1) fuel poisoning
2) open circut in heater

Check continuity on the heater.

I've never had a faillure. My OEM '97 sensor is still holding stoich in
closed loop. (confirmed by my wideband O2 sensor viewd on a laptop.)

Just a little over two years ago the O2 sensor on my 2000 Mitsubishi
Galant ES V6 had gone out (code P0155) & with lots of information from
this group, I replaced the sensor.

On having the recent "service engine soon" light checked out, I see the
same error code again, P0155 (sensor 1 bank 2) along with P0301
(cylinder 1 misfire).  I know I need to have my spark plugs replaced,
and I'm hoping that will take care of the cylinder 1 misfire issue.

However, I'm really curious as to why the same O2 sensor would give out
in two years, when having been replaced with the dealer bought OE
sensor, while the other sensors haven't given out yet on this car.

I noted in my follow-up (note 2 below) to the original post (note 1)
that I noticed the original O2 sensor was blackened indicating oil
contamination.

Could that be the culprit?  What cause should I check for while
replacing the O2 sensor?

It's interesting that the Denso sensor is still close to the price it
was selling for two years ago, but the price of the OE Bosch sensor has
dropped to US$76  I think I might go with that this time, especially if
I have to replace it in a couple of year again.

(1) Original thread:
(http://groups.google.com/group/alt.autos.mitsubishi/msg/61331e477dd522fd?hl=en&)

(2) Follow-up:
(http://groups.google.com/group/alt.autos.mitsubishi/msg/3300ff9373e6f635?hl=en&)

--
Himanshu