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2006 Forester
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Discussion Starter · #1 ·
Yeah, pot! That frigggin' 420 code.:yell:

I replaced my front O2 sensor and plugs, all OEM, on 1-17-10 and have had a consistent CEL code ever since. AND I get 1-2 MPG less than before. Ugh! I drive mostly highway miles to and from work and use the cruise control religiously. I haven't been able to use it since. For the last month, I've been getting that darn 420 code, as soon as it resets itself it kicks back in.

I did not reset the ECU figuring that it would reset itself in time. Heck, I shouldn't even have had to since my driving habit has not changed and figuring I'd get better gas mileage from the new O2 sensor and plugs. NOT!

Can anybody shed some light on my plight?
 

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2007 Forester Sports XT 4EAT
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There's alway the possiblity that the front O2 sensor you installed is defective & is causing your engine to run overly rich hence the P0420 CEL code. If the original O2 sensor was replaced as a preventative measure, put it back in. A PIA, but you need to start trouble shooting somewhere. :frown:

I'm looking more towards the front O2 sensor, since you aren't getting a misfire CEL code. :confused:

Bobby...

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Discussion Starter · #3 ·
Funny you said that...I told the guy who read my code today that I may put the old parts back in.

I had 104K miles on that original O2 sensor and figured it could stand to be replaced. I put the plugs in on 3-11-07 @ 27,425 mi. and I figured with ~75,000 mi they too could stand to be replaced. I may have replaced them early but heck I do that with everything, I replace my engine air filter religiously every 10K miles.

The plugs were Splitfire and my notes show them as Triple Platinum TP-392D 100,000 mile plugs. All 4 looked good when I replaced them, a little buildup on the tips but nothing that raised a red flag. I've heard bad things about these plugs after putting them in, but I had no problem with them. The new plugs are the recommended NGKs for my Fos.

I figured I'd get better gas mileage with the new plugs and sensor, but instead I got the opposite and a CEL code to boot. Go figure!

If my engine IS running rich, does this mean my cat is taking a beating?
 

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I'd bring the o2 sensor back and get another one. Entirely possible for them to be bad out of the box.

I'd try resetting the ECU if you haven't done that yet. It's possible it had adjusted the A/F learn or whatever to compensate for the old front O2 sensor being out. Or the rear o2 sensor could be going bad coincidentally.
 

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2007 Forester Sports XT 4EAT
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If my engine IS running rich, does this mean my cat is taking a beating?
The rear O2 sensor monitors the efficiency of the CAT, dropping a P0420 CEL code if it detects too much O2 in the exhaust flow. This can mean the engine is running way to rich & the CAT is overwhelmed, so the short answer is yes. :frown:

There are other possiblities, but a good place to start would be to put back the "known" good part (Front O2 sensor) & go from there. :wink:

Bobby...

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Discussion Starter · #6 ·
Would it be really be worth disconnecting the battery and resetting the ECU since it's been a month and I've filled up the tank 7 times so far since I replaced the parts? You'd thing the ECU would've compensated for the new sensor by now.

And I purchased the new OEM parts at a discount from an online vendor for this site. I think I need to be sure purdy darn sure it's the sensor before returning it.
 

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2004 Forester 4EAT
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Replace front o2 sensor with another new unit. That's what warranties are for. Reset ecu just for good measure. I would think the next place to look is the rear o2 sensor. When I had my 0420, I replaced the rear one and no more problems.
 

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Discussion Starter · #8 ·
I just pulled the neg from the battery, pumped the brakes, and will let it sit till this afternoon till I need to drive. Our band is playing tonight and I have to drive ~ 90 miles mostly highway. If it the CEL code kicks on tonight, I'll put the old sensor back in tomorrow.

There is a stealer near where I work that I pert'near refuse to go to anymore, but I may stop by there Monday and get a new rear sensor as it sure couldn't hurt trying it.
 

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I just pulled the neg from the battery, pumped the brakes, and will let it sit till this afternoon till I need to drive. Our band is playing tonight and I have to drive ~ 90 miles mostly highway. If it the CEL code kicks on tonight, I'll put the old sensor back in tomorrow.
Once you've pulled the negative battery cable & pumped the brakes a few times, you've drained the residual power & your ECM has been reset. Hint, when you go to start your Forester, turn the ignition to start, but don't start for 15 seconds. This will give the ECM to restore & your engine will start & not stall on the first start after the ECM reset. :wink:

There is a stealer near where I work that I pert'near refuse to go to anymore, but I may stop by there Monday and get a new rear sensor as it sure couldn't hurt trying it.
Normally the rear O2 sensor would also be suspect, but it won't cause poor engine performace. That's why I was looking upstream at the front O2 sensor. In addition, the rear O2 sensor is only used to monitor the CAT, so it's not critical for correct engine operation. I'd shop around & get a cheap one, if you want to replace it. Of course you can always do the $5 fix, but keep in mind, neither the replacement rear O2 sensor or the $5 fix will correct your poor performance issue. :frown:

Bobby...

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Discussion Starter · #10 · (Edited)
Well, after resetting the ECU yesterday morning, I drove to town and back yesterday afternoon and the car did not throw a code. Then drove 94 miles last night and no code was thrown.

I'm keeping my fingers crossed that this did the trick. And if so, I think there is a lesson to be learned here...don't hesitate in re-setting the ECU anytime an O2 sensor is replaced, just do it!

However, I'll be convinced of this fixing the problem only after this next week of driving to/from work. </fingers crossed>

Thanks guys for all your input.

EDIT: It's been 5 days and >300 miles since and I have not had this code since. I thing the ECU reset did the trick.
 
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