The question of whether an O2 sensor can be bad enough to cause drivability
or fuel economy problems without triggering OBDII codes has another data
In the alt.autos.subaru forum somebody asked for help sorting out very
troublesome hesitation after slowing in a 2006 Outback. A respondent had the
same problem in his car, and pressured the dealer to do *something*! The
dealer replaced the O2 sensor (front one, I presume) in spite of lack of
diagnostic codes and the symptom disappeared.
my experience is yes, it can definitely scratch that itch. with cheapo
aftermarket sensors, my '89, which has a slight tendency to hesitiation
anyway, is /way/ worse. no codes. but when i use a new oem sensor,
that problem all but disappears.
Am 89 is not OBDII compliant and does not monitor the sensors that close and
the sensors are totally different. The OBDII system monitors heater current
and voltage and switching times and voltage to very precisely determine what
they are doing. The chance of them causing a problem without setting a code
is extremely slim. They can be setting sub codes long before turning on the
light on the dash. The information in the computer should be thoroughly
analyzed as swapping parts is expensive and could just be masking the real
I would have thought OBDII would catch O2 weakness, too, but as the Subaru
experience illustrates it isn't necessarily so. I am puzzled what sort of
out-of-the-box defect the sensor would have that would cause it to behave so
badly and not be tattled on, though.
:-) Maybe the out-of-the-box defect was caused by the BOX ITSELF (like
had the word BOSCH on it)? I keep hearing horror stories about BOSCH,
and having owned Volvo and VW, I believe it.
monitoring heater current allows determination of element temperature if
taken to it's logical limit and simple burn-out at its most basic.
other than that, unless it's truly sophisticated, which i doubt in a
very noisy electrical environment like under the hood of a car,
diagnostics are going to be fairly basic stuff. the primary fault
detection mode afaik is to set differential limits between primary and
secondary sensors, and trigger if exceeded.
but most people take it to a shop that has testing tools. the obdc
computer therefore doesn't need to be that sophisticated. for the home
mechanic, and in the absence of these expensive diagnostics, a few parts
scavenged from a junk yard, which is where i got the sensors for my
comparisons, can be a /much/ cheaper way of determining the level of
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.