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If i just replaced the o2 sensor on my car and i currently have the stock exhaust, could installing a 3" down pipe and the saab sport exhaust possibly mess up the o2 sensors again or am i good. bc i have been reading on how the downpipe can mess up the sensors and the check engine light will come on and i really dont have any money to replace the sensor again
 

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it can sometimes cause a CEL, but it won;t damage the sensor, Usually the ECU needs a tweek, are you planning on ECU upgrades?
 

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I have had this very scenario occur. I have the mp 3" dp on stock ecu. I thought maybe I damamged the sensro during the install, but people only have problems with the front one, not the rear, so it is definatly the ecu. What to do if you don't get an ecu?

1. Buy an OBD II reader that can clear the code. Just clear it out every time it comes on. This will happen often, but if you have the reader, its easy.

2. Live with the CEL and check the code periodically. This is what I'm doing. I got to Advance Auto and they read the code for free. I'll probably do this every other week or so to make sure no new codes come up to trigger the light.
 

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I wonder why you should get a CEL? Does the sensor act slowly or too fast perhaps? Or (thinking) it's because the temperature in the exhaust is lower and it doesn't change as quickly as the alarm says it should?
 

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Temperature is my first guess.
 

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stromer said:
Temperature is my first guess.
O2 sensors have to be hot to work right. The reason why we have 4-wire sensors now is because there is a heater circuit to heat the sensor more quickly. Has to be over 600 degrees F just to start working.

If you make a change that allows more contamination in the exhaust, like fuel or oil, in combination with an already old sensor I suppose you can put it over the edge. On the other hand, if you do something while tuning the engine that allows unburned oxygen in the exhaust, the sensor will read it as "lean", and the ecu is fooled into putting too much fuel into the cylinder, etc...
 

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Thinking about this, I wonder if the sensor is actually getting too hot.

The only reason I say that is because there are tons of people on the boards with 3" downpipes and stock software who have NEVER thrown an O2 code, and it seems like only people with the MP units are having this issue.

MP mounts the first O2 sensor RIGHT after the turbo, while other 3" downpipes have the sensor farther down the line, closer to the stock location.

I am totally guessing on this of course, but since people with the MP unit are having this problem while others are not, and since the MP downpipe has the first sensor mounted very close to the turbo outlet, it wouldn't seem impossible to me that these things are related.
 

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If that is the case, then i was thinking you could probably make something up like the device used to trick o2 sensors....like the little metal tube the you screw your o2 into and then you screw this into the socket, so it creates a small distance between the sensor and the pipe, which is used for desensitizing the unit so it doesnt throw a CEL, but it could also take the sensor away from direct heat, so it may help, but maybe its just to hot anyway...
 

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I would be curious to see a show of hands of people this CEL has happened to and what brand downpipe they have. This information would be important to me seeing as I am going to purchase one shortly. I also had another question. Are all the front O2 sensors on the Mp Downpipes located directly after the turbo? I was told it would be close to my stock location, which is down by the transmission.
 

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The front O2 sensor is used to control how much fuel the ecu puts in the engine. Desensitizing the front sensor might be a bad idea for performance, because electronic engine control systems really depend on that input being accurate. An old sensor which is "sluggish" can also result in bad performance, because it reacts too slowly to changing engine conditions. The rear sensor does not matter as much, it's only purpose is to detect when the cat converter finally stops doing its job.

http://www.autotap.com/articles/Understanding_Oxygen_Sensors.html
 

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On my car, the MP downpipe actually moved the O2 sensor back in the line compared to stock. Stock had the O2 sensor pointed verticle on the level part of the downpipe. The MP had it further along, and its pointed at an angle because its on the first bend of the downpipe. Yeah this is only a short distance back, but it negates that theory.

brent, the two most common downpipes seem to be the MP for $399 with Cat; and the jak stoll one for I beleive $350 with cat. I have seen pictures of both and after a few thousand miles, the jak stoll one had some rust on the welds. My MP looks as good as when it went on, about 3,000 miles later.
 

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PMI said:
O2 sensors have to be hot to work right. The reason why we have 4-wire sensors now is because there is a heater circuit to heat the sensor more quickly. Has to be over 600 degrees F just to start working.

If you make a change that allows more contamination in the exhaust, like fuel or oil, in combination with an already old sensor I suppose you can put it over the edge. On the other hand, if you do something while tuning the engine that allows unburned oxygen in the exhaust, the sensor will read it as "lean", and the ecu is fooled into putting too much fuel into the cylinder, etc...
This makes sense about the unburned oxygen causing the ECU to think it's running lean. When my CEL went on, it was diagnosed as the front sensor being the problem. When the CEL was on, my mileage really sucked, worst ever. The shop I took the car to couldn't see a problem with the readings they took. Even though the CEL was read correctly, there was no indication that there was a problem. They eventually found that the flange wasn't tightened down enough and air was coming in at the connection with the turbo. Since they took apart the connection and redid it, it's been ok.

So far, so good, but I still cross my fingers every time I take my car out for a drive.
 
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