P0420 Code

Technical Area Engine, Transmission, Exhaust etc
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2002 1.8 Mk 2.5 with 54K miles.<o:p></o:p>

After a few miles of driving the engine MIL lamp lights up. Found it to be due to a P0420 code after checking with an MS509 analyser. Using the analyser to reset the code and extinguish the MIL light but it keeps coming back on.<o:p></o:p>

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The car appears to be running fine with a good steady idle and performance appears to be OK. No other codes are being set.<o:p></o:p>

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Used the analyser to check the front O2 sensor performance and a tick over the analyser shows the voltage swinging from 0.05volt up to 0.9volt in an oscillating waveform.<o:p></o:p>

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Using the analyser to check the rear O2 sensor initially without success there appeared to be no reading on the analyser. However after a good run making sure the engine was well warmed up got a reading for the rear sensor. This appeared to be following the same pattern as the front sensor i.e. oscillating from 0.1v up to 0.9volt. <o:p></o:p>

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Having spent a good few hours reading up about P0420 faults and O2 sensor data, conclusion seems to be a dead cat. Could it be something else causing the code, is it worth checking anything else or is it trip to the local exhaust centre.<o:p></o:p>

 

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If the rear sensor output is following the front one then the cause could be a failed cat. or it could be the air/fuel mixture is wrong so the cat. is not working. The cat needs an AFR of 14.7:1. If the upstream sensor output is fluctuating as you say then the chances are it is a cat problem as the front sensor is indicating the AFR is correct.

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 Here is a quote regarding the code P0420 - ‘There are a number of things that could be causing this p0420 catalyst efficiency code. The most common thing is the catalytic converter itself is no longer functioning properly. The other likely thing is the rear O2 (oxygen) sensor is no longer working properly. Other things could include exhaust leaks, damaged exhaust pipes, damaged O2 sensor wiring/connectors, plugged catalytic converter, etc.’

Agree with Robbie on this one.  Unlikely to be a problem with the air fuel mixure controlling precat lambda sensor as usually more specific error codes will appear.  Choice is usually between the cat and the post cat lambda sensor, assuming your exhaust is in otherwise good condition.  I would put my money on the cat.  The car will fail an MOT because of the engine light and in all probability excessive unconverted exhaust emission.    

 

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 Agree with the two previous replies.  An emissions test will confrm.  An MOT station could test it for you.  Examine the exhaust carefully, it only takes a small hole or poor joint on one of the flanges to cause the lambda reading to be out of limits

    Regards   Geoff Peace

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 Thanks for your comments will get the ehaust checked.

regards roger clay

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It won’t fail the MOT for the light being on, it’ll only fail if the exhaust emissions are above the limits.

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Never too clever to learn something eh RobbieWink

All those people that buy post cat lambdas off me to turn their engine lights off for the MOT could just leave then on…but seriously it never pays to leave the engine light on for a non critical fault as not only is the light annoying but it may well mask a major fault at some point in time. 

I think I am correct in saying that ABS lights, airbag warning lights and brake warning lights will all fail an MOT if illuminated but as the man says the engine light can glow through an MOT provided the tester does not find that the associated fault is reason for an MOT failure, emissions being a common failure. 

  

 

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  Been a while since I was last testing cars but airbag (SRS) and brake warning lamps weren’t a reason for failure along with engine management lights… The ABS light will definitely get you a fail though… Just to add I concur with the cat verdict, the fact that the lambda (o2) sensors are switching at the same voltage and frequency shows that the exhaust gases are the same on the way in to the cat as they are on the way out of it. :slight_smile:

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At the moment the only dash warning light that is a reason for MOT failure is the ABS lamp. The reason for rejection is that it doesn’t follow the correct sequence of operation, or indicates a malfunction in the system so not coming on is a failure as much as staying on or flashing. SRS and MIL can be on and still pass the MOT test. Word on the street is that more dash lights may become testable in the near furure though.

Check out this post, it’s the MOT test spelled out in black and white, makes great reading.