thanks for all the tips folks!
There is no media inside the cat so flow shouldnt be a problem. ;)
peeked under the car and the wires and sheath look pretty clean and dry - so reference air should be good.

I do loose maybe 1/2 an expansion tank of coolant over the course of a few months. Always thought it was the donut seal on the WP which I need to replace at some time.

I'll pull it and clean / test with propane per suggestions and videos.

yes 310,000 upper midwest miles and sadly I'm nearing the end.

If an 02 sensor lasts me 3 months ~ 5,000 miles thats still only $25 bucks a month! I've only put $300 into the car in 2014 so far. its been a fantastic maintenance year!!!

Matt

Hello,

First, the ODB code doesn't auto erase. Previous codes that appear when an item is faulty doesn't get automatically eliminated when the item is fixed. Clear your codes and drive the car to see if it comes again.

Second, the oxygen sensor uses the exhaust pipe as its grounding point. Check if your exhaust pipe grounds nicely. With a voltmeter set to resistance reading, put one probe on the sensor's body and one probe to the car's body or transmission (well..its too far to get to battery). Resistance should be almost zero.

Third (if the above checks out), them going bad should suspect coolant leak somewhere. Does your coolant level goes low?

You could remove the sensor and inspect it. You could rejuvenate the sensor by burning the sensing head inside a gas fire (butane or propane). There're many youtube videos showing how. Connect voltmeter (set to voltage reading) to the sensor's wire and body (for ground). You should see fluctuation of reading indicating functioning sensor (the sensor generate voltage in millivolts).

Here's such a video:
http://www.youtube.com/watch?v=BfJ_Jwf_Sa4

I've done this to my sensor at 50k km. Mine was preventive maintenance, no OBD codes. When reinstalled the engine runs much better.

Hope this helps,
Amarin.

I have read in some other posts that the sensor have an opening someplace that allows it to sample outside air for a reference. My understanding it could do it from the point where the wires go into the housing. It might "breathe" through the wires strands under the sheath.
I really don't know!

If the outside became soaked in oil it would have to be a whole bunch I am thinking.

310,000 is a lot of miles. How about your plug readings and the sensors vanes. They should tell you if its combustion fouling.

Is the engine running OK power wise?
This guy is in the cataylic converter way down under, right? The flow might not be right? Not getting hot enough on the side it mounted into?

A Lot of smut or gunk on it is a heads up. It should be a gray to whitish color or brown depending on additives in the gas you use.
You should be able to clean the sensor with a propane torch and read a signal with a voltmeter from it at the same time.

Since you have so many it would be worth a try to test them. Varing the heat and exposing the vanes to air varies the voltage. I use a spreading flame attachment on my torch.

It is very unusual to hear of such a high failure rate.

The answer lies within the sensor being contaminated or the converters flow is my best guess!

The code is leading in that direction. Maybe a new clean factory sensor can work fine while perfectly clean or lower flow rate for a little while?

I am definitely no expert in this field!

Phil

If a little coolant is getting to the O2 sensor that will surely ruin it.
A head gasket slightly leaking will do it.
sealers that are not sensor safe will ruin it too, like non safe silicone.