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Old 08-02-2012, 11:58 PM   #1
adamwilson301
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Subaru Stars 420 code questions

I had a mass airflow sensor problem and 420 code. I cleaned the MAF sensor and the codes went away for a week (200 miles) then the 420 code came back on. I examined the Cat and it appeared to have a few dent from hitting the ground. I am a newbie to this so I was wondering if it could be an O2 sensor or Backpressure problem? Will getting rid of the Cat and running a straight pipe clear the code or would the best thing be purchasing a new cat and installing it? I am trying to go cheapest route that wont effect performance. I appriciate any tips advice and help!

1999 Subaru Impreza 2.5 RS not sure if its stock headers and Cat but I am pretty sure it is. I have owned car for 2 weeks
Thanks, Brian!
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Old 08-03-2012, 12:08 AM   #2
RaceFaceXC
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A straight pipe (no cat) would probably guarantee the P0420 DTC. It may also cause other emissions related codes. A low quality replacement cat may also cause other codes to be triggered but it depends how sensitive the system is.
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Old 08-03-2012, 12:23 AM   #3
murrdogg24
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Old 08-03-2012, 12:41 AM   #4
Vrooom_Psh
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P0420 is a catalytic converter inefficiency code.
Is some cases, 02 sensors malfunction and this code is part of the result. BUT, since you say it looks like the cat has been hit, coupled with the age of the car, I'd say that you'd need to replace the cat. It's not going to be cheap though.
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Old 08-03-2012, 01:15 AM   #5
ManualOverAuto
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Quote:
Originally Posted by Vrooom_Psh
P0420 is a catalytic converter inefficiency code.
Is some cases, 02 sensors malfunction and this code is part of the result. BUT, since you say it looks like the cat has been hit, coupled with the age of the car, I'd say that you'd need to replace the cat. It's not going to be cheap though.
This^

Let me find my thread where someone explains the dreaded 420 code perfectly.

Edit: here's that post-
Quote:
Originally Posted by mod maniac
A p0420 code means that your rear 02 sensor is getting variable readings. your front 02 goes rich lean rich lean and your cat cleans that up so your rear 02 stays steady. When your cat isn't able to clean that up you get a p0420 code. That doesn't mean your cat is broken or plugged (but it could) it just means it's not working. Generally if the cat is plugged or broken you would be suffering power loss and hear a rattle under the car. With the fuel milage problem I would think you have a broken piece that is rattling around in there and blocking the exhaust sometimes. You can remove the cats and remove that piece,then put them back on and see if that works or remove them and install a after market cat.

Last edited by ManualOverAuto; 08-03-2012 at 01:22 AM.
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Old 08-03-2012, 03:34 AM   #6
adamwilson301
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Thanks guys well I will start with the cheapest thing and take a closer look at the current Cat. I wouldn't be able to tell about the power loss because the clutch is slipping very badly I plan on fixing that next weekend. I hope the $$1000's I have spent are well worth it in the end. I could of bought a used WRX for what I have thrown into this thing to make it drive-able.
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