I have a 2006 Lexus LX 470 and I’m getting the check engine light on. I have a reader and I am seeing P0430 “ catalyst efficiency below normal (bank 2). I took some data from the sensors and plotted it but didn’t really understand it that well. Was wondering if anyone could tell me if this reflects a bad catalytic converters or not. Not sure why the downstream sensor voltages would be different. My understanding was that the downstream sensor traces would be oscillating like the upstream of the cat was bad though. Thanks for any help. O2 Sensor Data

EDIT (adding catylytic converter temperature plot): Note: T0 on this plot is not the same as T0 on the top plot. This was taken after that data set. Catylytic Converter In/Out Temperature Data Sorry the plotting software ran the graphs together.

  • Welcome to Motor Vehicle Maintenance & Repair! How many miles are on the rear O2 sensors? Mar 16, 2018 at 16:43
  • Thanks Paulster2! Good point. Just a couple of hundred on the downstreams(I used Densos). Not sure on the upstreams. We just bought the vehicle a couple thousand miles ago but didn’t have that record. Was going to try replacing upstream but they’re significantly more expensive so wanted to try and get it right before throwing too much money at it.
    – clintsmith
    Mar 16, 2018 at 17:13
  • @csmithmaui could you give the label for the various colors on the graph? I am making some assumptions that the red and blue are the downstream O2 sensors but I am not sure...
    – Ukko
    Mar 16, 2018 at 17:19
  • @Ukko Hi Ukko, there is a legend at the top right. You are correct that the red and blue are the downstreams. Red is Bank 1, Sensor 2(B1S2) and Blue is Bank 2, Sensor 2(B2S2). Hope that helps. Thanks!
    – clintsmith
    Mar 16, 2018 at 17:27
  • Does this answer help you any? Mar 16, 2018 at 18:04

3 Answers 3


Bank 2 Sensor 2 seems stuck rich.

I'm more suspect of the downstream Bank 2 sensor - it's a flat line. As long as it's up to temperature, it should show swings back and forth relative to the upstream sensors.

I'm not sure you're hot enough (Not ECT, catalyst temperature). I'd love to see the same graphs after some spirited driving.

It appears they went closed loop at time 130, and Bank 1 responded as I would expect to a rich event around time 630.

Bank 2 seems anemic. Ukko's suggestion of a swap makes good diagnostic sense.

  • If the cats aren't warmed up after 10 minutes, I'd be highly surprised. Mar 16, 2018 at 23:54
  • 4.7 V8 at idle? I wouldn't be. I'm not referring to the [H]O2S, I'm referring to the entire cat(s).
    – SteveRacer
    Mar 17, 2018 at 0:00
  • Dunno, 'cause I've never ran one, but since the coolant has come completely up to temperature, it seems likely the cats would as well, considering they are in the direct flow of the exhaust. Would be an interesting test to see just how long it does take :o) I know my 5.3L V8 would be completely warmed up at that point, cats and all. Mar 17, 2018 at 0:04
  • Anyway, +1 because I agree with your overall assessment. Mar 17, 2018 at 0:09
  • 1
    Agreed on the heat differential and using it to determine health of the cat. \ Mar 17, 2018 at 0:50

Have a look at Test 2 in this answer.

According to it, Bank 2 Sensor 2 (B2S2) is pegged rich in the plot, which is fine.

Bank 1 Sensor 2 (B1S2) is not fine; it should not be oscillating up and down like that.


from the inlet and outlet temps of the cat which are about the same, i'd say the cat is not doing its job...there must be around 200 deg difference between the inlet and outlet temps-outlet being the higher one.

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