DMM and O2 sensor

[peter paul fenske]

Hi Guys

An O2 sensor is usually fed with a very hi impedance DC voltage
of about 0.45 volts from the ECM. In a fault condition if would remain
at 0.45 volts indicating a inactive o2 sensor. The impedance of the
voltage divider network is about 10 megs. Thus a 10 meg meter
would read about 0.35 volts. This has actually nothing to do with
the O2 sensor itself.

CU:peter

At 02:42 PM 3/28/97 -0600, you wrote:
>>
>>Ok, this may be a bonehead ? but here goes:
>>
>>When measuring the voltage from an O2 sensor with a digital multimeter, does
>>it give the true reading or a slightly lower voltage.  The reason I'm asking
>>is that the service manual says that the reading should be .45 V but "may
>>read as low as .32 V on a digital voltmeter"
>>
>>Ed Hilker aka "Chief"
>
>can't imagine what that means.  Typical DVM has 10 megohms
>input impedance -- ten times more (or better) than is recommended,
>so there should be no loading (i.e. the voltage should not read
>lower).
>
>If you're reading the O2 sensor while the system is under the
>control of a modern efi, the voltage will oscillate back and
>forth either side of stoich (which is about .5 volts).  That's
>probably why they say the reading you get may be off.  But,
>if you're not using an oem efi, I don't think the mixture
>will be varied by the ecu (maybe some do, doubt it, that's
>just to make the 3-way cat happy).
>
>Tom Cloud <cloud at peaches.ph.utexas.edu>
>
>