Well I got a check engine light P0420 bank 1, I am more less going with my thought that the car has 91,000 and it has never had the o2 replaced and it is now due for a replacement. This seems like the problem to me since the car is running a slight bit more sluggish than normal.

2009 Ford Mustang V6
 
Another thought: The sensor is OK and what it is suppose to sense is bad.
 
Im getting that code too. 
Pacesetter header, Hi-flo cat (magnaflow something), pacesetter catback 
I had the same setup on my sunfire and it didnt throw a code (had it on there for maybe 8 months... i think). Im thinking o2 sensor on mine, just cant spring for a new one quite yet.
 
I would doubt the sensor is okay at 91,000 they should usually be changed around 60,000 I thought.

2009 Ford Mustang V6
 
You may be right. Im not sure on a scheduled change or anything for those sensors, but if youre getting  an o2 code at 91k id assume the sensor is in need of replacing. 
I was at 73k on my 'fire and im at 75k on my Cav. 
However, i deleted mine a while ago and it took like 2 weeks (about 1000 miles) for it to come back. I just havent done anything about it since lol.
 
Well it came back so I used my friends live data scanner and my rear o2 is pretty much dead.

2009 Ford Mustang V6
 
I am also having this problem I have a Pacesetter header, Magnaflow cat and custom bent  2.5" exhaust all the way back. The car only has 38000 on it and the engine has like 9000. What can be the problem? The cat is only like 6 months old.
We Ride Together, We Die Together...... J-Bodies For Life!
SPONSORED BY: Showstopper Tuning, JM Graphics
2003 Cavalier LS Sport "SHOCKWAVE"
 
Quote:
Well it came back so I used my friends live data scanner and my rear o2 is pretty much dead
What did the data scanner show you?
The voltage of the rear sensor does not change much when it is OK. If it changes a lot like the front sensor, then the catalytic converter is bad.
On my cavalier which has no problems and just passed a DriveClean emission test, the rear O2 sensor behaves as follows: During city driving, the voltage is between 0.6 and 0.8 volts. When driving on the highway at a steady speed for 10 minutes, the voltage slowly drops to about 0.45-0.5volts. When coasting from highway speed to a almost standstill (no brakes) the voltage goes down to zero volts. And the voltage rises quickly to about 0.7volts when I start driving again.
 
 
i also recived this code i have no exhaust modifactions the car has like 124,000 miles and i do not know if eitrher 02 sensor has been replaced i don't have to run emmisions and the engine is running better than normal i resrt the code and other is there something i need to worry about or can i drive the car untill it becomes a problem 
there is no smoke coming out of tail pipe and oil maintains the same level
 
 
I got this after replacing my ICM since I was getting P0300 (Multiple Random Cylinder Misfire). It cleared the misfire up but I got this code after about 25 miles. What are the chances it got a bad reading to the O2 sensor after the cylinders were misfiring?
I only drove the car about a half of a mile after I got the CEL and before I replaced the ICM.
Dan Trump

2004 Cavalier LS Sport - Victory Red
 
I had to replace my ICM about a week ago.  I was driving down my driveway and it popped and dropped 2 cylinders.  Ever since then I had the P0420 code.  Last night I changed the front O2 sensor hoping that's what it means by bank 1.  When I pulled it out, it was more carboned up than any sensor I've seen.  The only thing I can think of is that when I dropped the cylinders I washed the sensor with raw gas and burned it up.  
I'll see what happens, so far 20 miles on new sensor and no code.  If not.......it had 65k on it....it was due for a change.
 
P0420 is catalyst below threshold efficiency, which basically means that the catalytic converter is not doing its job and filtering all of the stuff that its supposed to. Many times this will come on when you have a high flow cat or when you don't have a cat at all, as far as checking voltage on an O2 sensor it never surpasses more than 1volt it usually stays between .4 and .8 volts. 
 
 
Yep....light came back on.....so much for the easy fix....the cat is dead.
Time to get hpt .....
 
Could also be the sensor behind the cat 
I am looking in SI right now and it says it could be a bad sensor or cat
I would start with the 2nd sensor first then do the cat
 
 
Any one found a answer to this yet
 
When I had this code it was my downstream o2 sensor.  I put a Walker o2 in and now its all good and is now reading correctly.
 
