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O2 Sensors / Catalytic Converter Fault Finding

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Hi all

Sorry if this has been answered fully previously but is there a complete fault finding write-up on the numerous fault codes that arise from the o2 sensors / catalytic converter?

I have spent many hours reading forums and scouring the net for answers but all to no avail.

My wife's 2001 Avensis verso 2.0 VVT-i had the CEL indicator come on 3 weeks ago and has fault codes p0420 and p0430 simultaneously according to diagnosis.

I have cleared the faults/CEL (using torque app - see below) and they return again after 1-2 hours of driving.

The car seems to drive fine and has been very reliable in the 3 years we have owned it.

It hardly uses any Oil (rare I understand for this model) and the only issue recently was a faulty coil pack on one cylinder.

After reading so many horror stories and wasted time and money by so-called "technicians" in main stealers, etc. I thought it best to ask the true experts your thoughts as I do not wish to waste more time and money on their guesswork...

For fault finding I am using an eml327 bluetooth adaptor and the fantastic "torque pro" android app to fault find the issues.

I know my way around a multi-meter and the relationship between ohms, volts and amps.

However I am stuck in fathoming out the cause of my codes and would really appreciate any assistance in this.

For those of you unfamiliar with the EML327, it is a simple device that plugs into the OBD2 diagnostic port which almost all cars manufactured in the past 20 years. (not exactly sure on date but irrelevant).

Prices from £7 on fleabay or Amazon - beware of faulty/fakes

Torque Pro is an app on Android devices that displays the information produced from the many sensors dotted around a modern vehicle and can aid in diagnosis.

Price £3 I think.

After pairing the EML327 device and torque pro using bluetooth, I then went through all of the sensors and filled the screen of my Nexus 7 with oscilloscope type plotting graphs and gauges monitoring pretty much anything to do with the O2 sensors.

There seems to be next to no difference in the behaviour of the bank 1 to bank 2 sensors - indicating to me that it is not a faulty sensor but I would like to know if anyone else has ideas.

I did think the CAT may be at fault due to the recently faulty coil-pack but if this was the case would it not only affect either bank 1 OR bank 2??

Logic tells me the CAT is unlikely to be faulty due to bank 1 readings being similar voltages and tracking the behavior of Bank 1 when driven but I am willing to fault find further if anyone has suggestions.

I checked the heater elements out on all 4 sensors (black wires I think?) and both sensor 1's on bank 1 and bank 2 read about 1.5 ohms, both sensor 2's after the CAT on banks 1&2 read about 13 ohms each.

I think the torque app is a useful tool (BTW I am definitely not a salesman for it!!) and for little money removes a lot of mystery such as displaying the coolant temperature which gives me peace of mind on its own!

I would appreciate any tips and suggestions.

Thanks

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