error comes back replace the indicated converter. The O2 sensors are constantly checked for proper function. If one was bad there should be an error code. To replace any without an error code is probably a waste of money. But it is up to you if you want to go this route. If there is no O2 sensor error code you can try replacing the #2 sensor on the indicated bank. However my advice would be to replace the counterpart on the other bank in order that both banks have a new sensor to help prevent any imbalance in bank fueling arising from a new sensor on one bank vs. an old/used sensor on the other bank. I believe though after the sensor replacement you will be in for some disappointment as the converter error code comes back. The problem is the converter is not performing up to the necessary efficiency level. It is unable to store oxygen. The converter is either worn out or more likely -- this is my experience -- suffering from a mechanical failure. Possibly the converter brick is loose and the brick has shifted out of position and exhaust gas flow through the converter is no longer optimum which compounds the problem as then the converter doesn't stay as hot as it should to operate at the necessary efficiency level.

Thanks for the quick response to this question. i am headed to another shop for another diagnosis to be sure there are no sensor specific codes being registered. Next course of action will be to either replace an indicated sensor or find a converter as suggested. Thanks again, guys. John

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Depending upon the emissions rules/regs in your area you could source a replacement converter from a salvage yard, that is if you are OK with staying with the stock exhaust system. My Boxster manifested a P0430 error and I finally got tired of this - and the car failed one emissions test but a nice drive (half an Italian tune up) had the car passing with flying colors a few hours later but it scared me into action -- and I sourced two converters from a local salvage yard. I made sure the converters/exhaust manifolds didn't show any signs of impact damage and I gave both a shake to make sure neither one made any noise. Then I handled the units with care. The tech at my local dealer removed the old ones and install these and the error has not returned even once and the knocking (cold) and buzzing (hot) noises are gone. Ah peace and quiet... and no CEL.

Before you go throwing parts at the problem, swap the sensors side to side (the ones behind the cats first) and see if the problem follows the sensor. By doing this I was able to diagnose that I had a bad cat, and that the sensors were fine.

I swapped the two downstream sensors and retested. Sensors are fine and problem is the driver side cat. Now off to the market to find a replacement. CEL light has not come back on since swap and retest, but it will based on comments in thread. By the way, I removed the back bumper to help in removal of sensors, but found this would not have been necessary. A 22mm or 7/8" open end wrench did the job just fine from down under. There was also an obvious difference in the appearance of the sensor tip. The sensor from the good cat had a uniform grey color, but the one from the bad cat was black with one side of the tip looking like it had been recently sandblasted.

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The appearance of the O2 sensor after the "bad" cat is not surprising. The converter is not performing up to efficiency which means it is not converting the toxic exhaust gases into less toxic gases. When working properly this conversion produces heat which helps keep the converter hot so it works at peak efficiency. It also keeps the O2 sensor hot which it needs to be in order to work properly too. When I had the O2 sensors in my Turbo replaced all 4 came out with a very uniform very light grey color to the tip. The reason I replaced the sensors was one (one of the #1 sensors) was triggering an error pointing to the heater. Thankfully the converters were working just fine.

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Your bank 1 cat has a loose element inside. The shiny side should be towards the header. It's the loose element pounding on the sensor. The only way to correct it is replacement of the cat unfortunately.

One more question before I lay out the dough for a new cat. Has anyone had a bad cat opened up, repaired the loose element (welding I assume), weld back together and reused the repaired cat? If the catalyst is still active, this would seem to be a reasonable approach to avoid the expense of the new catalyst and the waste of good catalyst.

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That is correct if you can find a shop to do it for you. Btw, do you hear any rattling from the bank 1 cat during idling? A loose element should rattle.

No rattling at idle or at higher rpm. Only evidence of loose element is the shiny metal tip on half of bad cat O2 sensor tip. Do you know where to find a shop drawing of the cat to see where the element is located within the chamber we see on the outside?

The inside looks like this http://www.ocregister.com/articles/c...ions-cats.html with two elements on the ends and the post-cat sensor right between the two elements. Yes, the sensor only monitors the upstream element.

