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Discussion Starter #1
Ok so my PT got these codes
P0138-1/2 O2 SENSOR SHORTED TO VOLTAGE
this should be the downstream sensor.
P0172 FUEL TRIM SYSTEM RICH BANK 1
this would be the upstream sensor.
Anyway..were should i start?
It runs great but has had a tendency to smell gas like and old carbed engine when cold (cold start only)
Likes fuel but not more than expected, i didnt get the turbo for fuel economy....
 

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The downstream O2 sensor does have a hand in determining fuel trim (adaptives).
If the downstream sensor is shorted to voltage, it is pegged high and the PCM will keep adjusting the fuel trim (adding fuel) to bring the voltage down until it can adjust no further. It will then set P0172.
I would start with an OEM downstream sensor. Both upstream and downstream O2 sensor work together with the PCM to keep the fuel mixture happy.
Both sensor signal voltages need to be constantly swinging high/low and can't be pegged high (about 1.2v) or pegged low (about 0v) or staying at the center (about 0.5v) when in warm, closed loop.
The O2 sensor heaters provide quicker sensor warm-up so they can start operating sooner.
Explained here:
http://www.bionicdodge.com/bionic/index.php?topic=9863.0;wap2
 

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Discussion Starter #4
Thanks for the replys, im going to have a look at the car some day (its my sons)
and hook up my reader to it and se whats happening.
Interesting to learn that the downstream sensor changes the fuel trim, i always thought that
it was for checking cat con efficency.
 

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It does that too. Cat efficiency is basically determined by the PCM by comparing the oxygen in to the oxygen out with both sensors.
 

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ImperialCrown said:
It does that too. Cat efficiency is basically determined by the PCM by comparing the oxygen in to the oxygen out with both sensors.
For the techno nerds in the audience, the PCM looks at the switching rates between the downstream sensor and the upstream sensor. When the catalyst is new and working appropriately, it counts and then ratios the down stream switching rate (rich / lean switching) to the upstream switching rate (rich / lean switching) and calculates a ratio. When new the ratio may be at 30% between the two and as the catalyst ages the downstream switching rate approaches the upstream switching rate. When the ratio reaches 90% the PCM sets code P0420 and illuminates the CHECK ENGINE lamp.
 

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Discussion Starter #7
Lets see if i got this right..
The downstream sensor is "high" = it senses oxygen = Lean
Fuel trim is set at rich limit to correct the lean sensor.
The upstream sensor should read rich ("low")
If it behaves like that i can only assume that i have a sensor fault or an air leak into tha exhaust.
 
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