O2 Sensor Voltage Readings? - Subaru WRX Forum
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post #1 of 6 Old 05-29-2013, 03:52 PM Thread Starter
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O2 Sensor Voltage Readings?

Hey Guys,

I have an '04 WRX with a fresh full rebuild engine. I'm having a few issues though...

I feel that the car is laggy, and when I plug into the ECU with a diagnostics computer, I'm seeing that the upstream O2 sensor (#1) is reading about 3.8V at idle, while the downstream O2 sensor (#2) is reading between 0.5V and 0.8V. If i get on the throttle a bit, i can usually get the upstream to jump as far as 4.2V, but the downstream sensor remains about the same. Almost always under 1V.

Pressure at the tailpipe seems to be pretty good, and the cat's temperature (after giving it a good workout) seems to be about 400ºF at the top (motor side) and about 250ºF at the bottom (discharge side). So, I'm not entirely sure if it's a cat problem, or if it's the downstream O2 sensor.

There are no codes on the ECU, and no dash lights. I do get a very gassy exhaust smell in the car's cabin sometimes when I stop at a light.

The upstream O2 sensor was replaced right after the rebuild by the guys who did it, and the downstream O2 sensor is also fairly new. HOWEVER, the downstream, although new, did endure a lot of coolant when the motor blew. Ever after the motor was replaced I was told the car smoked for a good hour before the coolant cleared out of the system.

Does anyone know the voltages the O2 sensors should be reading? I feel like the sensor is the problem and not the cat, due to the pressure at the tailpipe, and then temperature readings.

Thanks for any help!!
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post #2 of 6 Old 05-30-2013, 05:11 PM Thread Starter
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Any ideas?? :-/
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post #3 of 6 Old 05-30-2013, 07:40 PM
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Most Stock "Narowband" sensors Normally read between 0-1V

(I haven't had the need to ever measure the voltage on my primary sensor in the subaru but anything above 1v for a narrow band sensor sounds pretty unusual to me).

The Secondary O2s is pretty much only there to monitor the cat so It doesn't really affect performance but if you have a problem with the primary sensor it definitely will.


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post #4 of 6 Old 05-31-2013, 11:08 AM Thread Starter
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Well it turns out that the front sensor is a wideband A/F sensor, not just a typical O2 sensor. Does this change the voltage range?
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post #5 of 6 Old 05-31-2013, 12:11 PM
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^Yes Definitely.

Depending on the Type of Wideband, it's often normal to read anywhere in between 0-7V

I recommend Checking the service manual for your specific model. It should have "Normal" operating ranges for this sensor.


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post #6 of 6 Old 05-31-2013, 04:46 PM
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This may be helpful.
Stop calling the stock front O2 sensor a narrowband... - NASIOC
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