Can you help me with a problem on my VW Polo?
The MIL is on and after diagnosis with VCDS (VAGCOM) it appears to be a problem with the catalyst (P0420)
"Efficiency Below Treshold"
I also captured the output from pre- and post lambda sensor and it seems to me that the catalyst is indeed not functioning as is should.
Can anyone take a look at the capture in the attachment and tell me if there can be another cause instead of the catalyst?
Not much info to go on, assuming ch A is pre cat and B is post it does not look good, if not already done recheck at a steady 2000 rpm eng temp up, Ch A should read as shown and B should show a steady 500 > 700mv confirm this with a steady light load cruise road test ? if post cat oscillates the cat is slow, confirm sensor response by introducing a rich and lean condition and check reaction. Check fuel trims to confirm no fuel or ign problems. Cheers Ron W
Fueling looks OK, confirm readings  with a cruise road test if you have a quiet length of road, use any scanner that will graph the signals and select OBD2 generic its often faster to find emission readings there than model specific, the high flat line the post sensor should produce does not mean a rich mixture as we generally term it, the cat is doing it job using oxygen to burn HC so no oxygen post cat the sensor reads high, oscillation means cat not doing its job. Cheers Ron
I have looked at the fuel trims and I think it looks OK, but the long term fuel trim at partial load is 3.5% and the short term fuel trim is pending between -8% and 0%. It that acceptable?
I've included a screenshot in te attachment.
You must search in measuring blocks the catalyst temperature(or for downstream oxygen sensor) ,this must be at least 400*C ...then make a road test with cruising,hard acceleration and hard deceleration with VCDS recording the selected values.After that test you can see how good is your catalyst mesuring oxygen storage capacity like in the attached graph
Today I did a road test as suggested.
Temperature at the post cat sensor is above 600 degrees and sensor values are included in the attachment.
The red line indicates the pre cat sensor and green the post cat sensor.
To me it looks like the sensors are responding just fine.
Hi Looking at your last capture there are no voltage/time scales? so I assume red .1>.9v and green around .6v. The distance/time between the red channel drop on decel and the green channel indicates the cat oxygen storage capacity a good cat is around 2>4 secs or better, assuming the red o2 is switching at 1hz ie once a second between peaks it would indicate say a .5 second storage and that would indicate a failing cat, check the times with your scope measurement. The cat is well up to temp so it must be working, however the code indicates below threshold, also the o2 post senor oscillates in time with the pre o2 not to the extent you would see with a failed cat but an indication of lower deficiency? The sensor reaction looks good but check the switching frequency is 1hz or better also recheck the fuel trims that minus 8 should not be permanent. Cheers Ron