Subaru Forester oxygen sensor readings

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Subaru Forester oxygen sensor readings

Postby mikeevans » Wed Sep 03, 2014 7:23 pm

I am trying to check the operation of pre and post cat sensors on a 2005 Forester 2.0. The pre cat has 4 wires, 1 Blue, 2 Black and 1 white. White is earth, blacks are heater wires and Blue output. The Blue has a constant Voltage of about 2.7V with the engine running. White is Earth. One of the black wires has battery voltage. The current reading on the Black supply wire is about 7amps. None of these figures are anything like the examples shown in the Pico automotive section. I am trying to work out what readings I should expect to get from this car to avoid missdiagnosing faults. I have also tested another 2.0 Forester and have had similar readings from this one as well. Any advice will be appreciated.
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Re: Subaru Forester oxygen sensor readings

Postby Robski » Wed Sep 03, 2014 9:02 pm

4 wire wide band ! Micro Amps clamp or scan tool data.

What DTC's do you have & what's the issue with the car ?
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Re: Subaru Forester oxygen sensor readings

Postby Alan » Thu Sep 04, 2014 8:17 am

Hi Mike,

As Robski says this is a wideband oxygen sensor which typically output a signal from -1mA to + 1mA. This is too small for the 20/60A clamp or 30A clamp to directly pick up. The reason we do not have a preset yet in PicoScope is that there are several different ways of measuring the signal and the best way depends on the vehicle you are testing and equipment you have. There may be other options but these are the ones I know of.

1. Scan tool

2. A microamps clamp. This might be an option if for some reason you frequently have to measure these, but they are an expensive investment for just wideband lambdas (and perhaps also magneto resistive ABS sensors). Expect to pay $600 / £400.

3. Use your existing low amps probe (either the 20A/60A in 20A mode or the 30A clamp). With a bit of care these can measure down to about 5mA so need some help. You need to wrap/loop the wire carrying the sensor signal through the jaws at least 10 times in order to amplify the signal. 10 loops of wire through the jaw means that the 1mA signal will appear as 10mA to the clamp. If you dont have the slack cable to do this you may have to improvise a bit - I have used the universal break out leads and cut the one carrying the signal in half. Into this I wired a coil of 100 turns of thin insulated wire. The 1mA signal now appears as 100mA so easy to see. If using this technique try enabling resolution enhancement from the channel options - this makes it easier to see tiny, noisy signals.

4. On some vehicles the small change in current does cause a voltage swing so probe both sensor signal wires on the lowest range that the signal remains in range (which varies from vehicle to vehicle depending on the ECU / sensor combination). Zoom in and often you can see the signal.

5. Break into the circuit and add a small resistor - measure the signal across the resistor. A value of 10 ohms seems to work - this will give 10mV for a 1mA signal through it so can easily be measured. Be careful not to fit a higher value resistor or you may trip a fault code.

One thing to also remember is that these sensors do not always switch high / low / high like other lambda sensors, they can settle at one value. A quick burst of WOT may be needed to wake them up.

If anyone has any other ways of testing these sensors please do add to this thread!

Alan

PS Most of the above is also true for the magneto resistive (active) ABS sensors which switch between 5mA and 15mA
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Re: Subaru Forester oxygen sensor readings

Postby mikeevans » Thu Sep 04, 2014 9:08 am

Hi, Thanks for the in depth information. The car has no stored faults but is hesitant on acceleration. It is one of our sales cars and has stood for some time. It has done about 65k but the service history is poor. I wanted to check the operation of the sensors out of interest and to eliminate them. My next move will be to check and replace the spark plugs as it is quite likely that these may not have been changed as they are difficult to reach. Previously we have had to rely on a certain amount of gueswork but we are trying to use the Picoscope to prove faults as often as we can. The more we use it the more confidence we are getting in identifying defective components.How can I tell the difference between a 4 wire wide band sensor and a normal 4 wire sensor?
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Re: Subaru Forester oxygen sensor readings

Postby Steve Smith » Fri Sep 05, 2014 10:14 am

In addition to the thorough testing above, there is a nice video on YouTube utilising a maths channel to identify change of state of a broadband sensor with reference to voltage when introducing an air leak. Here you can confirm switching speed and operation.

https://www.youtube.com/watch?v=q2_4eC-v5aM&feature=youtu.be

To determine at a glance the difference between a conventional O2 sensor and a Broadband O2 sensor, in my experience with Toyota vehicles we have sealed vent holes at upper section of the senor body for a conventional O2 sensor and additional holes for broadband sensors packed with a filter material. I do not know how this translates across to other brands of O2 sensor manufacturers and should not be used as a viable method to ID the style of sensor.

Take care......Steve
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Re: Subaru Forester oxygen sensor readings

Postby tode » Fri Sep 05, 2014 7:25 pm

i know that the target of the discussion is to use the Pico to control the oxygen sensor to broadband, but I think the best way is to use the diagnostic tester with software eobd, in real values, "current pump" where the ECU shows the values ​​in mA, in phases cutoff (much oxygen) or polluting the intake air with exhaust gas, through a tube (little oxygen) I can see if the current rises or falls.
If the sensor reacts I gather that it works, if it is not calibrated is more difficult, but I control through scann tool, long and short adaptation.
With Pico scope I control the heater.
The sensors Bosch possess four wires, but the connector contains a resistance that "transforms" the current signal into voltage signal, to the ECU, in fact the wires are five engine ECU side, on this sensor can be easily measured signal with the oscilloscope, although I think the best way is always the scan tool.
I hope I explained, correct me if I say stupid things.
greetings
:D
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