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Hello there!

I have been reading for hours on the P420 code as my 2008 Subaru Forester with 287,000 miles is throwing it now. I am thinking of trying to replace a sensor, not sure which O2 sensor to try to replace first. I have limited funds so can't really afford to have a mechanic charge me diagnostic money...which would probably be enough to pay for the sensors anyway. It almost seems as the comments are saying it's more often the rear one. Thoughts?

Through my reading, I have noticed the following from a post started over ten years ago. :nerd:
"I've be battling this code too (P0420) since the beginning of this year (car currently has about 140k miles). First there were two codes (forgot the other one, it was related to the rear sensor), so I replaced the rear sensor, but P0420 is still there once in a while. I kept on resetting it and it behaves exactly like others have described: it comes back, sometimes weeks, sometimes months. It looks like the cat converter is at fault in my case.

I'll try the $5 fix (mechanical fix) first."
I haven't been able to find out what the "mechanical fix" is. Can anyone give me any insight?

I am also getting a cylinder one misfire - ONLY WHEN IT RAINS - could this P0301 code be causing the P0420 code. I have replaced the coil pack and the spark plugs, which caused it to run good again, but that same problem has slowly come back to haunt me.

Looking to hit the 300,000 club, so any help or advise to get me through another 13,000 miles would be greatly appreciated! :grin2:
 

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2007 Forester Sports XT 4EAT
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@Allieem I see this is your first forum post, so welcome to the forum from Oregon! :biggrin:
I am also getting a cylinder one misfire - ONLY WHEN IT RAINS - could this P0301 code be causing the P0420 code. I have replaced the coil pack and the spark plugs, which caused it to run good again, but that same problem has slowly come back to haunt me.
My first thought was a bad coil pack. The test is simple, "mist" the coil pack & spark plug wires with water, while the engine is idling. If you notice the engine operation change, you could have defective spark plug wires or the coil pack is faulty. :confused:

The $5 fix is a "workaround"... not a fix. Assuming you have no other issues, just the P0420 CEL code, using the spark plug non-fouler can clear... prevent that P0420 CEL code.

► Disclaimer - The $5 fix will mask issues that should be addressed, so install it at your own risk!

The purpose of the spark plug non-fouler... spacer is to move the O2 sensor back out of the exhaust flow. If they use a tailpipe probe (sniffer) to check your emissions, you may fail, as the $5 fix doesn't change what's in the exhaust flow. :icon_eek:

As to not using the $5 fix on an '05+. Starting in '05, the rear O2 sensor is used for fuel trim, so the $5 fix should be used with caution on the '05+! You could get a P2096 CEL code - Post Catalyst Fuel Trim System Too Lean Bank 1. :frown:

On the positive side, some '05+ members have had good luck using the spark plug non-fouler... spacers. It doesn't hurt to try it, as it can be removed if other issues popup. :wink:

Also...
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Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]
 
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I had a persistent P0420 code on my 2008 and eventually bit the bullet to put on a front converter. P0420 code gone.
 

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2006 Forester X Prem 4EAT
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There are no sensors that need to be replaced in relation to the P0420 (an O2 sensor will have it's own specific fault), ultimately it's an indication of a poorly functioning primary catalytic converter, which is part of the entire exhaust manifold. A common fix for this is to install an O2 sensor spacer on the secondary O2 sensor. A misfire will definitely have an impact in causing this fault.

The fact that you have the misfire in the rainy weather is sometimes an indicator of a faulty ignition coil and/or spark plug wire, ideally you should only be using OEM or NGK parts for these, including the spark plugs.
 
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