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Discussion Starter · #1 · (Edited)
I've been browsing the forums for the code P0421 and it seems many people have this issue. I have read Desert Rat's post about this problem on the constantly asked questions but this is for when the SES light is on, and mine is gone. In addition to this, I have also recieved P0174 and P0171 but all have gone away for now. My main problem is that I have been getting crap MPG a rough idle below about 2.5k RPM and a smell of gas on a hard pull. I know Eclipse's have 2 cat's but I'm not sure if one or both could be causing my issues, or it could be something completely different. I'm hesitant to buy anything because I saw the prices and I'm not sure what is 100% causing it.

I have done:
  • Sprayed MAF and throttle body
  • Checked temperature on the underside cat and found the temperatures are close together
  • Filled up my tank and only got 240 miles on it. (2007 GS Automatic - Supposed to be 22mpg average - )
 

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Not Your Friendo
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First of all, kudos on doing your research. These issues are sometimes hard to pinpoint and can be somewhat of a headache.

To start: any modifications?

Is the P0421 always on or does it come on and off as it pleases? Sometimes if you're not sparking correctly, the exhaust mixture might be a bit "dirty" and set it off. Also, check whether or not you're burning oil excessively.

Lets focus on P0174 and P0171 (System too lean) first. You know they're on both banks so at least its consistent.

#1 Check your spark plugs. If you're lucky, your plugs are old and not blasting off properly. Its free if you have to tools so its a good place to start. Also, it'll give you some clues. If they're black, sooty and fouled. You know you're running rich for sure. Consider replacing and move on to finding the original cause.

#2 Check your exhaust gasket (Cylinder head to exhaust manifold) and consider replacing both the cylinder head to exhaust gasket and the exhaust manifold to downpipe gasket ($10+$2+shipping from rockauto). An exhaust leak could make you read lean, causing your car to compensate by dumping more fuel and still read lean, then fouling your plugs and giving your o2 sensors and cats trouble doing its job.

#3 Conveniently fouled all your o2 sensors and/or have a oily gunked up intake manifold? Try seafoaming it. Might be about $10 max at a local autozone/o'reiley's/pep boys. Unlike alot of gimmicky injector cleaners, this stuff works really well.

#4 Vacuum leaks in your intake system? You'll be able to tell from your spark plugs if its lean too. Doesn't explain other symptoms as well though.

I'm leaning towards an exhaust leak but as I said before, these problems can be tricky so I can't guarantee it. Replacing spark plugs and seafoaming will help regardless but may not address the underlying issue.
 

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Discussion Starter · #3 ·
First of all, kudos on doing your research. These issues are sometimes hard to pinpoint and can be somewhat of a headache.

To start: any modifications?

Is the P0421 always on or does it come on and off as it pleases? Sometimes if you're not sparking correctly, the exhaust mixture might be a bit "dirty" and set it off. Also, check whether or not you're burning oil excessively.

Lets focus on P0174 and P0171 (System too lean) first. You know they're on both banks so at least its consistent.

#1 Check your spark plugs. If you're lucky, your plugs are old and not blasting off properly. Its free if you have to tools so its a good place to start. Also, it'll give you some clues. If they're black, sooty and fouled. You know you're running rich for sure. Consider replacing and move on to finding the original cause.

#2 Check your exhaust gasket (Cylinder head to exhaust manifold) and consider replacing both the cylinder head to exhaust gasket and the exhaust manifold to downpipe gasket ($10+$2+shipping from rockauto). An exhaust leak could make you read lean, causing your car to compensate by dumping more fuel and still read lean, then fouling your plugs and giving your o2 sensors and cats trouble doing its job.

#3 Conveniently fouled all your o2 sensors and/or have a oily gunked up intake manifold? Try seafoaming it. Might be about $10 max at a local autozone/o'reiley's/pep boys. Unlike alot of gimmicky injector cleaners, this stuff works really well.

#4 Vacuum leaks in your intake system? You'll be able to tell from your spark plugs if its lean too. Doesn't explain other symptoms as well though.

I'm leaning towards an exhaust leak but as I said before, these problems can be tricky so I can't guarantee it. Replacing spark plugs and seafoaming will help regardless but may not address the underlying issue.
Okay I seafoamed and the rough idle seemed to go away. There was also instances of my RPM being erratic on neutral before I did this and that went away as well. I checked the spark plugs and they were fine. So far everything seems fine, I will have to check on how the MPG has been affected. If something is still wrong I guess I start that checklist you mentioned :( Thanks for the advice!
 

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Okay I seafoamed and the rough idle seemed to go away. There was also instances of my RPM being erratic on neutral before I did this and that went away as well. I checked the spark plugs and they were fine. So far everything seems fine, I will have to check on how the MPG has been affected. If something is still wrong I guess I start that checklist you mentioned :( Thanks for the advice!
Don't expect much of a change from the MPG for whatever reason the Auto GS gets horrible gas mileage from the research that I've done. NO reason why but a tune will get you back to what was promised from the factory if not better.
 

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Discussion Starter · #5 ·
Don't expect much of a change from the MPG for whatever reason the Auto GS gets horrible gas mileage from the research that I've done. NO reason why but a tune will get you back to what was promised from the factory if not better.
I see about how much would that end up being? I found a few performance labs around my area, I would just have to call or visit? Is it possible to just get headers and get a cat converter from RRE? I saw this on another post about someone with issues with their cat converter and it would end up cheaper then getting the 400$~ OEM exhaust manifold.
 

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Discussion Starter · #6 ·
First of all, kudos on doing your research. These issues are sometimes hard to pinpoint and can be somewhat of a headache.

To start: any modifications?

Is the P0421 always on or does it come on and off as it pleases? Sometimes if you're not sparking correctly, the exhaust mixture might be a bit "dirty" and set it off. Also, check whether or not you're burning oil excessively.

Lets focus on P0174 and P0171 (System too lean) first. You know they're on both banks so at least its consistent.

#1 Check your spark plugs. If you're lucky, your plugs are old and not blasting off properly. Its free if you have to tools so its a good place to start. Also, it'll give you some clues. If they're black, sooty and fouled. You know you're running rich for sure. Consider replacing and move on to finding the original cause.

#2 Check your exhaust gasket (Cylinder head to exhaust manifold) and consider replacing both the cylinder head to exhaust gasket and the exhaust manifold to downpipe gasket ($10+$2+shipping from rockauto). An exhaust leak could make you read lean, causing your car to compensate by dumping more fuel and still read lean, then fouling your plugs and giving your o2 sensors and cats trouble doing its job.

#3 Conveniently fouled all your o2 sensors and/or have a oily gunked up intake manifold? Try seafoaming it. Might be about $10 max at a local autozone/o'reiley's/pep boys. Unlike alot of gimmicky injector cleaners, this stuff works really well.

#4 Vacuum leaks in your intake system? You'll be able to tell from your spark plugs if its lean too. Doesn't explain other symptoms as well though.

I'm leaning towards an exhaust leak but as I said before, these problems can be tricky so I can't guarantee it. Replacing spark plugs and seafoaming will help regardless but may not address the underlying issue.
Turns out I spoke too soon and those issues came back
 

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Basically Stock
2009 GS Turbo. 2007 GT
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I'm not sure what your issue is, but in regards to tuning:

You can contact either Hackish (Fastworks tuning) or Stayer for a remote tune. Both can be done over the internet. You will need a laptop and Tactrix Openport 2.0 cable(169 USD). I think the price is about $350USD, but you'd have to ask them. It's been too long
 
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