Originally posted by Big Jim:
...I have no idea what was in the mind of the engineers when they put the downstream oxygen sensors (cat monitor sensors) in front of the main cat. It is a mystery to me.....
I can speculate.
If the downstream O2 sensor is behind (after) the main cat, then if you get a P0420 code, you can't determine if it is a bad Bank 1 precat, Bank 2 precat or Main cat issue. You have to change all three to fix the problem.
The way it is designed now, you can determine if it is Bank 1 or Bank 2 precat going bad. Being able to determine 2 out of 3 isn't too bad.
Of course, the problem would be solved if they had 2 precats and 2 "main" cats.
Then, with an upper O2 sensor being above the Bank 1 precat and a lower O2 sensor being below the Bank 1 main cat, if the code shows up, this bank needs a new precat and main cat. ditto for Bank 2.
I'm sure an additional main cat, piping and flex pipe would cost a little too much for it to be acceptable to the accountants at Ford.
"Always do the cheap and easy ones first."
1996 V6 ATX 96K miles
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