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2006-2009 Range Rover Sport
66 Posts
Discussion Starter #1
Hi All,

I have trick to pass inspection testing even if you get p420 errors.

I don’t know if this will work on all vehicles or just range rovers or just my vehicle . I have a 2006 Range Rover Sport SC 4.2 with 150k miles
I have been using this trick for 2 years now to pass inspection tests.

I started getting engine check light with an odb error 420 cat below efficiency about 2 years ago
I have tried all the flowing but nothing fixed it
1) non foulers / spacers on the downstream o2 sensor , this didn’t help
2) Check for vacuum / exhaust leaks , fuel trims etc all were good
3) I replaced both cats , but the error came back with in a few weeks
4) I replaced all the o2 sensors and checked all electrical
5) Fuel injector cleaner
6) Replaced PCV
None of these fixed it

I still don't know the exact reason my cats fail but I think I may have excess oil burning , but my exhaust smoke looks good to me , so I'm still not 100% sure, any comments welcome.
But I did notice that the 420 error would only occur under certain conditions, and I managed to narrow it down to engine load being about 60% or 70% being at around 50 to 70mph at constant velocity.
This was key to my discovery .
When the error would occur the o2 sensor 2 downstream ( yellow in the pic) would swing between 0.8 to 0.2 ,and it needs to swing about 7 or 8 times
I believe this is when its doing it cat test but toggling fuel trim as seen the o2 sensor 1 upstream ( green in pic) as it toggles as well.

So they key to this trick it to keep an eye on the o2 sensor downstream and when it starts to toggle then either accelerate hard or take your foot of the gas complete as seen in my diagram where yellow goes flat. If you can do this before it swings too many times then you don’t get an error.

So here is the full procedure

1) Full fuel up the tank
2) reset the check engine with odb tool ( I used iphone with cheap odb reader ODB fusion software)
3) Drive around for 100miles , whenever you drive need to keep an eye on the o2 sensor 2 and watch for toggling, and then either take your foot of the gas for a few sec or accelerate hard, just break the swing pattern. This only tends to happen above 50mph at constant speeds. You need to drive around at a variety of speeds and let the fuel go from full to less than 75% full because this is when it does the evap test. Every time I ended my journey I would check I have not hit a 420 , if you have you need to start again at step 1
4) I also check the monitor status every journey. After about 100 miles and half a tank , the evap system check should be complete
5) refill fuel to full tank
6) drive around for 100miles doing the same as in step 3 , monitoring o2 downstream , accelerating or foot off gas whenever it starts to swing
7) you will see catalyst test will complete , then take it for an inspection ASAP

I'm curios if this works for other people or anyone else had this problem. Like I say I have used it successfully for 2 years now.

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