Just replaced downstream after I put headers and catback upstream have not replaced yet
 
O2 sensors don't expire and quit working when they're old. Easy way to check what the problem is with emissions is this. Warm up your car thoroughly. Use an infrared thermometer and point it at your catalytic converter, front and back of it. There should be a 100-200F difference, the back should be hotter. This would indicate that the converter is catalysing Nox and such. If it's less than 50F difference then the converter is bad. Take the temperature readings with the car running.
 
I'm still trying to fight this problem on my car.  I used my scan tool and recorded the both 02 sensor readings.  This was recorded after the car had been run up to operating temp for a while.  I also had just changed the plugs and seafoamed the car before I recorded this.  I notice that the front 02 sensor seems to be working right.  Bounces from low to high.  The rear 02 sensor seems to telling the car I'm running rich.  There are also a couple spots where the rear 02 voltage goes over 1 volt and sometimes drops down to .1 or .2 volts Thoughts?
Recorded Data
O2S11 V        0.115
O2S12 V        0.700
Frame  -5 Time  -0.4
O2S11 V        0.125
O2S12 V        0.695
Frame  -4 Time  -0.3
O2S11 V        0.165
O2S12 V        0.690
Frame  -3 Time  -0.2
O2S11 V        0.505
O2S12 V        0.685
Frame  -2 Time  -0.2
O2S11 V        0.800
O2S12 V        0.680
Frame  -1 Time  -0.1
O2S11 V        0.880
O2S12 V        0.680
Frame   0 Time   0.0
O2S11 V        0.860
O2S12 V        0.735
Frame   1 Time   0.2
O2S11 V        0.820
O2S12 V        0.750
Frame   2 Time   0.3
O2S11 V        0.710
O2S12 V        0.750
Frame   3 Time   0.4
O2S11 V        0.250
O2S12 V        0.740
Frame   4 Time   0.4
O2S11 V        0.130
O2S12 V        0.735
Frame   5 Time   0.5
O2S11 V        0.115
O2S12 V        0.725
Frame   6 Time   0.6
O2S11 V        0.115
O2S12 V        0.710
Frame   7 Time   0.7
O2S11 V        0.125
O2S12 V        0.700
Frame   8 Time   0.8
O2S11 V        0.245
O2S12 V        0.700
Frame   9 Time   0.9
O2S11 V        0.570
O2S12 V        0.695
Frame  10 Time   1.0
O2S11 V        0.800
O2S12 V        0.695
Frame  11 Time   1.0
O2S11 V        0.855
O2S12 V        0.695
Frame  12 Time   1.1
O2S11 V        0.835
O2S12 V        0.725
Frame  13 Time   1.2
O2S11 V        0.745
O2S12 V        0.735
Frame  14 Time   1.3
O2S11 V        0.375
O2S12 V        0.730
Frame  15 Time   1.4
O2S11 V        0.145
O2S12 V        0.725
Frame  16 Time   1.5
O2S11 V        0.120
O2S12 V        0.715
Frame  17 Time   1.6
O2S11 V        0.120
O2S12 V        0.705
Frame  18 Time   1.6
O2S11 V        0.125
O2S12 V        0.700
Frame  19 Time   1.7
O2S11 V        0.190
O2S12 V        0.695
Frame  20 Time   1.8
O2S11 V        0.460
O2S12 V        0.690
Frame  21 Time   1.9
O2S11 V        0.745
O2S12 V        0.685
Frame  22 Time   2.0
O2S11 V        0.855
O2S12 V        0.685
Frame  23 Time   2.1
O2S11 V        0.845
O2S12 V        0.700
Frame  24 Time   2.2
O2S11 V        0.790
O2S12 V        0.730
Frame  25 Time   2.2
O2S11 V        0.635
O2S12 V        0.730
Frame  26 Time   2.3
O2S11 V        0.275
O2S12 V        0.725
Frame  27 Time   2.4
O2S11 V        0.130
O2S12 V        0.720
Frame  28 Time   2.5
O2S11 V        0.115
O2S12 V        0.710
Frame  29 Time   2.6
O2S11 V        0.115
O2S12 V        0.700
