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Premium Member
1995-2002 Range Rover P38A
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1,115 Posts
Discussion Starter #1
Greetings Gang -
I'm not in the trouble zone with this yet, but am trying to make a decision and need a little input.

One of my post cat oxygen sensors is switching, but in very tiny voltage variances. Volts are low (-1.5) at idle and jump to high (6.5) when accelerator is pushed, then decay back to low again when left at idle. The expected minus to plus to minus voltage variance is not happening. The upstream sensor is switching nicely and no problems are noted with it's function.

My question is, is this indicative of a bad sensor or bad catalytic converter? If it's just a sensor, I figure I'll replace the two pre-cat sensors with new and move the used but good sensors to the post-cat positions? :think: The pre-cats are the key feedback sensors for fueling, yes? The post-cats are an NAS feature for converter condition monitoring?

Any wisdom/advice is most appreciated.

Thanks!
 

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Premium Member
2010-2012 Range Rover MkIII / L322
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4,635 Posts
Yes, it is a NAS thing to have post cat sensors to check the cats are working.

It looks like the voltages are incorrect, so I'd be looking there first of all. Do chance for rattling cats though first of all, or even consider some sort of emissions test.

What's the best price you've had on sensors? Are both banks reporting similar readings? Can you get them on a scope?
 

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Premium Member
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1,036 Posts
Bloody hell,some people just look for problems ! The downstream sensors are meant to drift up and down slowly,not switch rapidly like the upstream ones.If they did the CEL would be on as its a good sign that if the downstreams follow the upstreams then the Cats are doing zilch.
 

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Premium Member
1995-2002 Range Rover P38A
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1,115 Posts
Discussion Starter #4
allyv8 said:
Bloody hell,some people just look for problems ! The downstream sensors are meant to drift up and down slowly,not switch rapidly like the upstream ones.If they did the CEL would be on as its a good sign that if the downstreams follow the upstreams then the Cats are doing zilch.
:shock: Apologies, and thanks... sage advice with an edge.
 

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Premium Member
2010-2012 Range Rover MkIII / L322
Joined
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4,635 Posts
allyv8 said:
Bloody hell,some people just look for problems ! The downstream sensors are meant to drift up and down slowly,not switch rapidly like the upstream ones.If they did the CEL would be on as its a good sign that if the downstreams follow the upstreams then the Cats are doing zilch.
Do the voltages switch between 0 and 5v still on the NAS downstream sensors?
 

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Premium Member
1995-2002 Range Rover P38A
Joined
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1,115 Posts
Discussion Starter #6
Yes Rich, the post cats had previously switched regularly, similar to the upstream. Just recently noted that one was "sluggish". I'm noting this with a Rovacom unit which doesn't exactly provide scoped detail, but never-the-less it shows that the change in voltage, on one, has lessened.

New sensor price USA is $109.00, on sale right now. (OEM setup with harness plug)

Thanks :)
 
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