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What are these codes saying?

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I have a 2000 2.7 manual with 151K miles.  The car has been running fine but shows a couple of CEL error codes. P0430 Code 45- Cat conversion bank 2 and P0420 Porsche fault code 40.  What do these codes mean? And what is causing them?


Thank you,


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P0420  TWC Conversion, Bank 1 – Above Limit
P0430  TWC Conversion, Bank 2 – Above Limit


Probable causes:

– Catalytic converter
In the case of a catalytic converter fault in the Boxster (2.7l and 3.2l), only the pre-catalytic converter is to be replaced.

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If it is the pre-catalytic converter that needs to be replaced, what will need to happen?  Does this mean I take it to a exhaust shop and they remove the old one and weld in a new one?  What typically happens in this case?   

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More questions here.  I'm trying to figure out exactly what's going on.  I've done a little research about catalytic converters and how they work.  I've also started looking at actual values on my Durametric software for pre and post O2 readings.  What should I be seeing? I know these are vague questions but I'm just starting to learn about this stuff.  Any help would be greatly appreciated.



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Credit to Brett


The O2 sensors do just that, sense the amount of O2 in the exhaust gas relative to the amount of O2 in ambient air. Perfect combustion of a perfect mixture of air and fuel (around 14.7/1 air/fuel ratio) leaves behind only CO2 and water as products of combustion. All the oxygen gets consumed in the combustion and combines with all the carbons and hydrogens. If there is not enough fuel (lean mixture), then all the fuel gets burned leaving some oxygen left over. Conversely, if there is too much fuel (rich mixture), then all the oxygen gets burned leaving behind extra hydrocarbons (fuel). Now an oxygen sensor outputs a voltage between 0 and about 1 V depending on the difference between the amount of oxygen in the exhaust and the amount of oxygen in normal air. If there is a lot of oxygen in the exhaust (lean mixture condition), the sensor outputs close to 0 volts. Conversely, if there is no oxygen in the mixture (rich condition), then the output is close to 1 V. These O2 sensor voltages are read by the computer. This is the feedback loop that tells the computer how the engine is performing with regard to air/fuel mixture. It's impossible for the computer to hold the exact perfect air/fuel mixture constantly, so the way mixture control is designed is for the computer to continually adjust the mixture from very slightly rich to very slightly lean and back again using feedback from the pre-cat O2 sensors. This means that the pre-cat O2 sensor signal will oscillate back and forth from high to low to high to low voltage as the computer adjusts the mixture. In a normal running engine at idle the signal goes from low to high voltage and vice versa about every 1 second, with a transit time from low to high (or vice versa) being about 200-300 milliseconds. This transit time is important because as an O2 sensor ages, the transit time gets longer, and eventually it can get too long such that the computer will call it a malfunction and signal a check engine light and fault code for a slow responding O2 sensor. O2 sensors need to respond to mixture changes quickly so that the computer can keep up with the proper mixture adjustments.


So the bottom line is that the pre-cat O2 sensors should oscillate between about 0.2 to 0.8 volts regularly (about every 1 second at idle) in a healthy engine.


The post-cat O2 sensors are identical to the pre-cat O2 sensors (same voltage outputs). They are there only to monitor the performance of the catalytic converters. So, as discussed, the pre-cat sensor signals are oscillating between 0.2-0.8 volts. Once the exhaust gasses pass through the catalytic converter, most (all, in theory) excess fuel (hydrocarbons) will be combusted thus reducing hydrocarbon emissions. The cat uses oxygen in the exhaust to combust the fuel. So what you end up with in the exhaust after passing through the cat is a gas mixture that is reduced in hydrocarbons and reduced in oxygen relative to the mixture entering the cat. The post-cat exhaust gas mixture should be CONSTANTLY low in oxygen if the cat is doing its job of burning excess fuel. Therefore, the post-cat O2 sensor signal should be a constant lower voltage reading (not oscillating). So, if the post-cat O2 sensor is seen to oscillate just like the pre-cat O2 sensor, that means that the post-cat sensor is seeing the same gas mixture as the pre-cat sensor meaning that the catalytic converter isn't doing its job of burning excess fuel. The computer monitors the post-cat sensor and compares it to the pre-cat sensor. If the signals are similar, it assumes the cat is bad.

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Thank you Mikefocke for the great info on how this all works.  I knew the purpose of the catalytic converter but now I better understand how it functions.  I need to check some readings with my Durametric, hopefully I find a solution.  

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1 hour ago, Flat6speed said:

Ok, so what do these new codes mean?


Try listing them, the photo is very small and my eyes very old...……….

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Porsche fault code 4-Heating power O2S behind cat. c. bank 2



Porsche fault code 14-Heating power O2S behind cat. c. bank 1



Porsche fault code 361- Oxygen sensing adaptation

idle range bank 2



Porsche fault code 17- Sensor aging behind cat.

conv. cylinder (1-3)



Porsche fault code 23- Sensor aging behind cat.

conv. cylinder (4-6)





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OK, here goes:


P1117 & 1118:  Circuit resistance high on the sensor's heating elements, interruption in the heat circuits, interruption of the sensor's signal wires.

P1130: A/F ratio is rich at idle on bank 2, MAF signal issue, high fuel pressure, leaking injector, EVAP purge valve stuck open.

P0139 & 0159: Both sensors are not changing values as they normally should.


I would do the following:  Using your Durametric, check to see if one sensor on each side is show the normal voltage oscillations at idle.  As you seem to suddenly have sensor signal issues on both banks, I would be looking for a wiring issue.

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