Frame  30 Time   2.7
O2S11 V        0.120
O2S12 V        0.690
Frame  31 Time   2.8
O2S11 V        0.215
O2S12 V        0.680
Frame  32 Time   2.8
O2S11 V        0.575
O2S12 V        0.680
Frame  33 Time   2.9
O2S11 V        0.715
O2S12 V        0.675
Frame  34 Time   3.0
O2S11 V        0.745
O2S12 V        0.670
Frame  35 Time   3.1
O2S11 V        0.735
O2S12 V        0.670
Frame  36 Time   3.2
O2S11 V        0.610
O2S12 V        0.670
Frame  37 Time   3.3
O2S11 V        0.220
O2S12 V        0.670
Frame  38 Time   3.4
O2S11 V        0.125
O2S12 V        0.670
Frame  39 Time   3.5
O2S11 V        0.120
O2S12 V        0.665
Frame  40 Time   3.5
O2S11 V        0.130
O2S12 V        0.665
Frame  41 Time   3.6
O2S11 V        0.145
O2S12 V        0.665
Frame  42 Time   3.7
O2S11 V        0.360
O2S12 V        0.660
Frame  43 Time   3.8
O2S11 V        0.705
O2S12 V        0.660
Frame  44 Time   3.9
O2S11 V        0.820
O2S12 V        0.655
Frame  45 Time   4.0
O2S11 V        0.820
O2S12 V        0.655
Frame  46 Time   4.1
O2S11 V        0.795
O2S12 V        0.660
Frame  47 Time   4.1
O2S11 V        0.640
O2S12 V        0.665
Frame  48 Time   4.2
O2S11 V        0.250
O2S12 V        0.675
Frame  49 Time   4.3
O2S11 V        0.130
O2S12 V        0.670
Frame  50 Time   4.4
O2S11 V        0.115
O2S12 V        0.670
Frame  51 Time   4.5
O2S11 V        0.115
O2S12 V        0.660
Frame  52 Time   4.6
O2S11 V        0.120
O2S12 V        0.640
Frame  53 Time   4.7
O2S11 V        0.125
O2S12 V        0.635
Frame  54 Time   4.7
O2S11 V        0.215
O2S12 V        0.635
Frame  55 Time   4.8
O2S11 V        0.605
O2S12 V        0.635
Frame  56 Time   4.9
O2S11 V        0.835
O2S12 V        0.635
Frame  57 Time   5.0
O2S11 V        0.890
O2S12 V        0.635
Frame  58 Time   5.1
O2S11 V        0.835
O2S12 V        0.635
Frame  59 Time   5.2
O2S11 V        0.775
O2S12 V        0.635
Frame  60 Time   5.3
O2S11 V        0.390
O2S12 V        0.635
Frame  61 Time   5.3
O2S11 V        0.150
O2S12 V        0.635
Frame  62 Time   5.4
O2S11 V        0.120
O2S12 V        0.635
Frame  63 Time   5.5
O2S11 V        0.115
O2S12 V        0.620
Frame  64 Time   5.6
O2S11 V        0.115
O2S12 V        0.605
Frame  65 Time   5.7
O2S11 V        0.125
O2S12 V        0.600
Frame  66 Time   5.8
O2S11 V        0.160
O2S12 V        0.605
Frame  67 Time   5.9
O2S11 V        0.470
O2S12 V        0.610
Frame  68 Time   5.9
O2S11 V        0.755
O2S12 V        0.615
Frame  69 Time   6.0
O2S11 V        0.875
O2S12 V        0.615
Frame  70 Time   6.1
O2S11 V        0.890
O2S12 V        0.615
Frame  71 Time   6.2
O2S11 V        0.885
O2S12 V        0.615
Frame  72 Time   6.3
O2S11 V        0.820
O2S12 V        0.625
Frame  73 Time   6.4
O2S11 V        0.485
O2S12 V        0.625
Frame  74 Time   6.5
O2S11 V        0.165
O2S12 V        0.630
Frame  75 Time   6.5
O2S11 V        0.120
O2S12 V        0.630
Frame  76 Time   6.6
O2S11 V        0.115
O2S12 V        0.630
Frame  77 Time   6.7
O2S11 V        0.125
O2S12 V        0.625
Frame  78 Time   6.8
O2S11 V        0.180
O2S12 V        0.625
Frame  79 Time   6.9
O2S11 V        0.510
O2S12 V        0.625
Frame  80 Time   7.0
O2S11 V        0.735
O2S12 V        0.625
Frame  81 Time   7.1
O2S11 V        0.800
O2S12 V        0.625
Frame  82 Time   7.1
O2S11 V        0.765
O2S12 V        0.625
Frame  83 Time   7.2
O2S11 V        0.680
O2S12 V        0.630
