General Technologies ST05 Oxygen Sensor Tester and Simulator - Manual (Page 7)

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4.1.4 Wide Band (Air/Fuel Ratio) dual cell sensors:
These sensors, with the Bosch LSU4 being the most popular, use two Zirconia cells, one
is used as a conventional Zirconia sensor (reference or Nerst cell), and the second is used
to ‘pump’ oxygen into the reference cell in order to keep it at or near the stoichiometric
output. The PCM measures how much oxygen (current) it needs to pump into the refer-
ence cell to keep it at a set output ( approximately 0.45 Volts), and from this calculates
the actual mixture in the exhaust.
A basic test can be performed on this type of sensor by simply measuring the output signal
of the reference (Nerst) cell, and observing that a good sensor should output a signal level
which is steady and near stoichiometric (0.45 Volts).
4.2 Oxygen sensors testing
Follow the steps as described in “3.4 OxyGEN SENSOR MEASUREMENT” to select sen-
sor type and to connect the test leads to the sensor’s wires.
4.2.1 On vehicle testing of pre-catalytic converter sensors
Below are the typical readouts of a good (pre-catalytic converter) sensor:
Sensor type
Bargraph display Alphanumeric display
Crosses per second
Test
conditions Lean Rich
Zirconia and
Titanium 1
Volt
< 0.3 Volt > 0.6 Volt Feedback Carburetor: 1
Throttle body injection: 2-3
Multiport fuel injection: 5-7
Engine at 2500
RPM
Oxygen sensor
at approximately
800 °F or 425 °C.
Titanium 5
Volt
> 3.5 Volt < 1.5 Volt
Zirconia and
Titanium 1
Volt
< 0.2 Volt
when
forced
lean*
> 0.8 Volt
when
forced
rich*
N/A
Oxygen sensor
at approximately
800 °F or 425 °C.
Titanium 5
Volt
< 4.5 Volt
when
forced
lean*
< 0.1 Volt
when
forced
rich*
* To force the mixture to go rich inject some propane into the intake manifold. To force
the mixture to go lean, disconnect a vacuum line temporarily.
4.2.2 On vehicle testing of post-catalytic converter sensors
Post-catalytic converter oxygen sensors are used primarily to monitor the emission after
the exhaust gases have been processed in the converter, to insure that any residual
hydrocarbons (fuel) and carbon monoxide present in the exhaust are completely burned
before reaching the tail pipe.
A good catalytic converter and oxygen sensor will exhibit a voltage output which is steady
(not switching or fluctuating between rich and lean) and near the 0.45 V level for Zirconia
and Titanium 1 Volt, and 2.5 Volt for Titanium 5 Volt sensors, and the crosses per second
should be 0 (in the alphanumeric display).
A fluctuating output signal (similar to a pre-catalytic converter sensor) from a post-catalytic
converter sensor may indicate a damaged or non operating converter.
A constant rich or lean output may indicate a damaged oxygen sensor or faulty wiring.
Before testing a post-catalytic converter sensor, insure that the pre-catalytic converter
sensor is working properly.
4.2.3 Off vehicle testing of oxygen sensors (for Zirconia sensor only) :
1- Remove the sensor from the vehicle.
2- Connect the test leads of the ST05 to the sensor wires as described in section ‘3.4.2
Sensor signal measurement’.
3- Hold the sensor using a vice or some insulated pliers, and with a propane torch, apply
the flame directly to the sensing element until the bargraph display shows a voltage
output of 0.8 Volt or above, if this voltage level can not be reached, it may point to a
defective sensor.
4- The propane torch performs two functions, first it heats up the sensor to operating tem-
perature, and second when directed at the sensor element, it removes the oxygen from
it, so the sensor “sees” a rich mixture.
5- When removing the flame from the sensing element, the voltage output from the sensor
should fall to 0.2 Volt or below, as the sensor detects the oxygen in the air around it.
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Fig. 5 - Off vehicle testing