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2005 Outback 2.5i - 5 speed - 191K
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I suffered through my first P0420 error codes. In my efforts to diagnose the issue, which I never really diagnosed, I came across questions about the O2 sensors.

Since the O2 sensors could be the culprit I acquired a Bluetooth OBDII reader plug and purchased the pro versions for DashCommand and Torque Pro.

My car has upstream and downstream O2 sensors. The OBDII Apps did not really have an option for BOTH sensors (they are called banks from what I can tell).

I did take a screenshot of the reading and included below.

My question come from confusion. I have had members explain the O2 sensor should be a flat line and other say it should be up and down with a steady "heartbeat."

Does anybody have a screenshot for their O2 sensor? What should the voltage be for the sensor with the engine heated, at idle and with the AC off.


I DO NOT have any current CEL codes and the CEL was off when I recorded the O2 readings in the attached image. The car runs fine.

What else should I look for that the OBDII reader shows? MAF? Short Term Fuel Trim? Etc...

Thanks for any help!!
 

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Bank 1 and Bank 2 refer to the sides of the engine, Bank 1 being the right side (passenger side in LHD cars), and Bank 2 being the left side.

Depending on the exhaust configuration, there could be two, four or even five exhaust sensors. The basic 2005 2.5 non-CA spec has one catalytic converter, and two sensors, one before the cat, and one after. The 2005 2.5 CA spec has three catalytic converters, and five sensors (the third cat has only one sensor after the cat, none before).

The sensor that is before the first cat in an exhaust is "Sensor 1", the one after the cat is "Sensor 2" etc. Each sensor is also identified by the "Bank" it's in, i.e., the exhaust pipe. For a car with a single cat converter, all sensors are "bank 1". If the car has two converters, one for each bank, then there will be sensors identified as "bank 1" or "bank 2".

Putting it together, the sensors in a single cat system would be "Bank 1 Sensor 1" and "Bank 1 Sensor 2". In a dual cat system, there would be additional sensors identified as "Bank 2 Sensor 1" and "Bank 2 Sensor 2". And, in the CA-spec with the third cat and one sensor after it, that sensor is "Bank 1 Sensor 3".

The actual OBD readouts from the sensors before and after the cat could be different. Typically, the readout for the sensor before the cat, in our Subarus, is a number around 1.00 ("lambda"), an air-fuel readout around 14.7, or a voltage around 2.7 V. The readout for the sensor after the cat is usually in Volts, and ranges between zero and one.

The first thing to do is identify which sensors the scanner can indicate (i.e., identify what type of exhaust setup the car has), and from the sensor descriptions, which are pre-cat and which are post-cat.

Depending on the engine operating conditions at the time, and whether or not it's running properly, the sensor indications could be steady or varying.

Typically, with a warmed up engine at idle that is running properly, both the pre-cat and post-cat sensor signals would be steady, with the post cat sensors indicating something in the range of 0.6 - 0.9 Volts. The most important sensors are usually the Sensor 1 and Sensor 2. Where there's a Sensor 3, it's signal might be helpful in some cases.

It's good to identify which other sensors or signals the OBD reader can display, but which ones to select might depend on the reason, e.g., is there a particular problem being diagnosed and for which certain signals could be more helpful.

Looking at your screenshot, that's a post-cat sensor (Bank 1 Sensor 2) which is varying between roughly 0.1 and 0.7 V. But on its own it doesn't tell us much. For example, what's the time frame -- is the scale at the bottom milliseconds, seconds, or minutes? And during that time was the engine continuously idling on its own, or was it revved up at times, or is that a graph of the sensor while driving the car?

If you haven't already, it might be worthwhile looking at these threads:

http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diagnosis.html

http://www.subaruoutback.org/forums...spective-p0420-catalytic-efficiency-code.html
 

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2005 18psi supercharged U5 Outback w/207k+ miles
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from what I've read, I believe the graph should look like front o2 sensor #1 in my graph, not front o2 sensor #2 which is way up and down.

The scale here is about 30 seconds at around 2500 RPM held constant as best as I could (would be cool if I could tell the computer to hold an RPM...possible???).

I've read that an O2 sensor could read this way for a few reasons. Exhaust leak introducing fresh air, bad o2 or bad cat, but I could be wrong. My understanding though, that , it would take quite an exhaust leak to make an o2 read this way and it's unlikely to happen at 2500 RPM when there is a good bit of pressure coming from the exhaust (??).
 

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