Frame  84 Time   7.3
O2S11 V        0.345
O2S12 V        0.635
Frame  85 Time   7.4
O2S11 V        0.155
O2S12 V        0.640
Frame  86 Time   7.5
O2S11 V        0.125
O2S12 V        0.640
Frame  87 Time   7.6
O2S11 V        0.120
O2S12 V        0.640
Frame  88 Time   7.7
O2S11 V        0.125
O2S12 V        0.635
Frame  89 Time   7.8
O2S11 V        0.210
O2S12 V        0.635
Frame  90 Time   7.8
O2S11 V        0.575
O2S12 V        0.635
Frame  91 Time   7.9
O2S11 V        0.775
O2S12 V        0.635
Frame  92 Time   8.0
O2S11 V        0.840
O2S12 V        0.635
Frame  93 Time   8.1
O2S11 V        0.805
O2S12 V        0.635
Frame  94 Time   8.2
O2S11 V        0.705
O2S12 V        0.650
Frame  95 Time   8.3
O2S11 V        0.375
O2S12 V        0.655
Frame  96 Time   8.4
O2S11 V        0.145
O2S12 V        0.660
Frame  97 Time   8.4
O2S11 V        0.120
O2S12 V        0.655
Frame  98 Time   8.5
O2S11 V        0.115
O2S12 V        0.640
Frame  99 Time   8.6
O2S11 V        0.130
O2S12 V        0.630
Frame 100 Time   8.7
O2S11 V        0.255
O2S12 V        0.630
Frame 101 Time   8.8
O2S11 V        0.635
O2S12 V        0.625
Frame 102 Time   8.9
O2S11 V        0.790
O2S12 V        0.620
Frame 103 Time   9.0
O2S11 V        0.800
O2S12 V        0.620
Frame 104 Time   9.0
O2S11 V        0.755
O2S12 V        0.620
Frame 105 Time   9.1
O2S11 V        0.470
O2S12 V        0.620
Frame 106 Time   9.2
O2S11 V        0.180
O2S12 V        0.625
Frame 107 Time   9.3
O2S11 V        0.125
O2S12 V        0.625
Frame 108 Time   9.4
O2S11 V        0.120
O2S12 V        0.620
Frame 109 Time   9.5
O2S11 V        0.130
O2S12 V        0.615
Frame 110 Time   9.6
O2S11 V        0.300
O2S12 V        0.610
Frame 111 Time   9.7
O2S11 V        0.625
O2S12 V        0.615
Frame 112 Time   9.7
O2S11 V        0.815
O2S12 V        0.610
Frame 113 Time   9.8
O2S11 V        0.845
O2S12 V        0.610
Frame 114 Time   9.9
O2S11 V        0.805
O2S12 V        0.630
Frame 115 Time  10.0
O2S11 V        0.765
O2S12 V        0.650
Frame 116 Time  10.1
O2S11 V        0.330
O2S12 V        0.655
Frame 117 Time  10.2
O2S11 V        0.150
O2S12 V        0.655
Frame 118 Time  10.3
O2S11 V        0.120
O2S12 V        0.650
Frame 119 Time  10.4
O2S11 V        0.115
O2S12 V        0.645
Frame 120 Time  10.4
O2S11 V        0.130
O2S12 V        0.630
Frame 121 Time  10.5
O2S11 V        0.155
O2S12 V        0.625
Frame 122 Time  10.6
O2S11 V        0.400
O2S12 V        0.620
Frame 123 Time  10.7
O2S11 V        0.720
O2S12 V        0.620
Frame 124 Time  10.8
O2S11 V        0.830
O2S12 V        0.620
Frame 125 Time  10.9
O2S11 V        0.860
O2S12 V        0.620
Frame 126 Time  11.0
O2S11 V        0.830
O2S12 V        0.660
Frame 127 Time  11.0
O2S11 V        0.755
O2S12 V        0.685
Frame 128 Time  11.1
O2S11 V        0.420
O2S12 V        0.685
Frame 129 Time  11.2
O2S11 V        0.145
O2S12 V        0.685
Frame 130 Time  11.3
O2S11 V        0.115
O2S12 V        0.675
Frame 131 Time  11.4
O2S11 V        0.115
O2S12 V        0.635
Frame 132 Time  11.5
O2S11 V        0.120
O2S12 V        0.620
Frame 133 Time  11.6
O2S11 V        0.145
O2S12 V        0.610
Frame 134 Time  11.7
O2S11 V        0.345
O2S12 V        0.605
Frame 135 Time  11.7
O2S11 V        0.625
O2S12 V        0.605
Frame 136 Time  11.8
O2S11 V        0.755
O2S12 V        0.605
Frame 137 Time  11.9
O2S11 V        0.800
O2S12 V        0.605
