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Discussion Starter · #1 ·
As above, I'm getting DTC P0420... I tried clearing it and it came back on (at cruising highway speeds) within a few days. I've read previous posts on this but no one really seems to know the cause.

Before I go putting spacers on my rear O2 sensor or replacing sensors I want to check for vacuum leaks. Any common places for leaks on the 2.4i?
Anything else that can cause these codes to pop up?

Thanks,
Phish
 

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What's the general description of that code again? Recently had a "system too rich" CEL on our S40 and it turned out (I think) to just be a dirty air filter. I replaced the FPS first, it came back, then replaced the filter and it's been clear ever since.
 

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Discussion Starter · #3 ·
What's the general description of that code again? Recently had a "system too rich" CEL on our S40 and it turned out (I think) to just be a dirty air filter. I replaced the FPS first, it came back, then replaced the filter and it's been clear ever since.
Catalyst system efficiency below threshold bank 1.
 

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Running nearly 60k miles with the same engine code. Car has a oil leak near the engine to transmission mating. Car isn't used as a daily anymore.

Car is running out of life, but the engine doesn't seem unaffected.
 

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Discussion Starter · #9 ·
As above, I'm getting DTC P0420... I tried clearing it and it came back on (at cruising highway speeds) within a few days. I've read previous posts on this but no one really seems to know the cause.

Before I go putting spacers on my rear O2 sensor or replacing sensors I want to check for vacuum leaks. Any common places for leaks on the 2.4i?
Anything else that can cause these codes to pop up?

Thanks,
Phish
Oddly the light went off on its own. Hoping it stays off but I'll continue to push any fixes off until absolutely necessary.
 

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My experience with the P0420 on other cars is it's not uncommon for it to go on and off.

The car periodically tests the catalytic convertor by comparing the O2 sensor readings before and after the cat. The after-cat chould show smaller variations in O2 if the catalyst is working.

I think when the cat gets marginal, it will fail the test sometimes and pass at other times, for whatever reason. There's even a can of stuff you can put in your gas tank to supposedly clean out your cat, and some people have had success with acid washing the cat.

The "spacer trick" pulls the rear O2 sensor out of the airstream, so it no longer registers the change in O2 that is symptomatic of a failing catalytic convertor, so the CEL goes out.

Downsides to running with a P0420 code:

Environmental damage (not sure if significant)
Harder to sell the car, especially to a region with emissions testing
Driving around with a CEL on, you will not be notified of other failures in the emissions system


IMO, if you are losing coolant, the first concern is a small leak - that could become a big leak unexpectedly
 

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Sounds like a cat. Just short of 200k P0420 would come on and off, often coming on in the rain. Eventually it'd pop on in minutes after a reset. O2s were fine, the cat had just reached the end of its life.
 
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