Btw, also pay attention to the smog visual inspection rules, if any.

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Can the converter brick really be contacting the O2 sensor tip? That seems a bit out there to me. Based on my admittedly limited experience with sensors the tip doesn't look to be up to much of any contact with anything before it would be past the worn shiny point and coming apart. Not the same of course but even though the converter brick in my Boxster's exhaust was loose enough to knock when cold and buzz when hot the #1 and #2 sensor on that side were just fine with no signs of any contact on the tip of either sensor. I agree if the brick is loose the converter needs to be replaced although as has been mentioned in other posts cutting apart the converter housing and securing the brick then welding up the housing again is one possible solution to the problem. I have read of other owners having this done and with successful outcomes but how to find a place to do this... Ideally someone who has had this done and can recommend a place would be one way but one might be the first in his area.

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It happened to my '03 so it's definitely not "out there".

See the shiny tip and flat spot on the left postcat sensor. 0317142039 (Large).jpg

Assai, my post bad cat sensor had the exact same appearance as yours, but mine had not worn a flat spot yet. I have a buddy who can tig weld the converter shell back together after I open it up, so that is my approach. I will ask him to reattach the brick if there is an obvious failure of the original attachment mechanism. I will post pictures of what I find. Thanks for the confirming photos of your sensors. Jonhums.

So the shiny tip is truely due to the converter brick touching the sensor tip? Learn something every day.

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Damn, I need a buddy like that Once you take off the cat, you should be able to shake it and feel the brick rattling inside. Keep us posted.

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Yes, once the brick is loose, the post cat sensor is the only thing stopping the brick from shooting further down. The honey comb material of the brick just polishes the tip of the sensor from vibration, kinda like those vibrating toothbrush.

I opened up the upstream weld on the bad cat and was not able to separate the shell at the weld. I got it to move about 3/8" but it seems to hang up on an internal sleeve that connects the two bricks. That sleeve has a hole in it for the sensor and is free on the downstream side of the shell but somehow attached to the upstream head of the shell. I can't see how the sleeve could have been welded to the upstream head and am soaking that joint with PB blaster overnight. Any experience with taking one of these guys apart would be appreciated.

Maybe someone knows but I've never seen a pic of one opened up. I'm sure it can be done though as I had Carnewal put in some 200-cell bricks on mine. Is the upstream brick loose?

It slides within the inner sleeve, but does not rattle. I probably caught the problem early enough to avoid more damage. Do you have a name at Carnewal? I would like to pick his brain for a clue as to what to try next. Thanks.

Gert from Carnewal in Belgium. He offers exchange sport cats http://www.carnewal.com/products/P96...nge-Sport-Cats

Post cat sensors. Top driver side; bottom passenger side Driver side sensor port. Note sensor contact area on edge of brick. Update on problem solution. My buddy welded up the bad cat converter shell and I attempted to solve the problem of the shifting brick by installing a stainless steel leaf spring retainer through the sensor port. The spring was placed up against the second brick and anchored on the body of the sensor while exerting pressure against the first brick in the middle of the spring. The spring was a 6" rule pictured below. The first 200 miles with this solution have been CEL free. 6" stainless steel rule with clip on top to distribute spring load on second brick edge and radius on bottom to lock into the sensor body.

Exact same failure as mine. A very novel fix! Glad you got it sorted. Just wondering since you opened up the shell already, was there any way to stabilize the brick by welding a piece of metal inside the shell or something?

No - the two bricks are contained within an inner sleeve which must be welded directly to the inlet nozzle of the converter shell. When I cut the shell apart at the inlet head, I could not move that inner sleeve by opening up the gap at the cut. That inner sleeve is free to expand on the outlet end, but is welded solidly on the inlet end, so cutting the inlet head did not reveal the way the first brick was attached to the inner sleeve. When I saw the first brick slide back and forth in the sleeve, I came up with the idea of a leaf spring to keep it separated from the second brick and away from the sensor body. Nothing lost by trying this McGiver approach!

Thanks for the explanation.