P0420 Fault Code

[devonlad66]

Hi Folks I have a 2002 Toyota Avensis Petrol estate.

EML light came on. Reset. Came on again after a month. Garage attached Scanner done tests and the Snap on scan tool said The P0420 Bank 2 02 Sensor was working ok. He checked the figures (voltage??) on the snap on and working ok. They said the fault is down to putting the incorrect fuel in tank. Need to use super unleaded. How bizarre. I think they were using this as an excuse as they didnt really know how to solve this problem.

Cat is ok. I used Cataclean and used Super Unleaded still light came back on. Replaced the little air flow sensor in the air box. This engine has 4 02 sensors.

Please could this code be down to a undetected miss fire. Not sure what to do. Wondered if anyone has experienced this

Checked forum could find anything related. 

Vehicle runs perfect. I know it's an old banger and want to keep it. I guess a Toyota Main dealer is the best option

[devonlad66]

My car is a Mk1 Avensis with the 2 litre 1AZ-FSE. 

if the wrong fuel is the problem why would this cause the fault code to come up. I only used super unleaded on a couple of occasions.

[ents]

Hi

The P0420 indicates that the cat is not performing as it should, not that it isn't working at all. The O2 sensors are not the problem! I had that same problem with my old 2003 1.8 Avensis and it even passed MOT 3 times with this problem (i cleared the EML light before the test). In my case, i think the problem cause was the oil burning engine, it sooted the cat and this slowly lowered it's performance. 

As for the fuel, i don't know what the super unleaded is in your country (does it have higher octane number like 98 vs normal 95?), but you can't use E10 (10% ethanol) petrol. The max allowed ethanol % in the petrol for your car to use is 5.

[Gerg]

I've not had this problem and I don't own this car but, I seem to remember that another Avensis owner had a broadly similar problem that turned out to be a small, barely visible, hairline crack in the area around the oxygen sensors.

The crack was allowing air into the exhaust mixture and causing 'false' EML operation.

I think the theory on this runs along the lines of; every positive gas pressure pulse (an exhaust stroke) is normally followed by a negative pressure pulse.  This negative pressure can pull some air into the exhaust system, and if it's in the region of the O2 sensors then incorrect (false) readings can occur as it dilutes the mix.

I'm not saying that this is what you've got, just something unusual to bare in mind, especially given the age of the exhaust pipework.

If I understand this correctly, the first O2 sensor is used to adjust the air fuel mixture for the correct engine operation.  This is routinely changed, quite quickly, from slightly too lean to slightly too rich and back again, over and over, at a set frequency.

The second O2 sensor is looking for that same pattern, but with a 90 degree phase shift in the timing, which shows catalyst operation/efficiency.  An air leak can throw those readings out.

The second O2 sensor is there to show catalyst efficiency as a legislative requirement; to show the engine emissions are under control, not because the engine needs it to run 'well', as such.  I think on cars older than yours, but still fitted with a catalyst (after 1991?), there was no second O2 sensor fitted, at least until the emission legislation changed, which forced the need for one to be designed-in.

Happy to be corrected, naturally.