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P0138 error code on a 2004 B P.T. cruiser

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Old 10 Oct 2010, 09:43 am
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Post P0138 error code on a 2004 B P.T. cruiser

I am troubleshooting a 2004 B (non-turbo) P.T. cruiser with 61,711 miles on it and I am getting a P0138 DTC. The downstream oxygen sensor signal is maxed out at 1.27 volts and does not move whether the car is cold or running. What, other than a bad oxygen sensor, could cause this code? I say that because my this is my Mothers car and when the check engine light came on back in December she took it to a muffler shop and she THINKS they replaced this O2S that is throwing up the DTC. She remembers them telling her that the sensor was bad and they did not have one so she went to Autozone and picked up a new one. They installed it and the check engine light came back on shortly after that. The muffler shop told her that if the check engine light came back on after that the catalytic converter was the problem and needed to be replaced.

From my understanding we can get the catalytic converter replaced for free from the dealership since the car is less than 8 years old and has less than 80,000 miles on it. If it is not the catalytic converter we will have to pay the $79.00 diagnostic charge. I just want to make sure it is in fact the catalytic converter before we take it to the dealership.

I bought a scanner with the ability to record, playback and graph data so here is some additional info:

2004 PT Cruiser

DTC (Codes)

P0138 ENG
O2S Ckt High Volts

Battery Disconnect
Within Last 50

MIL Code 18

Not all supported
On-Board Readiness
tests are complete.

Misfire Monitor ok
Fuel System Mon ok
Comp Component ok
Catalyst Mon inc
Htd Catalyst n/a
Evap System Mon ok
Sec Air System n/a
A/C Refrig Mon n/a
Oxygen Sens Mon inc
Oxygen Sens Htr ok
EGR System Mon n/a

Playback Data

ABSLT TPS($10) 14.5
ABSLT TPS($18) 14.5
ENGINE SPD($10) 722
ENGINE SPD($18) 724
CALC LOAD(%) 27.8
MAP("HG) 9.4
IAT(F) 95
ST FTRM1(%) -3.9
LT FTRM1(%) 7.0
O2S11(V) 0.055
ST FTRM11(%) 2.3
O2S12(V) 1.270
MIL DIST(mi) 0
FRAME: 18 TM: 50.6

The P1684 is because the battery was dead and I had to charge it before I could do anything.

One thing that does concern me though is that the voltage output from the upstream oxygen sensor is bouncing all over the place. It ranges from 0.055 - 0.86 volts. Is this normal for an oxygen sensor to fluctuate that much in such a short period of time?

Here is a picture of the graphed data of this sensor:

I am not familiar enough with oxygen sensors to know if what I am seeing there is normal or not. That just looks kind of funny to me.

So to boil it all down to a few simple questions:
1. Could something other than the catalytic converter be causing the high voltage from the "bad" downstream oxygen sensor or could it be something else?

2. Are the voltages coming from the upstream oxygen sensor normal or does that look like it will probably be failing soon?

A few other side notes:
1. When my Mother got the new sensor she bought it from Autozone. I have read a lot of posts from various forums talking about how bad these generic sensors are compared to factory sensors. Do you guys feel the same way?

2. I forgot to mention that the car runs fine. It idles around 750-850 RPM. You can rev it up and it drops back down to idle without missing a beat. It does every 20-30 seconds stumble a little.

Any help would be greatly appreciated!
Thanks in advance!
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"bad oxygen sensor", "check engine light", "error code", o2s, p0138

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