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Hello, for a while I have started to get p0420 code. After removing it with OBD reader it will appear again at random intervals, mainly when doing longer journeys. Intervals can be random 800 miles to 60 miles and so on, just random. Might appears more often when engine working harder like on North wales roads. Any ideas what that could be? On MOT done last year November it says there is small exhaust leak, could that be the case? 

 

Also, not sure if problem is related, but sometimes I am getting rough idle where rp falls bellow 700 rpm, and car doesnt feel very smooth, on rare occasions it goes to 500 rpm or even 400 rpm (only had that at cold winter morning before changing spark plugs) and car start to shake. It doesnt happen all the times, noticed it occurs more often on humid days, and colder humid mornings. I have cleaned valve throttle body and changed spark plugs. While ECU is learning there was no rough idles, but soon after learning it start to rough idle again. Any ideas? BTW, car got around 120k miles. 

 

Thank you. 

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There is a good chance given the mileage that the O2 sensor is on its way out however the exhaust leak may also be contributing.

Unless you have live data capabilities to confirm 100% repair the exhaust, replace the O2 sensor with a genuine part and reset the codes. 

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I had a p0420 on my IS200 sport years ago when I owned it and mine was triggered by a tiny pinhole exhaust leak near the o2 sensor 

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You could try a temp repair of the hole in the exhaust and reset and see what happens, very cheap bit of 1st option fault finding if you know that the exhaust has a leak and this code can relate to this.

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Two things normally cause this, either a small leak in the exhaust or a faulty O2 sensor - not the sensor reporting the fault but the upstream sensor.

It could be something else like the MAF sensor, or actual a Cat but normally its exhaust or one of the two upstream O2 sensors.

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31 minutes ago, ColinBarber said:

Two things normally cause this, either a small leak in the exhaust or a faulty O2 sensor - not the sensor reporting the fault but the upstream sensor.

It could be something else like the MAF sensor, or actual a Cat but normally its exhaust or one of the two upstream O2 sensors.

Ok, will try to go for fixing exhaust. After all it was picked up on MOT, however on normal visual inspection we could not see any soot. On app it reads as

OBDII: Catalytic converter system, bank 1 - efficiency below threshold. 

Which side is that?

 

Also any suggestions for that rough idle? 

 

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34 minutes ago, samanaslt said:

Which side is that?

 

Also any suggestions for that rough idle? 

Bank 1 is always the bank that contains cylinder 1. On the IS250 that is the left of the engine (when standing at the front of the vehicle looking at the engine bay (drivers side for a RHD car)).

Given that you also have an idle issue it points more towards the problem being fuel/air related rather than the exhaust, and because you also don't have a P0430 error it seems to be just on one bank so unlikely to be an item common to both banks such as air filter/MAF/intake air leak etc. Therefore Bank 1 Air/Fuel Ratio (O2) sensor is most likely but you should ideal check/test it rather than just swapping it out as it could be a fuel injector or some other item.

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