If you have a Japanese Domestic Market E7 (chassis no. starts CT9A....) then the car is only fitted with one oxygen sensor and therefore, a cat replacement pipe will not cause any problems in terms of the ECU. If on the other hand you have a European Spec. E7 (chassis no. starts JMY....) then the car is fitted with two oxygen sensors, one before and one after the cat. Therefore, removal of the cat in this case will result in the ECU thinking that the cat has failed and will subsequently illuminate the engine check light and possibly cause irregular engine running.
The second sensor on the European Spec car (after the cat) is there to check cat efficiency, if you remove the cat the computer will obviously think that the cat is not working correctly. If you have one of these cars (quite a rare model at the moment) and want to remove the cat, then the only simple remedy would possibly be to remove the second sensor from the pipe but leave connected in the loom securing it underneath the vehicle. You would then have to blank off the hole left by the sensor in the pipe, this way the sensor would be in fresh air and fool the computer into thinking the cat was working correctly. This is obviously not and ideal solution.
The twin Lambda sensor system isn't anything new, it was used on the Eclipse. As Elvis said the second sensor after the CAT is used to work out the CAT effiency (in conjunction with the first sensor) and if it goes below a set effiency level (70% effiency on the manual transmission Eclipse, 90% on the Auto) it will throw up a warning light.
I am not sure what effect it will have on the engine running, no doubt it will go into a 'safe mode' and prevent over fueling if the CAT is thought to be knackered. I bow to Elvis's wisdom on that one as she definitely knows what she is talking about! (good to hear from you again Elvis ).
By the way this is different to the E4 which only had a High Temperature sensor in the CAT and not a Lambda sensor.
I have also been told there will be more or different emmission controls on the 'official' CCC E7 which no doubt means the CAT sensors (plus the EGR system).
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