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I am posting on this page as a site member, although the fault relates to a different vehicle that we own and I am searching for an answer. 


It all started with a cam sensor code and I detected a continuity fault which I have fixed.  This was soon followed by an Oxygen sensor code.


The car is a year 2000 Toyota with a 1000cc engine done 90k miles.  It has a single Oxygen sensor before the CAT.  The code says faulty sensor.  On erasing the code it generally returns within about 40 miles. The car idles well, but just a little too slowly at about 600.  It's smoother at 650/700. 


I have changed the 02 sensor twice to make sure the first changeover was not faulty.  The fault returned.


There are no exhaust leaks between the sensor and the exhaust manifold.


I have cleaned both the mass air flow sensor and the throttle body. 


I have changed the air filter.


I cannot find any vacuum leaks.


Fuel consumption is very good.


Engine monitoring says the car is not ready for testing, even after a new 02 sensor and the engine up to temp. 


A voltage is being detected from the 02 sensor.


Spark intensity is good so are the plugs.


I have not yet done a compression check and I am beginning to suspect a leaking inlet valve.


It passes its emission tests at annual testing time.


Any pointers gratefully received?







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Agree with Electrict. Check mode 6 TID $01 CID $01 (catalyst efficiency) and if the number fails, that's the catalytic converter. Are you sure it's not P0420 though? P0430 is bank2 which I don't think the car has.


The car uses a regular O2 sensor (non-wideband) so you can also monitor the pre-cat and post-cat sensor signal. The post-cat should stay pretty steady at 0.7 at idle after car is fully warmed up. If the post-cat signal swings similar to the pre-cat, that's an indication the cat is bad.


Have you checked for exhaust air leak upstream of the O2 sensors?

Edited by Ahsai
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Have you looked under the car on the exhaust al the way from after cat to the tail pipe to find the post-cat O2 sensor? Is the exhaust stock or is it possible someone has modified and removed the second sensor?


If the car truly has one sensor only, I don't see how it can detect if the cat has gone bad, which P0420 is referring to.

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This is what has foxed me.  I can't match the code to the symptoms. The exhaust looks as if it's an original part and there are no after CAT wires hanging about looking for a home, from what I can see so far.  Furthermore, the exhaust smells OK and the car passes its emission test (UK MOT annual testing).


I will just keep looking to see what turns up.  I am reluctant to spend a lot of money on the project as it's a very low value car.


Thanks you for your replies and if I eventually fix the problem I will let you know.



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