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Old 12-07-2008, 05:02 AM
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Default Help with O2 Readings Needed

Background - this is my daughters 2001 Nissan Sentra GXE that she drove with a missfire due to a faulty ECM. I replaced the ECM and now the check engine light is on. I originally thought she fried one or both of the catalytic converters. The fault code is P0420 catalyst efficiency bank 1 but, aren't these good readings for B1?



B1 S1 .13 to .72 VDC
STFT B1 S1 -2.3 to - 5.4%

B1 S2 .13 to .14 VDC
STFT B1 S2 99.2% (cycles between rich and lean but mostly lean)

B2 S1 .11 to .61 VDC
STFT B2 S1 -.7 to -3.1% (reads lean)

B2 S2 .12 to .16 VDC
STFT B2 S2 99.2% (cycles from rich to lean)

Can anyone advise on the condition of the O2 sensors and catalytic converters given the above info? The car has two converters and I assume the first is a precat or bank 1. Thanks for the help.
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Old 12-07-2008, 05:48 AM
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The convertor is attached to the exhaust manifold on this model. You want to monitor the sensors in a line graph to see what the pattern looks like. If the pattern on the rear o2 sensor start to mimick the front then you have a bad cat. The rear sensor pattern should be almost a straight line until you snap the throttle and should only increase momentarily and go back to a straight line.
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Old 12-07-2008, 09:47 AM
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Doesn't the steady voltage output of sensor 2 indicate the cat is doing its job? Thanks.
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Old 12-07-2008, 01:16 PM
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Quote:
Originally Posted by Walt911 View Post
Doesn't the steady voltage output of sensor 2 indicate the cat is doing its job? Thanks.
In theory. However monitoring by #'s does not give you a clear picture of what the sensor is doing. In a graph form you can see in greater detail the fluctuation of the rear sensors. If the computer is showing a p0420 code, there is almost definately a problem with the convertor.
This vehicle has B1 and B2 convertors side by side directly off the manifold. They can not be separated.
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Old 12-07-2008, 07:02 PM
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Since the ECM has been replaced on the car would be of any value to have the unit reprogrammed. I understand the ECM can become over sensitive and give false PO420 codes.

Thanks for all your helpful answers.

Walt

Last edited by Walt911; 12-08-2008 at 01:49 AM.
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Old 12-08-2008, 02:23 AM
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Quote:
Originally Posted by Walt911 View Post
Since the ECM has been replaced on the car would be of any value to have the unit reprogrammed. I understand the ECM can become over sensitive and give false PO420 codes.

Thanks for all your helpful answers.

Walt
Honestly, I have never seen a nissan throw a p0420 code that didn't need cat convertor.
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