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Old 04-06-17, 06:43 AM   #1
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Default Bypassing the catalytic converter / Eliminating Downstream O2 sensor

Hello

I hope we can make this sticky.


Today I was able to bypass my downstream O2 sensor after a lot of trials and experiments. Now the downstream sensor is gone with all it's errors and the check engine light from a bad downstream sensor will never come back. And of course you will never need to change it.

There's a lot of methods on the web, but this one is what worked for me. Here's the diagram:



Just follow the diagram, be careful to cut the wires after the connector from the O2 sensor side. Here's an image for the job, I know it looks messy but I did it quickly for testing.



Make sure to isolate the 5 ohm resistor from anything as it gets very hot.
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Old 04-06-17, 08:54 AM   #2
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Seems pointless to me as you need it to work to pass the emissions each year. They are not that expensive either.
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Old 04-06-17, 12:17 PM   #3
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As above why would you do it. Was it the sensor that was faulty if not you may end up with a destroyed CAT or other problems from over/under fueling
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Old 04-06-17, 12:25 PM   #4
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There's a lot of reasons to do it:

- It is not that important to the system.
- You may end up having errors that you can't get rid off, even though your CAT is fine.
- It can be used to test whether the problem you have is from the sensor or from the harness.
- You may just like not to pay for a new sensor.


Cheeeeeeeeeers
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Old 04-06-17, 07:08 PM   #5
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The pre and post sensors work in tandem to control the emissions. So I would have thought that was important!
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Old 05-06-17, 06:04 AM   #6
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The downstream sensor only purpose is to check the catalyst performance. In other words the efficiency of the catalytic converter itself.

It is there to meet certain regulations.
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Old 05-06-17, 08:04 AM   #7
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So by taking the downstream sensor out of the system you have no way of monitoring the cat performance therefore making the ecu unable to adjust parameters when needed. Possibly also making your car no longer compliant with regulations making it unraodworthy by failing to meet said regulations.
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Old 05-06-17, 11:58 AM   #8
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You need to read up on how the system works.
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Old 05-06-17, 02:59 PM   #9
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Unless I've gone colour blind overnight, the schematic doesn't relate to what you show in the image.......

The schematic shows the 25w resistor across two black wires and the resistor & capacitor across The white and grey wires.

You have the 25w resistor across two white wires and the resistor & capacitor across a grey & black wire. Have you got the right connector ?

I'd like to see an MOT testers face if he came across that lot on a test. If I was an MOT tester and saw that I would definitely scrutinise the car just in case there was any other nasties hiding away. There also looks to be another cable that has been butchered. Make me wonder what kind of state the rest of the car is in.
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Old 07-06-17, 06:18 AM   #10
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YES Storeman

You are correct, it was an honest mistake

The 5 ohm-25 watt resistor should replace the heater wires, which are the two white ones.

While the parallel 1 Mohm resistor and 1 uf capacitor should go across the grey and black ones
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