Frame 138 Time  12.0
O2S11 V        0.745
O2S12 V        0.605
Frame 139 Time  12.1
O2S11 V        0.485
O2S12 V        0.600
Frame 140 Time  12.2
O2S11 V        0.180
O2S12 V        0.600
Frame 141 Time  12.3
O2S11 V        0.125
O2S12 V        0.595
Frame 142 Time  12.4
O2S11 V        0.115
O2S12 V        0.590
Frame 143 Time  12.4
O2S11 V        0.115
O2S12 V        0.520
Frame 144 Time  12.5
O2S11 V        0.175
O2S12 V        0.490
Frame 145 Time  12.6
O2S11 V        0.455
O2S12 V        0.540
Frame 146 Time  12.7
O2S11 V        0.720
O2S12 V        0.550
Frame 147 Time  12.8
O2S11 V        0.830
O2S12 V        0.565
Frame 148 Time  12.9
O2S11 V        0.830
O2S12 V        0.560
Frame 149 Time  13.0
O2S11 V        0.790
O2S12 V        0.560
Frame 150 Time  13.0
O2S11 V        0.560
O2S12 V        0.565
Frame 151 Time  13.1
O2S11 V        0.190
O2S12 V        0.565
Frame 152 Time  13.2
O2S11 V        0.125
O2S12 V        0.570
Frame 153 Time  13.3
O2S11 V        0.115
O2S12 V        0.525
Frame 154 Time  13.4
O2S11 V        0.115
O2S12 V        0.450
Frame 155 Time  13.5
O2S11 V        0.125
O2S12 V        0.420
Frame 156 Time  13.6
O2S11 V        0.355
O2S12 V        0.460
Frame 157 Time  13.7
O2S11 V        0.755
O2S12 V        0.510
Frame 158 Time  13.7
O2S11 V        0.905
O2S12 V        0.520
Frame 159 Time  13.8
O2S11 V        0.910
O2S12 V        0.565
Frame 160 Time  13.9
O2S11 V        0.900
O2S12 V        0.635
Frame 161 Time  14.0
O2S11 V        0.865
O2S12 V        0.700
Frame 162 Time  14.1
O2S11 V        0.820
O2S12 V        0.710
Frame 163 Time  14.2
O2S11 V        0.700
O2S12 V        0.705
Frame 164 Time  14.3
O2S11 V        0.305
O2S12 V        0.700
Frame 165 Time  14.4
O2S11 V        0.140
O2S12 V        0.685
Frame 166 Time  14.4
O2S11 V        0.115
O2S12 V        0.660
Frame 167 Time  14.5
O2S11 V        0.115
O2S12 V        0.570
Frame 168 Time  14.6
O2S11 V        0.120
O2S12 V        0.545
Frame 169 Time  14.7
O2S11 V        0.135
O2S12 V        0.570
Frame 170 Time  14.8
O2S11 V        0.295
O2S12 V        0.595
Frame 171 Time  14.9
O2S11 V        0.565
O2S12 V        0.600
Frame 172 Time  15.0
O2S11 V        0.775
O2S12 V        0.595
Frame 173 Time  15.0
O2S11 V        0.840
O2S12 V        0.595
Frame 174 Time  15.1
O2S11 V        0.855
O2S12 V        0.600
Frame 175 Time  15.2
O2S11 V        0.820
O2S12 V        0.600
Frame 176 Time  15.3
O2S11 V        0.685
O2S12 V        0.600
Frame 177 Time  15.4
O2S11 V        0.255
O2S12 V        0.600
Frame 178 Time  15.5
O2S11 V        0.130
O2S12 V        0.595
Frame 179 Time  15.6
O2S11 V        0.115
O2S12 V        0.595
Frame 180 Time  15.7
O2S11 V        0.115
O2S12 V        0.570
Frame 181 Time  15.7
O2S11 V        0.115
O2S12 V        0.520
Frame 182 Time  15.8
O2S11 V        0.145
O2S12 V        0.525
Frame 183 Time  15.9
O2S11 V        0.430
O2S12 V        0.555
Frame 184 Time  16.0
O2S11 V        0.725
O2S12 V        0.560
Frame 185 Time  16.1
O2S11 V        0.815
O2S12 V        0.565
Frame 186 Time  16.2
O2S11 V        0.810
O2S12 V        0.560
Frame 187 Time  16.3
O2S11 V        0.755
O2S12 V        0.570
Frame 188 Time  16.4
O2S11 V        0.460
O2S12 V        0.570
Frame 189 Time  16.4
O2S11 V        0.145
O2S12 V        0.570
Frame 190 Time  16.5
O2S11 V        0.115
O2S12 V        0.565
Frame 191 Time  16.6
O2S11 V        0.115
O2S12 V        0.375
