2011 Aygo - P0420 Catalyst System Efficiency Below Threshold

[Swordtail]

I recently bought a 13 year old Toyota Aygo for my daughter to learn in and the engine light came on. I bought an OBD reader (I have some basic mechanical knowledge but would like to learn more). The reader shows the above DTC code but also shows two other "fail" reports.

1. There is an O2 sensor error "O2 bank 1 sensor 1 Low for switch" showing test value 0, min 0.005.

2. There is a Mode 6 error "Exhaust gas sensor monitor bank 1 sensor 1: Rich to lean sensor switch time (calculated): Test value 119, max 64"

There aren't any DTC codes showing specific to oxygen sensors. Would it be worth changing sensor 1 to see if this resolves all these errors?

For information, my other daughter has a Citroen C1 of the same age, same engine. That isn't showing any DTCs or the Mode 6 error but both oxygen sensors show the same fail message with a low for switch value of 0. I don't know if I should worry about that.

Cheers for any help.

Phil.

[flash22]

it will be a failing o2 sensor B1S1 (pre cat) monitors the air-fuel ratio - if you do not rectify the issue it will kill the cat, if it's not already done for

 

don't buy cheap o2 sensors, a Denso or Bosch item will be £50-60

Denso - DOX-2028

[Swordtail]

Cheers for this.  I've replaced sensor 1 with a cheap one for now.  If this resolves the issue once the car completes a drive cycle then I'll buy a proper OEM one.  I'd not seen one as cheap as £50 for a decent make or I'd have got it.

However, I have been investigating further and checking the real time graph data for the voltages of both sensors.  I understand the downstream sensor 2 is supposed to stay relatively steady at about 0.45V regardless of load.  It does seem to do this for a period of time but then the voltage drops (even to zero) and stays very low but spikes when revved, sometimes very high, sometimes just slightly even with a similar amount of revving (see pic).  I don't know if this is a broken cat or sensor 2 is faulty.  I'd have thought if the cat was faulty it wouldn't read 0.45V at all unless it only becomes apparent once heated up.  At least this is an interesting learning exercise!  I'm trying to spend as little as possible on a cheap car which we probably won't have all that long.

 

[flash22]

there are a lot of factors inc fuel trims and open and closed loop, as you rightly said the car needs to run a drive cycle let it correct trims and ignition, OEM sensors are Bosch or Denso the toyota items are around £140-160

[Vandals01]

https://www.autodoc.co.uk/denso/7024887  £55.31 + Shipping

https://www.gsfcarparts.com/dendox-2028 £71.14 with 15% off right now 

Stay away from Amazon or eBay for obvious reasons (especially for electronic sensitive components).

EuroCarParts also supply genuine parts.  However, they became a lot more expensive over the years. (Bosch one sold at £83.99 right now)

Yeah, Toyota oxygen sensors are basically a re-badge Bosch or Denso with Toyota price on top.

Apart from Bosch and Denso, NTK (NGK) are also top quality.  I also had very good success rate with FAE which I believe they are the OEM supplier for Porsche.