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Discussion Starter #1
I have recently fitted a stainless steel cat back system to my 2001 1.8 16v. Since doing this my engine light has now come on. I have put it on the ethos snap-on reader and it is coming back with a o2 sensor fault, erased code but light came back on after approx 1 mile. Any ideas ? The sensor is in the cat on the manifold so upstream of the new exhaust so I can't understand why it is faulting. Seems a bit strange that the sensor faulting coensides with the exhaust replace.
 

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You might find your new exhaust box provides a greater back pressure, presenting an environment where there is a build up of gases that the O2 sensor will pick up. As the engine tries to change the fuel input to adjust the gas output to the correct level (maintaining the correct fuel to air ratio - stoichiometric rate) it invariably can't cope with the rapid changes or can't control the fuel input quick enough to maintain an average stoiciometric rate and flags up a faulty sensor. So rather than pointing to a particular component going wrong it will point to a condition that the ECU can't control.

It could also be a clogged sensor. The o2 sensor heats up to around 316'c and when o2 passes over it, it compares it to outside o2 and the difference produces a voltage. If the sensor becomes coated in carbon as it can do with poor fuel quality, or higher back pressure this can also throw up faults. Equally the loss of pressure can also create issues. As the probe needs to warm up to full temp the lack of pressure means the probe isn't getting immersed in hot gasses to heat up.

Try turning your engine on for 10 minutes to warm up. Reset the codes then give it some beans. If the code comes up again immediately or you can't reset then it's too much BP. Turn off your engine and leave to cool for an hour then start again. If the code appears there will be too much back pressure.

Has there been any noticeable difference in power of the vehicle, loss or gain?

Sorry for the late reply. Been viewing loads of posts last night. Report back your findings and good luck.
 

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fastjetfixer said:
Try turning your engine on for 10 minutes to warm up. Reset the codes then give it some beans. If the code comes up again immediately or you can't reset then it's too much BP. Turn off your engine and leave to cool for an hour then start again. If the code appears there will be too much back pressure.
I meant to say not enough back pressure on that last sentence. Sorry chap.
 

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Discussion Starter #5
I will give them ideas a try, huge thanks for reply, was starting to think this was a ghost forum. I have noticed a sooty build up around the tailpipe which could indicate overfuel, I had put this down to the car having done 87k and burning a little oil.

Also with regard to the sensor not getting warm enough, would this still apply as my o2 sensor is situated right on the exhaust manifold in the top of the cat so would have though it would get very hot quickly and also plenty of backpressure as the cat restriction is right below it.

very puzzling I know, have fitted this type of system to cars in the past without fault but then again they have all had sporty engine whereas this is'nt,lol.
 

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It's the back box (muffler) that supplies the back pressure, the cat plays a very little part in it as it can block up with carbon deposits. If it clogged up totally then your car wont run at all. The position of the sensor is purely so it measures the o2 readings as soon as the gases enter the manifold and can tell the ECU to adjust the fuel input. If the sensor is cold (which can for all intense and purposes have 'burnt out' or have been caked in carbon), then it will tell the ECU to flood the engine with fuel. Higher level of fueling will inevitably lead to more carbon deposits and a poor exhaust emissions.

If you kept the old muffler, stick it back on and have a look at the lambda. Make sure its clean. There is a way to check if it works but can't remember how or where I saw it. Sorry. But I'm sure google will give you some good info. I'd look for you but rather busy at the mo pal.

Again, let me know how you get on.
 
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