Hi guys how this sounds crazy but....my radiator cap was bad it sucked my hoses vacuum....
Besides i solved my P0420 code its just for the info i ve monitored the second 02 sensor and it stays stable....so maybe this info helps for you guys outthere

 

I don’t understand. How did it suck your vacuum hose?

 

Well the valve in the cap did not function i looked it up on the net and found out during cooling down it can happen

 

i think he means the cap was causing a vacuum to occur on cooldown and collapsing his coolant hoses. Dont know how that affects the obd2 system other than delayed warmup or overheat causing the temps to change rapidly and the ecm fueling that the O2 sensors read

 

This is what i ment with the cap and for the OBD system.....
Well me neither but since then i have no MIL that's why I came to this conclusion... I drove the car for then till now 300 kilometres in miles I think around 150ish???

 

Ah I see. Glad to hear that you’ve fixed the problem.

 

well, don’t be too positive yet because that fault code requires two successive drive cycles in a row to flag the fault and you have to maintain very specific driving conditions in order to run the test that would flag the fault. Not trying to be a Debbie downer here, but I thought I cured my P0420 code 100 times only for it to come back a week or two later when I finally got the conditions right for 2 drive cycles to run the test for the fail and then it flagged the code again.

 

Yes I agree that it's hard to see how a radiator cap or anything in the cooling system can give you a low catalytic converter efficiency code? Plus why only one side? The car has 2 cats and they both should be effected if this is the cause?

But it is fixed so good job and watch what Aarcuda said. I have also fought low cat efficiency codes and so far 100% of the time you will need a new cat!
You did not mention this but I have found that it's very common to have that code as a "pending" code before it turns the light on.

Have you checked for pending codes?
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Well not yet i use a universal obd reader and i have the Chinese mongoose but with the software I have some issues which I need t to tend to

 

And back to the drawing board......so with the cold engine started it sprayed brake cleaner to check the gaskets on leaks engine reacts on it so ....time to renew them.....nah it's not that hard but it's a intense job