Frame 192 Time  16.7
O2S11 V        0.115
O2S12 V        0.240
Frame 193 Time  16.8
O2S11 V        0.130
O2S12 V        0.210
Frame 194 Time  16.9
O2S11 V        0.395
O2S12 V        0.340
Frame 195 Time  17.0
O2S11 V        0.790
O2S12 V        0.430
Frame 196 Time  17.1
O2S11 V        0.900
O2S12 V        0.500
Frame 197 Time  17.1
O2S11 V        0.910
O2S12 V        0.555
Frame 198 Time  17.2
O2S11 V        0.895
O2S12 V        0.605
Frame 199 Time  17.3
O2S11 V        0.865
O2S12 V        0.670
Frame 200 Time  17.4
O2S11 V        0.835
O2S12 V        0.700
Frame 201 Time  17.5
O2S11 V        0.785
O2S12 V        0.710
Frame 202 Time  17.6
O2S11 V        0.680
O2S12 V        0.705
Frame 203 Time  17.7
O2S11 V        0.265
O2S12 V        0.700
Frame 204 Time  17.7
O2S11 V        0.130
O2S12 V        0.690
Frame 205 Time  17.8
O2S11 V        0.115
O2S12 V        0.675
Frame 206 Time  17.9
O2S11 V        0.115
O2S12 V        0.625
Frame 207 Time  18.0
O2S11 V        0.115
O2S12 V        0.585
Frame 208 Time  18.1
O2S11 V        0.120
O2S12 V        0.605
Frame 209 Time  18.2
O2S11 V        0.235
O2S12 V        0.615
Frame 210 Time  18.3
O2S11 V        0.625
O2S12 V        0.620
Frame 211 Time  18.4
O2S11 V        0.820
O2S12 V        0.610
Frame 212 Time  18.4
O2S11 V        0.835
O2S12 V        0.610
Frame 213 Time  18.5
O2S11 V        0.800
O2S12 V        0.605
Frame 214 Time  18.6
O2S11 V        0.670
O2S12 V        0.610
Frame 215 Time  18.7
O2S11 V        0.370
O2S12 V        0.615
Frame 216 Time  18.8
O2S11 V        0.145
O2S12 V        0.610
Frame 217 Time  18.9
O2S11 V        0.115
O2S12 V        0.610
Frame 218 Time  19.0
O2S11 V        0.115
O2S12 V        0.545
Frame 219 Time  19.1
O2S11 V        0.120
O2S12 V        0.540
Frame 220 Time  19.1
O2S11 V        0.145
O2S12 V        0.570
Frame 221 Time  19.2
O2S11 V        0.455
O2S12 V        0.580
Frame 222 Time  19.3
O2S11 V        0.765
O2S12 V        0.580
Frame 223 Time  19.4
O2S11 V        0.865
O2S12 V        0.580
Frame 224 Time  19.5
O2S11 V        0.860
O2S12 V        0.600
Frame 225 Time  19.6
O2S11 V        0.785
O2S12 V        0.635
Frame 226 Time  19.7
O2S11 V        0.545
O2S12 V        0.660
Frame 227 Time  19.7
O2S11 V        0.205
O2S12 V        0.635
Frame 228 Time  19.8
O2S11 V        0.115
O2S12 V        0.530
Frame 229 Time  19.9
O2S11 V        0.125
O2S12 V        0.500
Frame 230 Time  20.0
O2S11 V        0.255
O2S12 V        0.555
Frame 231 Time  20.1
O2S11 V        0.670
O2S12 V        0.570
Frame 232 Time  20.2
O2S11 V        0.050
O2S12 V        0.955
Frame 1287 Time1350.8
O2S11 V        0.555
O2S12 V        0.390
Frame -9881 Time-1990.3
O2S11 V        1.065
O2S12 V        0.185
Frame -23375 Time-346.5
O2S11 V        0.635
O2S12 V        1.145
Frame 12135 Time-1676.0
O2S11 V        0.985
O2S12 V        1.185
Frame -23887 Time-2064.8
O2S11 V        0.740
O2S12 V        0.855
Frame -2244 Time 930.4
O2S11 V        0.545
O2S12 V        0.875
Frame -4231 Time-565.9
O2S11 V        0.160
O2S12 V        0.780
Frame 15835 Time3208.6
O2S11 V        1.225
O2S12 V        0.150
Frame -6863 Time-2108.7
O2S11 V        0.035
O2S12 V        1.165
Frame -20253 Time-1667.6
O2S11 V        0.035
O2S12 V        1.220
Frame -12325 Time-351.8
O2S11 V        0.930
O2S12 V        0.065
Frame -24638 Time-2548.9
O2S11 V        0.080
O2S12 V        0.080
Frame -19115 Time-2503.6

