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Old 01-22-2011, 06:12 AM   #1
coondogg
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Default PO420, but only above 1/2 tank... O_o

Since my last trip into the garage, I've been watching my check engine light (po420) spending long periods on and off. Honestly, I didn't recognize any differences in my driving style so I was trying to see if there was any pattern and what was causing the cel to go on and off like that. As an attempt to address the light for the po420, I picked up those spark plug arrestors and still plan on installing them on the back o2 as a possible "fix". Almost 800ish miles later and I realized that the light is only on till my tank gets to ~1/2 level then it goes out until I fill the tank again. I've been able to confirm this theory over my last 3 tanks of gas.

Any clue as to why the fuel level would effect a catalyst efficiency code? I'm ruling out an issues with the fuel or the tank its self due to the length off time burning up each tank is not consistent but possible fuel filter? Well, I'm going to replace the fuel filter anyways just for maintenance since I have no clue when it was last done and it looks easy as shiet to do.

I hate this code because there are so many expensive parts that could be the real problem and I don't want to blindly throw parts at it and not repair the issue.
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Old 01-22-2011, 12:01 PM   #2
yarrgh
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Here's the next test, only fill up to halfway for a few "tanks" and see what comes. How many miles on the car? It could be coincidence that during your normal routine it just so happens that you're at 1/2 tank when the computer decides to turn off the code.
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Old 01-22-2011, 12:13 PM   #3
coondogg
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I just went under 1/2 yesterday when the cel went out for a 3rd time and at 1/4 of a tank now. About 99.9% of my driving is open country roads between 35-55mph. Normally in 40-70 mile jaunts. I'll take your suggestion and only add about 5-6 gallons and see how it reacts from there. Car has 223k on it and just bought it used so unaware of any previous maintenance/repair work done to it.
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Old 01-22-2011, 05:16 PM   #4
coondogg
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Theory is bonked. After 23 miles, cel came back on. Filled it up with 91 this time and will watch my mileage as to when/if it goes back off...
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Old 01-23-2011, 11:19 AM   #5
yarrgh
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Well, the 2 most obvious causes would be:
1. You cat is dying
2. The rear O2 sensor is dying

A less obvious cause would be the front O2 sensor going bad, causing bad fueling, causing causing a not expected gas mixture to pass through the cat, causing confusion w/the rear O2....but that's kind of a long shot.
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Old 01-26-2011, 08:30 PM   #6
Escape-performance
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Yeah, the gas tank has nothing to do with p0420. What year is the car? Sorry to say but you need cats. The factory test consists of running the car at 50mph for 2plus minutes. Then watch the rear o2 voltage when you go from steady throttle to decel. You should see the sensor go full lean when you let off the throttle, this tells you that the o2 is functioning properly. Then stay at 50+mph and steady throttle, voltage should stay constant if the cat is firing properly. If the cat is bad you will see the voltage drift. This said you need a graphing multi meeter and should watch the front o2 or af sensor at the same time. Depending on the year, the fed gov gives a long warranty on cats. Sorry to say but it is going to be the cat, subaru cats go bad all the time. You can punch out the cat and get a o2 simulator to trick the ecu.
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Old 01-27-2011, 09:41 AM   #7
GEE-OTTO
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Well the reason this happens is b/c the ECU only monitors O2 voltage above 3/4 of a tanks and below 1/4 a tank.
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Old 01-27-2011, 10:06 AM   #8
XanRules
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Quote:
Originally Posted by GEE-OTTO
Well the reason this happens is b/c the ECU only monitors O2 voltage above 3/4 of a tanks and below 1/4 a tank.
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Old 01-30-2011, 08:18 PM   #9
Escape-performance
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What? Where on earth did you hear that?
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Old 01-30-2011, 09:34 PM   #10
GEE-OTTO
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Quote:
Originally Posted by Escape-performance View Post
What? Where on earth did you hear that?

I had the EXACT same problem on my 2008 Legacy when I went in to Stohlman Subaru in Tysons Corner Va the Master Tech explained to me why the P0420 was so troublesome. In that explanation this is what he told me you can ask him:

8433 Leesburg Pike
Vienna, VA 22182

P: 866-311-4735
Email: Contact Us
Fax: (703) 893-0737
Internet Sales: (888) 325-4207
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Old 01-30-2011, 10:34 PM   #11
Wrencher86
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Are you sure he didn't mean that those are the parameters for the cat monitor to run? The only time a fuel injected vehicle isn't looking at the O2s is after start-up when it's in open loop. Otherwise it's got no fuel control.
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Old 02-01-2011, 05:44 AM   #12
GEE-OTTO
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Oops yes sorry about that, above 3/4 and below 1/4 it doesn't care about the cat between those marks it checks and throws the light. I was at the dealer yesterday getting a quote on new carpet for my legacy and asked the tech again. Sorry for the confusion but that is why the light comes on and goes off.
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