So as I mentioned before when the exhaust shop replaced the cat they used a universial cat and they placed the O2 sensor right bofore the converter. I thought this was odd so I asked them about that. They said the secondary sensor did not have to be after the converter just close to it. Should I have them mount the sensor after the cat? Could this be causing some of my problems? I don't want to just ignore the light, but I also don't want to spend a bunch of money replacing parts that are actually good. From what I can tell those o2 sensors are not cheap. Can you elaborate on how to actually check the sensors with a volt meter. That I can do..
Thanks guys.

 

I'm sure the 2nd sensor has to to after the cat. The whole reason for having a 2nd sensor is to measure whether the cat is actually working. Can't do that if it's upstream...

The old-school way to check is to use a voltmeter on the signal wire, while the sensor is still hooked up. In other words you tap the wire without disconnecting it. The first sensor should rapidly swing from 0.1v to 0.9 volts or something like that. The actual voltage isn't so important, but how fast does it swing. It should swing several times per second; the most common way for a sensor to go bad, is that it will get sluggish.

It's not easy to follow the swinging with a digital meter, especially since the affordable ones don't update fast enough. An analog meter works, as does some good ones that have a bar-graph in the display (like my Fluke).

I haven't tried this with an OBD-2 car yet, 'cause I haven't gotten an O2 sensor failure. I've been told that these newer ECU's are actually pretty good at figuring out whether the sensor is actually bad.

 

I'm not real big on the technicals but, some of the o2 gets reused. A lot more oxygen enters the cat than will ever be used. The cat also has the ability to store oxygen, quite alot of it. The ecu tries to keep the 2ndary o2 sensor at 0.6 volt. So it does have some affect on the fuel trim in closed loop. Some of the newer models that use a primary wideband sensor have an adaptive system that helps to smooth out the ups and downs, but it shoots for a flat line at .6 volt.

 

That's probably your problem, the o2 sensor needs to be after the cat.

 

just like everyone else said, that looks like the problem. The ECU is seeing the o2 reading before the cat so it is thinking the cat is bad.

 

Well I had the shop move the O2 sensor to the rear of the CAT. and they reset the ECU. So far no light but we'll see in a few days. One thing I did notoce it the was the first sensor actually had a Negative voltage reading of like -3.4V the second sensor was pretty stable at around 0.9V. So I guess the first sensor could be going bad. Any suggestions on what type of sensors to get? And should i replace both at the same time or one at a time. This time I got the error code it is a P0420 error. Thanks guys for all the replies..

 

Now you know why the 2nd o2 sensor has to be behind the cat. It has to monitor the efficencey of the catalytic converter. If you place the sensor before the cat, the ECU cannot determine if the catalyst is functional.

 

you are correct the P0420 codes does refer to the second O2 sensor. It can show a bad reading if there's an exhaust leak as well... I'm assuming that everything sounds fine since you got the cat replaced? If that's the issue then I'd assume the reason it was giving the error is because it was reading nearly the same to the first sensor which would indicate to the car something like you having just a straight through exhaust (I.E. No cat) so it would error as being a defective cat. Long story short. Should resolve your issue.

 

do you think that a bad O2 sensor could create a P0420 error?


rj

 

I recently replaced converter on my 99 Subaru Forester due to a persistent P0420 and 2nd O2 sensor pattern that essentially matched the primary O2 sensor, swinging from -0.7 to -0.2 V in phase w/ primary sensor.

I checked two other cars ( 97 Acura CL, 97 Acura 2.5TL) and they generally show a high, smooth voltage (-0.7 V = low oxygen) on the 2nd O2 sensor, but will change to low voltage at some times.

After the new converter my Subaru 2nd O2 sensor stays high for some time (1-2 minutes) at steady speed ~ -0.7V, and then gradually reduces to about -0.2V (high oxygen). My understanding of this from other sources is one of the 3-way catalysts (cerium) is storing oxygen during high voltage and yielding low O2 to sensor, and when capacity to aborb O2 is reached, the O2 content goes high. Purpose of storing oxygen is to allow catalyst to continuously supply oxygen to converter exhaust products during rich engine conditions when normally most O2 in exhaust stream is consumed.

After removal of my old Subaru converter, I examined the element though the end opening and could see no visual faults, such as melting or breakup of the element. It looked OK, but performance was definitely subpar, as measured by the 2nd O2 sensor.

regards