P0420 code after replacing cat and sensors

[dennis10]

Hi all, previous owner had a decat pipe on my corolla tsport which I've just replaced because I didn't want the hassle of emissions failure come MOT time.

I replaced with a whole new Klarius mid section, which includes the catalytic converter and center silencer. I also replaced both o2 sensors with Denso items.

I've driven the car about 50miles and the EML plus VSC+TRC have come on again. My code reader shows P0420 again.

I work for a fire and rescue authority and we have a workshop with emissions tester so just yesterday I asked if they could test it for me to make sure it was all working as it should. They had to get the cat hot, but it went through and would've passed an MOT. Then I get this code again today and I'm now stumped.

 

Is there anything else that can cause this code, dodgy MAF maybe?

[AisinW]

Toyota catalytic converter is very good in term of lowering emission. I never have any good luck with aftermarket Catalytic converter with modern hybrid toyota.  The emision treshold for Toyota hybrid is much lower than regular cars and there is a reason why the original cat is so much more expensive. It is because it contains much higher catalyst quantity and quality. 

The only way to solve P0420 is installing original catalytic converter.  Otherwise, we will waste too much money on garbage aftermarket metal pipes.  The O2 sensors are rarely failed and P0420 is 99.99% is always the catalytic converter itself. The O2 sensor code is very specific and different. 

A catalytic converter that may be good enough for other brands are never good enough for Toyota Hybrid. 

[dennis10]

I guess it could be an exhaust manifold leak? So definitely a faulty MAF sensor wouldn't affect the output of exhaust?

I understand what you're saying about aftermarket catalytic converters, but I had it tested at my works workshop and it passed, although they did have to get it hot.

[Gerg]

2 hours ago, dennis10 said:

I guess it could be an exhaust manifold leak?

As I understand it, an exhaust leak, particularly near the O2 sensors can cause this type of problem.

2 hours ago, dennis10 said:

tested at my works workshop and it passed

I'm just a Toyota owner, with little practical experience of replacing cats and O2 sensors, so allow for that in what follows! 

Passing the emissions test to MoT standards does not mean that the engine is meeting its own references judged from the the car's O2 sensors.  For example, the MoT emissions test is performed at tickover (isn't it?, it's ages since I stood and watched one being done!).  The car will be judging the O2 sensors when in normal operation, not just at tickover.  

If the catalyst is undersized/poorly-specced, then this will become more apparent at higher gas flows, so an Mot-style test might flatter the performance of the cat.

The first O2 sensor is used to adjust fuel mixtures, the second is used to test for catalyst function (by looking for a phase-shift in the oscillating mixture adjustment, that the car is endlessly performing after warm-up).  There are some exceptions to this but it is rare, and I don't think Toyota breaks this convention.

I've just plucked this below link off the internet as an illustration.  It's not your car, I realise, and the owner is using an O2 sensor extension unsuccessfully, and this car has three O2 sensors, apparently!  But, if you can see beyond those (!), then might be something to be learned from what this bloke's doing?

https://mechanics.stackexchange.com/questions/67740/is-it-possible-for-an-oxygen-sensor-spacer-to-cause-a-car-to-stall

An O2 sensor extender is cheap, and might get you around your problem.  But, I've not used one myself, so buyer beware!

Very happy to be corrected if anyone knows differently, of course!

[AisinW]

@dennis10  I understand your argument. It is true that MOT test will be fine but not your own car ECU test.  HSD ECU required much stringent tolerance for the emission and always give us headache with aftermarket catalytic converter.  Your code is P0420, it is absolutely the catalytic converter.  The O2 sensors sense higher NOx or CO on the 2nd O2 sensors.  if it is MAF related, it will be P01xx. 

@Gerg Placing any spacer in post Toyota 2008 will not do any trick.  The ECU periodically send rich-lean-rich-lean mixtures intentionally to test if anyone cheat the O2 sensors.  By doing that, if the 2nd O2 sensors does not sense rich-lean-rich-lean but constant reading, the P0420 and other codes about O2 sensors will be permanently ON. It will never work either unless we have much older Toyota. 

[dennis10]

Many thanks for your replies and advice, both!

My T-sport is the same as an XRS, the 2ZZ-GE engine, 2005 model.

Would you think this engine will have the anti tamper checks you speak of, rendering the use of an o2 sensor spacer redundant, or would it work with my model?

The other issue is come MOT time. I'm not sure if the cat would be inspected and then failed if a spacer was discovered during an MOT in the UK, I suspect yes.

So my options look like either living with the problem and then clearing the codes before MOT time or spending a lot of money on an older car to buy a genuine TOYOTA cat.

Thanks for advice

[AisinW]

2005 Model may work with the O2 spacer, we only need spacer on the 2nd O2 sensor. Do not put anything on the 1st O2 sensor, it is very important for the close loop mixtures. MAF, 1st O2 sensor, and the EGR work together to get perfect mixture but keep the temperature at ideal range. 

Many of us gutted the main Catalytic converter in 2002-2006 QR25DE engines with nutorious crumbly Catalytic conveter on the exhaust manifold.    We still have 2nd catalytic converter after the 2nd O2 sensor and just enough to pass any MOT.  But if the MOT saw the spacer, we may fail too.  It will definitely fail in Germany, the TÜV employee are very diligent and careful.  They even failed my perfect Auris with a pending code (no check engine lights).  It was very annoying because I have to pay $18 extra caused by that pending code C1248 (12V Battery loose connection).  So, from now on, I always check with OBD2 scanner just in case I have a pending code. 

By the way, the P0480 code can be cleared periodically every 100 miles or about 10 days. I am not sure if you can get away just before the MOT test.