HO2S faulty code P0420

RickyG · Post by **RickyG** » 14 Jan 2013, 04:37

I have a XC90 2007 and not long time ago the engine light came on. I took it to a Volvo mechanic and told me that one of the Lambda (HO2S) sensor was bad not sure which code was at that time. Few days after been replaced the light came on again, so I decided to get an OBD2 tools to read the ECU codes. Now, I'm getting a faulty code P0420, the mechanic told me that the sensor was replaced but no sure why the light is on again. Is there any way that the other sensor is also bad? Or could be the cat converter?

jpbrooks81 · Post by **jpbrooks81** » 28 Sep 2019, 08:15

I have a 2011 XC90. Having the same issue. Did you ever find out what was causing the problem?

Post by **jimmy57** » 28 Sep 2019, 08:51

p0420 is faulty catalyst function code. The checking of catalyst is by rear O2 sensor with the control of fuel mixture that makes catalyst able to function being signalled to engine control module by front O2 sensor. Exhaust leaks before or at catalyst also can make for this code. I assume you have a 2011 3.2 XC90. The oxygen sensors can be checked by introducing a vacuum leak while having the both O2 sensors showing as live display values. The 3.2 has no vacuum lines so I loosen the intake manifold bolt to head and when I want it to leak I slide a screwdriver between head and intake to create the leak. WHen the leak is started BOTH sensors should indicate lean in 2 seconds. Lean for front is value going over 1 and for rear it is voltage going to 0.1V or even lower. The front sensor is shown as lamda value while rear is raw voltage from the sensor. You leave the leak for over a minute and then pulled screw driver out and push intake against head while watching the values. The ECM adjustments with leak present will have richen mixture a lot and when leak is closed it will be rich. The front sensor should drop to .75 or close and the rear will go to .8 V or close. Again the response should be immediate when the leak is closed. If the sensors check and there are no leaks in exhaust then that leaves catalyst as the cause. I am discounting a fuel injector as that being the cause also would make the engine run rough and that will usually set a misfire code. It is a good diea to check that there is no high vaccum in crankcase due to ruptured diaphragm in the breather on top of head. That diaphragm will create a leak from intake to inside engine cavity. WIth it idling pull out dispstick and put finger over the tube. If it strongly pulls finger, better yet if it has high vacuum on a gauge connected there, then crankcase breather cover needs to be changed first.

jpbrooks81 · Post by **jpbrooks81** » 29 Sep 2019, 08:53

Wow, thank you for the highly detailed response, jimmy57. I have to admit, i'm a complete newbie to the xc90, so my knowledge on the vehicle is very limited at this point. Ive only had the vehicle for 2 months. When testing the O2 sensors. What tool/software should I use. Like i said, i'm completely new at this and have much to learn. Thanks again.