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2001 Outback base
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24 Posts
Discussion Starter #1
I had the CEL illuminate and so picked up a scan tool to further diagnose the issue. The code was P0420.

The first thing I noticed in checking readings from various sensors was that the scan tool was not receiving any signal from front oxygen sensor (got good readings from the rear one though). I plugged the unit into my Dodge Ram and received signals from the front oxygen sensor in it and so I don’t think it’s a scan tool issue.

I would like to believe that the front oxygen sensor is bad or not connected in the OB (and so is responsible for the CEL and code) but don’t know what should normally be expected (is there really no code for no signal from a oxygen sensor?). I plan to check the wires and connector coming from the front sensor, but was curious if others had any experience with this issue.

Thanks in advance for any clues or advice!
 

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257 Posts
I wouldn't rule out the scan tool. If the O2 sensor wasn't reporting at all, I would expect a different fault code.

Does your scan tool show the loop status? Is the car in "closed loop" when you are testing it?
 

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01 Outback LL Bean
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2,429 Posts
What do you mean it's not showing any readings?

These cars have air fuel sensors in the front which hover around 4 volts. Some older OBDII scanners do not report the values of these sensors correctly.
 

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03 H6 OBW & 06 WRX Sportwagon
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17,711 Posts
yeah, not sure in this instance but, in the past, I have definitely read that some inexpensive ebay or harbor freight scanners fail to work with the ISO protocol in Subarus.
 
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