Re: O2 Sensor Diagnostics

["Graydon D. Stuckey" <graydon@apollo.gmi.edu>]

On 2 Sep 1996, Paul C. Waterloo wrote:

> It has 12 VDC to the O2 sensor (I take it this is for the heater) and the
> current goes from about 1.7 amps to 1.0 amps as the car warms up. The Bently
> says the current should increase, not decrease, as the car warms up.
Paul,

	Your reading is correct.  The heater draws more current when the 
sensor is cold since it has to heat up the sensor to get it working.  
After it is hot, it takes much less current to keep it hot, because the 
exhaust gases also keep it hot.  Bentley is wrong if that is indeed what 
they said.

> The green (signal? I guess) wire has 455 mVDC with it disconnected and the
> ignition on -- O.K.
> 
> But when I pick a voltage off of it when connected to the sensor and the engine
> running, it varies from 0.8 VDC to 0.29 VDC at idle. If I then rev the car with
> no load on it, it changes from 0.08 VDC to 1.4 VDC. Seems like an awfully large
> change.

	Yes, 1.4v does seem rather high, but I usually think that if the
sensor is oscillating back and forth with a healthy spunk, then that would
seem to indicate that the sensor is OK.  I do think that is high enough to
warrant trying a known good sensor though.  Your idle spread sounds
perfect.  My 5KCSTQ will idle with no leds showing (less than .1v) after
everything settles down into a steady idle. 

	Also, what kind of meter are you using?  It is possible that a
digital meter might be aliasing, giving weird readings.  (Is that the
term, digital gurus?)  Also, the meter must have a very high impedance so 
as not to draw any significant current, or this will screw up the 
signal that gets sent to the engine while you are reading it.

> My question: Is the O2 sensor toast and the ECU is changing the air/fuel mixture
> as told to, or is it something in the ECU and the O2 sensor is just providing
> feedback to what is actually going on?

	If the O2 sensor is wrong, the ECU will be deceived, and the final
mixture will also be wrong, which will cause all sorts of drivability
problems. 

> The car has 95K on it, I suppose it is the original O2 sensor. I know, you'll
> tell me to replace it. But I just thought I would ask first.

	I've seen 'em last past 100K, but Bosch only guarantees them for
60K.  I think that Bosch has some new ones coming out soon that are good
for 100K.

Later, ---------------------------------------------------------- 
Graydon D. Stuckey 	'85 Mazda RX7 GS, no toys 
graydon@apollo.gmi.edu 	'86 Audi 5000 CS Turbo Quattro, has toys
Flint, Michigan USA	'89 Thunderbird SC, lotsa toys