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Posted by Darren Bamford on February 23, 2007, 11:45 pm, in reply to "Re: Low cat efficiency question. "
I'm with Alan on this subject, If the Upstream sensor it not working correctly the the vehicle will drop into open loop and Catalyst monitoring will be inactive. Also the drive cycles needed to complete the CAT efficiency test's are very complex and if the upstream is not perform correctly it would flag a upstream code well before it logged P0420. I can honestly say that Every P0420 code I have come across as been down to the CAT efficiency being low or ECU software weakness.
when the vehicle is fitted with a down stream sensor, the ECM will use the signal to monitor the efficiency of the catalyst. The state of the catalyst is assessed in line with its ability to ‘ hold’ Oxygen. In a serviceable unit, the ‘ excess’ Oxygen in the exhaust gas is held on the surface of the precious metal coating of the ceramic blocks within the catalyst. This Oxygen is used to convert the harmful elements produced by incomplete combustion (particularly during acceleration and conditions where the engine requires a rich air/fuel ratio) into Carbon Dioxide, Nitrogen and water.
By comparing the signals received from the pre-catalyst sensor with those received from the post-catalyst sensor, the ECM can calculate how much Oxygen has been retained by the catalyst and
can, therefore, determine its condition.
If the ECM determines that the catalyst requires replacing,it will store a fault code.
HTH
Darren............................
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| Message Thread:
- Low cat efficiency question. - Joe McGeoghan February 21, 2007, 1:32 pm
- replies - Jim Watson February 21, 2007, 3:29 pm
- replies - Graham Bullock February 21, 2007, 5:22 pm
- replies - Ed Kaps February 21, 2007, 5:50 pm
- replies - JOe McGeoghan February 21, 2007, 6:04 pm
- replies - Michael Whitworth February 21, 2007, 8:05 pm
- replies - mark parkinson February 23, 2007, 8:31 pm
- replies - Alan Kane February 23, 2007, 9:32 pm
- replies - Tom Yates February 23, 2007, 9:47 pm
- replies - Alan kane February 23, 2007, 10:01 pm
- replies - Darren Bamford February 23, 2007, 11:45 pm
- replies - Mark Parkinson February 26, 2007, 5:58 pm
- replies - Darren Bamford February 26, 2007, 9:06 pm
- replies - Alan Kane February 26, 2007, 11:00 pm
- replies - Mark Storey February 27, 2007, 12:05 pm
- replies - Ian McCulloch February 26, 2007, 12:14 pm
- replies - john smithson February 26, 2007, 10:43 pm
- replies - Ian McCulloch February 27, 2007, 9:03 am
- replies - John Kennard February 27, 2007, 12:38 pm
- replies - john smithson February 27, 2007, 2:57 pm
- replies - UKAT5576 February 27, 2007, 4:58 pm
- replies - UKAT2680 March 5, 2007, 11:16 pm
- replies - Joe McGeoghan February 28, 2007, 7:50 am
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