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06 Forester, 02 Tacoma
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I have an 06 5 speed with 170 on the clock. Had it a few months, passed inspection a few weeks ago the P0420 started. Scanner shows me that both O2 sensors are, from idle, responding to pretty much instantly to the gas pedal. The seem to be tracking pretty much perfectly, that is spike up and down in sync with each other and the pedal.

This is a Federal standards car.

Is this saying the Cat is bad? If so, which one? Any comments on the Bosal line?

Am I reading the FSM correctly? I have not checked the physical layout on the car, but it seem to be saying the front and rear O2 sensors are on the front exhaust pipe, before and after the front CAT, but BEFORE the rear CAT. Is that correct:?

I checked some parts illustrations and yes, the sensors bracket the front CAT. I guess the rear is just there for "clean up".

So, If I have the two sensor "tracking" and no exhaust leaks up front, I can presume the CAT is done, or needs a good cleaning? Davico advises using something called Sledgehammer to "decarbonize" before changing the CAT.

I now recall the previous owner telling me a bout changing a coil pack. From the way she said it I infer it had run poorly for some time. That may have carboned things up quite a bit?
 

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2007 Forester Sports XT 4EAT
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36,360 Posts
@joea99 the rear CAT is not monitored... which I thought was odd, figured it was added for States that have stricter emissions & uses a tailpipe sniffer as part of the emission testing.

On the diagram below, [6] is the front O2 sensor & [3] is the rear O2 sensor.

I always thought moving the rear O2 sensor after the rear CAT would prevent the likelihood of getting a P0402 CEL code.

Bobby...
528416

['07 FSXT Member Journal] ['03 X Member Journal]
 

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2006 Forester X 2.5
160k miles

This one has my stumped and I can't seem to find a answer. Got this code and started digging with my reader and got some confusing readings.

2.799v on upstream O2 sensor
Downstream O2 had large voltage changes (Which to my knowledge indicates a bad cat)
Used thermometer on cat and showed 300 inlet and 500 outlet (something close to that it looked good to me)
replaced front O2, same voltage reading
tested old o2 with torch got typical .000 to .900v
now that upstream O2 still shows that higher voltage when it is hot, cold, off and even unplugged which I am not tracking why its reading that.

P0420 is the only code I had (I reset it)
Car drives great, no performance issues or funny smells just that strange voltage reading I am used to quick movements from low to high on that front sensor
 

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2012 Forester 4 speed auto
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981 Posts
@xA2D9 That's because it isn't a normal O2 sensor. It is a Air Fuel ratio sensor and the voltage can be 2.4 to 2.8.V The down stream O2 sensor is a normal O2 sensor and should vary between 0 and 1 Volt. Check your fuel trims next.
 

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06 Forester, 02 Tacoma
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66 Posts
@joea99 the rear CAT is not monitored... which I thought was odd, figured it was added for States that have stricter emissions & uses a tailpipe sniffer as part of the emission testing.

On the diagram below, [6] is the front O2 sensor & [3] is the rear O2 sensor.

I always thought moving the rear O2 sensor after the rear CAT would prevent the likelihood of getting a P0402 CEL code.

Bobby...
View attachment 528416
['07 FSXT Member Journal] ['03 X Member Journal]
I only work on my own vehicles but have replaced a few O2 sensors. Everything I have worked on at least 2000 and newer, has at least two CATs. Some transverse V engines have one for each bank and one "rear". On Toyota, the rear O2 is after the rear most CAT

Does seem like Subaru is making it hard on themselves doing it this way.
 

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2007 Forester Sports XT 4EAT
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2005 Forester X and XT Manual and Auto
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803 Posts
@NinoS I don't believe those spark plug non-foulers are designed for constant exposure to the elements... like salty winter road spray, so that might have been the reason yours rusted & seized?

No winter road salting here where I live, so the one we installed in my son's '03 X is still rust free.

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
I think you are spot on with this. We dont get a ton of snow but we do get one or 2 events a year, plus living near the ocean too doesn't help. We are year round humid as well. Certainly closer to rust belt conditions than not.

On the generic cat, i checked it out when replacing the starter last week and it looked surprisingly good...pretty much like new...but again has not been long. Will serve its purpose for us at least.
 

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06 Forester, 02 Tacoma
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@joea99 the earlier XT had 3 CATs! What surprised me is the "unmonitored" rear CAT. Even more curious, I have seen some aftermarket replacement that do have a bung on the rear CAT.

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
Wonder how moving the sensor would work? Stainless Bungs are available, have one on hand, so I could try pretty easily, if the cable reaches.

I'm running some cleaner in the gas right now to see if it takes longer to set, or if the O2 voltages look different after. A few miles after the cleaner went in, I could swear it is running smoother and revving better. But I am sorta gullible . . .
 
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