87 Maxima 3 Wire Oxygen Sensor Circuit

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edge - 08 Jul 2008 16:48 GMT

I have a 87 Max where I am trying to test the 3 wire O2 sensor circuit
before I replace the sensor. The car has 60K miles and has poor fuel
mileage. The cyl head temp sensor connection was repaired and the ECM
flashes 44 code that it is operating correctly in mode 3. But, the
ECM does not flash the proper code that the O2 circuit is operating
correctly in operating mode 1, so there is a problem here. One of
the tests the manual says to check for is ground back through the
harness with the O2 connector disconnected. But if you check the
circuit diagram the harness returns this lead back to pin 152 on the
ECM computer , not to ground. I measured open circuit on this lead.
I understand the case of the O2 sensor is grounded, but this is on the
sensor side. Should this harness lead be to ground or to the
computer?

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edge - 16 Jul 2008 17:02 GMT

I am told the ECM switches the ground on and off to control the heater
circuit. This shouldn't affect the O2 sensor, it will just take
longer to heat up during warm up and idling.

For the 02 sensor at 2000 rpm and operation temp I get .23 - .77 V
and a full cycle .23 -.77 -.23 takes about 1 sec. At idle at
operating temp the O2
stays at 0.8V . When I unplug a vacuum line to make it
lean the voltage drops to .19 V responds quickly, less than 1/2 sec.
The heater circuit is bad, but
as said it should only affect it during warm up and idle. This range
seems to be just under the range for a O2 sensor .2 -.8 V . Is this O2
sensor is marginal and should it be replaced?

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