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2012 Forester Automatic
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Discussion Starter · #1 ·
Hi All,

Just replaced the upstream O2 sensor on our 2012 Forester (150k miles), due to the following codes:


  • P0420 Catalyst System Efficiency Below Threshold
  • P0171 System Too Lean
  • P014D Manufacturer Defined - O2 Sensor Slow Response - Lean to Rich (Bank 1 Sensor 1)
  • P014C Manufacturer Defined - O2 Sensor Slow Response - Rich to Lean (Bank 1 Sensor 1)

They were kicking on the Check Engine and Traction Control lights, and flashing the Cruise light. I also noticed that the O2 B1S1 voltage was reading 0.00 all the time. B1S2 showed different various readings within normal range. P0171 was showing set as a permanent code.

I replaced the sensor with a Denso unit and reset the codes, and all seems well with the car (better, lower idle, etc.) -- but the voltage on O2 B1S1 still always shows 0.00V, even though the P0171 cleared. Also tried a different reader with the same result.

I didn't disconnect the battery to reset the ECM as of yet... but I wondered if there was something that was either keeping the system in closed loop, or causing it not to show the B1S1 voltage.

Any ideas, hypotheses or suggestions?

Thanks!

Any ideas?
 

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2012 Forester Automatic
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Discussion Starter · #2 ·
Additional info... OBD Fusion shows under "Readiness Standard" Test Results that the Catalyst test is 'available' but not 'complete'. I am guessing this is related. Does anyone know of a way to kick in that test, or is it just a 'number of cycles' kind of thing? Thanks.
 

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Discussion Starter · #3 ·
Ok, found how to kick in the monitor test... but is that any reason why the O2 sensor would not be showing any signal?
 

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2012 Forester 4 speed auto
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1,146 Posts
Your O2 sensor won't produce voltage if the exhaust is TOO LEAN. Do you have a hole in the exhaust manifold, bad gasket, engine misfiring?
 

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2012 Forester Automatic
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Discussion Starter · #6 ·
Thanks for the reply, that's good to know. I am unaware of any leaks & it's very quiet as well. It is running better/smoother than before I changed the front sensor, though the front sensor was reading 0's before the change as well. The rear sensor is pulling readings, and currently not throwing any codes except for he Heated Catalyst Monitor status). Would the rear still show a non-zero reading if it was too lean? Thanks for the help!
 

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2012 Forester 4 speed auto
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Check the wiring and the connector on engine harness. The engine is still in OPEN loop. That is, it isn't using the O2 sensor to regulated fuel and air. Or every thing is fine and your reader isn't reading. Check the contacts on your OBD reader
 

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Discussion Starter · #8 ·
Thank you again. I did check again the lower wiring connector near the sensor, and it is in good condition and secure. I also tried a few different readers with no luck. By chance do you know if there is an 'upper' connector somewhere for that sensor that could be the issue?
 

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2012 Forester 4 speed auto
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Did you replace the Upstream sensor with part dense part 234-9109? Are you sure its the upstream and not the downstream sensor. People have been known to confuse the two. The plugs are different so you cannot replace an upstream with a down stream sensor or visa a versa.
 

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Discussion Starter · #10 ·
Thanks -- yes, Denso 234-9109 Air Fuel Sensor. Green wire, gray plug, with a clip to hold it in place. I'm going to try and get under there again and make sure the connection is good and secure and that no pins have pushed out of the connectors. I' ll also try and clean and inspect the wiring going up for any damage.
 

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You might try this. Drive the car until the engine warms up and should go into closed loop. Stop the engine, disconnect the Upstream O2 sensor, make sure the connector and wires don't get ruined by the rotating belts, exhaust manifold, and restart the engine. The engine should go into open loop, run rougher, may even stall. Only run the engine for a very short time. You will ruin the Cat.

What have you learned? The O2 sensor is working, the engine was in Closed loop and your OBD reader isn't working.

If nothing changes then the car is in open loop, the sensor doesn't working and your reader is working.
 

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Discussion Starter · #12 ·
Still no voltage readings from the upstream sensor through OBD, but the Catalyst Monitor is now in a ready state. I'll just take that as a win for now. Thanks Easy Fixer for the help!
 
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