jonhums

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.

5CHN3LL

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

jonhums

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)?

Quadcammer

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.

5CHN3LL

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.

FRUNKenstein

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.

Macster

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.

jonhums

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

Macster

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.

kpl

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.

jonhums

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.

Macster

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.

Ahsai

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.

jonhums

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.

Ahsai

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.