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[feeutfo]

on the v engine, and 99% sure the 4pot as well, the order is... sensor, pre act, sensor, main cat.

The acknowledged fix is to move the 2nd sensor from between the 2 cats to behind main cat. Usually done by welding in a new 02 sensor boss and plugging the old hole with a sump plug or similar.

I guess the part Proz has is pre modified with the sensor pre positioned after the main cat to save welding. Should work if Albatros's fix is anything to go by.

There are some theorys as to why the code is raised in the first place. The usual guess is the pre cat is damaged or no longer efficient. On the v6 there was a theory that a batch of faulty valve stem  seals where allowing oil into the combustion chamber and tripping the code, either on emissions or by cat contamination. See the off with its heads thread by mark dtm on mick dundees car.

Anyway, in theory, that part should work. :-)

[Iain]

I guess my theory is a bit off the mark then.........

However it has to do with the temperature i think.....the pre cat will heat quickly,where as the pipe without the cat wont,therefore the sensor wont get a true reading...
The reason i say this is because,when i went from my house to Hotel21's,which is about a hours drive,we got about halfway there,the EML went out on its own....
So i think it has something to do with how hot it is/going to be

[Proz]

However it has to do with the temperature i think

Yes i think you are right .... mine failed the mot on natural idle but passed the fast idle when nice and hot ..... anyway if this doesnt cure it i will change the O2 sensors even though i dont think there is anything wrong with them and if that fails then its swan vesta time 

[feeutfo]

or drill and plug your original cat..?

[Proz]

Well new cats been on for 2 days now and I/M monitor reports cat monitor ok and no light so hopefully .... where as before the instant readiness wouldnt complete and it would put the eml on with the cat code .... so looks like this might cure my P0420 .

Praying no more light 

[Marks DTM Calib]

Ok,i did a bit of online research and also spoke with a guy over here who makes stainless systems

The first sensor is effectively no use because...... on the factory set up the first sensor takes the measurement from the 'swirling' gases from within the pre cat box.
After it has been through the main cat and treated the measurement is then taken from the 2nd sensor.

As the aftermarket pipe has no pre-cat the gases travelled too fast 'past' the 1st sensor for it to take an acurate measurement.....

Both sensors measures in voltage so because the first sensor doesn't get an acurate reading the voltage fluxuates too wildly to be 'within tolerance' for the system of the car...

The set up should be...

1st sensor....voltage with rise and fall wildly
2nd sensor......voltage will be more constant

Because the 1st sensor is not getting a proper reading then the 2nd sensor will rise and fall wildly because it has nothing to compare it with (from the 1st sensor)

The EML will come on because the car's system registers that there is an emmision problem due to the fact that the first sensor is not getting a proper measurement reading,however it wont flash up a fault with the sensor because it is there and it itself is functioning properly

Well he needs to stick to maiking exhausts then as he clearly has no clue to what lambda sensors do!

The first sensor is THE most importnat one, it is providing feedback to the ECU to effect mixture control.

The second oen is teh redundant one, it does nothing more than monitor the state of the pre-cat....

[Marks DTM Calib]

I guess my theory is a bit off the mark then.........

However it has to do with the temperature i think.....the pre cat will heat quickly,where as the pipe without the cat wont,therefore the sensor wont get a true reading...
The reason i say this is because,when i went from my house to Hotel21's,which is about a hours drive,we got about halfway there,the EML went out on its own....
So i think it has something to do with how hot it is/going to be

Lambda sensors do not work until hot and the cats dont work until hot either. But, lambdas have heaters in them to get them upto temp much quicker.

The idea of a pre-cat is its small and gets hot quick so works faster and hence meets emissions requirements on the heat up cycle

[Iain]

Ok,i did a bit of online research and also spoke with a guy over here who makes stainless systems

The first sensor is effectively no use because...... on the factory set up the first sensor takes the measurement from the 'swirling' gases from within the pre cat box.
After it has been through the main cat and treated the measurement is then taken from the 2nd sensor.

As the aftermarket pipe has no pre-cat the gases travelled too fast 'past' the 1st sensor for it to take an acurate measurement.....

Both sensors measures in voltage so because the first sensor doesn't get an acurate reading the voltage fluxuates too wildly to be 'within tolerance' for the system of the car...

The set up should be...

1st sensor....voltage with rise and fall wildly
2nd sensor......voltage will be more constant

Because the 1st sensor is not getting a proper reading then the 2nd sensor will rise and fall wildly because it has nothing to compare it with (from the 1st sensor)

The EML will come on because the car's system registers that there is an emmision problem due to the fact that the first sensor is not getting a proper measurement reading,however it wont flash up a fault with the sensor because it is there and it itself is functioning properly

Well he needs to stick to maiking exhausts then as he clearly has no clue to what lambda sensors do!

The first sensor is THE most importnat one, it is providing feedback to the ECU to effect mixture control.

The second oen is teh redundant one, it does nothing more than monitor the state of the pre-cat....

Well actually that was my theory really......
I'm no expert on exhausts or Lambda's...so,
sorry

[Iain]

I guess my theory is a bit off the mark then.........

However it has to do with the temperature i think.....the pre cat will heat quickly,where as the pipe without the cat wont,therefore the sensor wont get a true reading...
The reason i say this is because,when i went from my house to Hotel21's,which is about a hours drive,we got about halfway there,the EML went out on its own....
So i think it has something to do with how hot it is/going to be

Lambda sensors do not work until hot and the cats dont work until hot either. But, lambdas have heaters in them to get them upto temp much quicker.

The idea of a pre-cat is its small and gets hot quick so works faster and hence meets emissions requirements on the heat up cycle

So am i correct in thinking then that, because the pipe on mine just now wont be hot enough to get a reading then?

Hence the reason the light is coming on?

[Proz]

The reason i was getting it was the cheap crap VX fitted .... it had no  pre-cat .... mines been fine since the one above was fitted     .... best £130 ive ever spent to get rid of that light ...

[Iain]

Aye,i know the solution/fix for it mate....

Was just trying to get a reason behind it that was all and MarksDTM seemed to no more than me..