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140K mile. 2nd owner....Car has been good to me. Just recently it started coughing at start-up and with any 'climbing'. Flat freeway I am just fine. I avg about 28mpg.

Had to dump $700 in brakes today and the guy did a quick code read for me. I got P0301,303 and 303 plus P0420. He recommended replacing the O2 sensor, and doing a complete tune-up with coils for $700. I had researched that changing the plugs in these beasts are knuckle scrapers. :(

I am think to just replacing the O2sensor first. Good or bad idea? And, some of what I have read is the local parts stores don't always list the correct match. So, is it better to buy from the dealer this particular part? Can an average guy with average tools replace the O2sensor? I think there are two sensors....what would this one be called?

Then, to the subject of the plugs. I am running lean funds. It seems like an average guy can change the plugs? What are coils? Are those the spark plug caps? Since I cannot really see the plugs is there any chance the spark plug caps have come loose? I am used to be able to see a distributor cap with plug wires.

My first time posting here cuz the Outback has been good to me. Thanks in advance for any help.

ONE LAST UNRELATED QUESTON. It seems the bearing on my driveshaft has gone bad and making noise. I have been driving with that noise for about 6 months. The brake guys recommended a reputable tranny shop to get that done. But, is it a packed unit and any idea what the typical cost to repair that is?
 

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Find another shop.

The misfires and the lack of power on hills is typical of a clogged filter. 301 is cylinder 1 misfire, 303 is cylinder 3. 1 and 3 are on the same side of the engine, passenger side.

The 420 code is on because of the misfires. Too much fuel entering the CAT and the CAT can't keep up.

Change the fuel filter and plugs only. Plug number BKR5E-11 or 6E-11. If the coil is bad, have the tech prove it. I don't change many coils on a Subaru. I think I did one in the last 6 years and I see a lot of Subarus.

You can do this yourself if you are strapped and have about an hour of time. The fuel filter is on the driver strut tower in front of the master cylinder and once the hoses are removed can be taken out of the clamp easily. The fuel filter is marked for flow direction. I find WIX filters work best.

The coil is that square box with the four cables, 2 on either side. You needn't worry about it at this point.

When you start on the plugs, give the ignition cable ends a bit of a twist one way and the other to loosen them from the spark plug. Remove the plugs and let us know what the tips looked like. Take a pic if you can and post it.

BEFORE installing the plugs you need to make sure the "gap" is correct. You can get a gap gauge at the same time you pick up the plugs and fuel filter. Cheap. Set the gap at .040. You could also ask the parts clerk if he/she could show you how to set the gap without damaging the plug tip.

Just for information purposes: The front sensor is the Air/Fuel sensor and the rear sensor is the O2 sensor. The front sensor feeds voltage data back to the computer to help with fuel trim settings to keep the ratio at stoichiometric. The rear sensor feeds data to the computer relating to the functioning of the Catalytic Converter. Since you have misfires, the fuel content in the exhaust is higher than normal so the feedback from the rear sensor will also be high indicating the CAT is not functioning efficiently. This does not mean an O2 sensor is bad, or the CAT. See my post http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diag.html .

Start with the simple things first, then move on if necessary.

I believe the plugs and filter will result in a dramatic change in the engine performance. When you are done, remove the negative cable from the battery for a minute, then reconnect it. This resets the ECM memory and clears the MIL (check engine light). If a problem remains in the system, the MIL will come back on after its driven a bit.
 

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wow...thanks for such a comprehensive reply. I am going to replace the fuel filter tonight. And, then try the plugs. I really appreciate your help.
 

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I like to be more of a contributor than I have been...but, wanted to report back in....I got a guy to change my plugs for a $100....fortunately he was experienced because he really had to massage the old plugs out. But, he got it done. The engine light went away and it ran great for a week. Then, during idles things got real rough. But, very erratic. He did a read with his little gadget and it said #2 cylinder is misfiring. Could that be a spark wire or coil or both? I was surprise to learn how expensive coils are...especially if I am just guessing. I was able to pass smog so that is good but would like to get the idling under control. Any ideas....?
 

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I agree with CARDOC. I had EXACTLY the same problem with backfires while using a little extra power to climb even the slightest hill. I also got the same codes. Mine was P0302, cylinder 2 misfire. I replaced the plugs, wires, and coil. I probably could have gotten away with just replacing the plugs. They are a pain to do, but not impossible for a typical shade-tree mechanic. You'll need a socket extension for your spark plug socket because they are pretty far down in the engine. You'll need to remove part of the air intake and the washer fluid bottle. No big deal and all you'll need for those is a Phillips head screw driver.

OK, just saw you've had the plugs done. Next is the wires. Fuel filter is a likely culprit and is easy to get to and not as costly as the coil. A bad coil can cause a lot of other symptoms as well. Anything else acting up with the ignition?

The P0420 code could be caused by the several things. I'd do the plugs, wires, fuel filter, coil, and PCV valve in that order. Clear the codes and re-read them. If the P0420 goes away it was probably related to the misfires.
 

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plug wires could easily be bad if original or cheap aftermarket. Denso is preferred, other brands all seem to have some bad reports.

cardoc has a great thread about the p0420 code.

I'd think a throttlebody cleaning and a PCV valve would be good ideas but.....
 

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Thanks David...I had replaced the fuel filter first. Then I did the plugs. Ok, I will look at the wires next. Are the wires pretty obvious, ie. cracked etc? It fires up fine and runs fine but has the erratic erratic idling issue. I will look at the wires this weekend and report back.
 

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A good test for the wires is to run the engine after dark with the hood opened. Take a spray bottle and spray a fine mist over the engine around the wires. If you see a blue sparking then the wires are definitely shot. You can also unplug one end from the coil and the other from the plug - be sure to listen that they "pop" back on when putting them back on the plug. Use a multimeter to check continuity between the ends. They should all be about the same. Obviously, if you notice they have physical damage when testing them then they might be bad. You'd think the mechanic that did the plugs would have noticed that though. I used NKG wires and haven't had any problems. Some folks here insist on using OEM Subaru wires or other brands. You can search here for their advice. Good luck and let us know how it goes.
 

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BTW the oem replacement wires that you'd get from the dealer would be NGK's which you can probably get at any parts store. I think DavidPeab meant NGK not NKG..
 
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