Does the Rear O2 Sensor Effect the Air/Fuel Ratio; No, and Yes - The Saab Link Forums

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Old 03-02-2011, 04:55 PM   #1
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Does the Rear O2 Sensor Effect the Air/Fuel Ratio; No, and Yes

We all know that the front O2 is for fuel trim and the rear is for sensing the catalytic converter's condition. But the rear O2 sensor can effect the Air/Fuel mixture.

I've been trying to fix several CEL codes for the last week; P0132, P0135, P0138, P0141. They would come on shortly after clearing and in general indicate a problem with the O2 sensors or a lean condition. What made diagnosing this problem much simpler was having a wideband O2 gauge. What I was seeing was 17/1-18/1 ratios during idle and light cruising, but when floored it would go open loop and show 11/1-13/1 ratios.

Clearly it was not a mechanical problem since I was seeing the correct fuel under heavy load. First thing I did was replace the front O2 sensor, but this did not change anything. Checking temp sensors with the OBD2 reader also showed all numbers in spec, except the O2 voltage numbers seamed very slow to change.

I measured lots of sensor resistances and tried unhooking them one at a time then cleared the ECU to see how they effected the A/F. The solution came when I unhooked the rear O2 sensor, pulled power to clear the ECU again and started it up. The A/F settled down quickly to 14.7/1.

The rear O2 sensor can effect the A/F when it's heating element shorts or grounds out and stops the front O2 sensor's heating element from working. The front O2 then reads incorrectly and the closed loop goes lean.

Hope this can help someone else in the future. Took me 5 days to figure out.

edit: Thanks NeedForSwede
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Last edited by ERP; 03-02-2011 at 07:59 PM.
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Old 03-02-2011, 04:58 PM   #2
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Oh Yea,.
I had a similar problem several years ago with my 9-3
Took me a little while to figure out as I had no Wide-band

Good info Eric, hopefully it helps someone out in the future.
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Old 03-02-2011, 05:37 PM   #3
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Good info, as i had the p0133,p0130 codes and replaced with brand new bosch front o2 and still getting the CEL... Also gas mileage has been pretty shitty lately too
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Old 03-02-2011, 05:45 PM   #4
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Just to clarify, open loop is when your car is under heavy load aka WOT, or when it's warming up from a cold start. Closed loop is when your car is under partial throttle (normal load driving) or when the car is warmed up at idle.

BTW, T7 suite has 2 ways of turning off the rear o2 sensor. I used one of those ways to turn mine off. Therefore, my afr's should be fine, correct?
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