 
 
If your downstream 02 is bad, your car will not run right. This is becuase the sensor will not be sending the correct info to the computer to make changes. HOWEVER, if your cat is bad, the car should still run relatively fine. The reason for this, is becuase the computer is getting signals from the downstream o2 saying "WOW something is wrong" and correcting it. A bad cat will probably lower ur gas milage a bit too.
I have the same code right now, and when I scanned it, i got a printout of things...this link shows exactly what the print out said.
P0420 Print out from AutoZone after scanning my car
 
 
So for the hell of it I checked my sensors again.  The rear was now bouncing around like the front 02 does!  I'm confused as hell and I don't know if I should replace that sensor or not.  It may have to go to the dealer just to have them check it out.

 
 
a cat is cheaper then an 02 sensor....so figure out what one you think it is most, and start there...i would personally start with the cat, because a faulty cat can cause the 02 sensor to do funny things.
 
 
seriously?  Cat is cheaper?  A GM one is over 500 bucks.  Where did you find it cheaper?  An o2 sensor from GM is $184.  I can get a bosch one from autozone from autozone for 80 something.  The cat you put on in your link I can't use because I'm on a stock exhaust.  Using magnaflow's website I found a universal 2" cat but that's still over 200 bucks.  I hate cars.  I'm buying a horse.

 
 
Ah ok then. well mine was under 80 bucks shipped for the Magnaflow 2.25 that i bought...my bad...
but on that note, somethign weird jsut happened....I got in my car, turning it on NO CODE....turned it off and back on...still no code...HMMM...nothign has been replaced yet, and i DID NOT clear the cod the night i had it scanned
