Martin Joseph said:
Thanks for the help! A visual inspection of the intake and hoses show
everything to look ok. no visible or audible leaks. The drivability of
the car is still perfect also.
I went to an autoparts store today and used an OBDII scanner, which now
shows P0136 - O2 Sensor Circuit malfunction (Bank 1 Sensor 2).
This seems odd since I just replaced sensor 1? What are the odds of them
both going bad at the same time?
The replacement was a straight across swap with the original. it took me
four tries at the Auto parts store to get the correct part, but
eventually I got the proper Denso model that looked identical to the
original and had the proper connector also.
Thanks again for any help and or suggestions with this.
The code that points to the front O2 sensor in almost any OBDII car (and
probably the OBDI cars, too) means that the sensor is not providing usable
output. That's the rub, since anything that causes the ECU to be unable to
find the correct approximate fuel ratio will set the code.
Think in terms of things like dripping injectors, vacuum leaks, exhaust leak
(but I'd expect you to hear that), MAF sensor adjustment/defect... there are
a fair number of suspects and tracking them down is not always easy. The one
time I had to do that (on my son's misbegotten Taurus... as though there any
Tauruses that aren't misbegotten) I put a scope on the O2 sensor lead and
watched it. The ECU would start at the rich side of the range as shown by
voltage near 1 volt and lean it out to where the sensor transitioned low.
The sensor would vacillate between high and low normally - many times a
second - for a couple seconds and then go high for most of a second. The ECU
tried a leaner mixture and the O2 sensor responded with low output, which
ended when the range sweep ended after maybe 15 seconds. Turned out to be a
dripping throttle body injector that was messing with the mixture.
I expect the new sensor is okay. A close visual inspection of the intake
system sounds like a good (easy) place to start. A fuel pressure test will
tell you if the fuel pressure regulator has gone haywire, and a look at the
plug color should also give you some useful info one way or another. Of
course, a bottle of fuel injector cleaner is cheap, easy, and can't hurt.
Same applies to cleaning the crankcase ventilation system.
Finally, even a digital - not analog because of the loading effect -
voltmeter can tell you something about the O2 sensor operation... although
not as much as a scope can. If you can get the meter between the sensor
output and ground you should read about .45 volt DC when the sensor is
cold - that confirms the connection from the ECU to the sensor. In a few
minutes the voltage should start to change, and (if everything were working
right) you would get a continuous jumble of voltage readings between 0 and
approximately 1 volt. If the voltage does not change from .45 volt, the
sensor is stone dead. If the voltage is around 1 volt most of the time, the
mixture is too rich to start with. If it is around 0 volts most of the time,
the mixture is too lean. In either of those cases, coming off idle should
disturb the status quo and produce a high reading for a moment and a low
reading for a moment, in no particular order. If the reading doesn't change
something is very wrong with the mixture and will probably be visible on the
plugs. If the reading is a jumble as described, the ECU must be complaining
about mixture under some other conditions, like cruise.
Mike
