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Subaru outback check engine - bad tank of gas?

Hi all-
My 2000 outback wagon has only 67k miles. About a month ago, the check engine light came on. I took it to an Auto Zone and although unforutunately I didn’t write down the code, the person who checked the car seemed to think it was probably just a bad tank of gas. I ran through that tank of gas and filled up with premium gas. Light didn’t go off until THAT tank was near empty. Filled up a third time and light remained off until tank was near empty and it came on again. It has not yet gone off. I’m not sure how many more tanks of gas I should run through! The car isn’t running poorly so I thought the bad gas was a good explanation.

A side note: about 6 months ago I had to have the head gasket replaced. I argued with Subaru and got some extra work done in exchange for the head gasket payment since my car missed the recall by a few digits but I was obviously stuck with an inferior product. I’m hoping they didn’t do an inferior job. Even though it was 6 months ago, it wasn’t that many miles ago - you can see my the year/mileage on my car that I don’t drive a lot.
Any ideas other than to take it in? Does that gas explanation sound reasonable? Anything about a head gasket related?

Thanks in advance for your comments.

Go back to Autozone, get that code re-read and post it here.
WIthout it, we’ll just be guessing.
For instance, it could be as simple as the gas cap being bad or as complicated as the engine having serious issues.

Edit: And, whatever you do, don’t let them sell you anything, including service. Say “Thank you” and leave.
Autozone is to car repair what captain Ahab is to the whales.

+1 to Remco’s advice.

Just as “patriotism is the last refuge of scoundrels”, the “bad gas” mantra is the first refuge of those who know little or nothing about cars.
Is it possible to get a bad tank of gas? Sure!
Is this a frequent occurrence? Nope!

But, if you had gotten a tank of gas containing some water, the first thing that you would have noticed was a badly running/stalling engine. You tell us that your engine never ran badly, so it is fairly clear that gas was never the issue. And, many tanks of gas and many months later, it is all the more obvious that bad gas was never the issue.

The presence of the CEL means that the car’s OBD system has stored one or more trouble codes. Those codes could relate to virtually any engine problem (from trivial to serious), so rather than having us engage in random guessing, you really need to have the codes read, and to come back to this thread to report them.

Fair enough - I will go back and get the code and repost.

I don’t think it is simply “bad gas”. You have low miles, but the car is also a 2000 and is now 12-13 years old. It could be a whole bunch of things, but bad gas isn’t likely.

I guess the gas story was what I was hoping for (i.e. cheap). I’m disappointed in this car - lots of money already spent on it for repairs.

Anyway, here is the code:
P0420
Rich A/F ratio
Ignition system malfunction
Faulty catalytic converter

Any insight would be appreciated.

No doubt they’ll want to replace the Catalytic converter, along with both both O2 sensors. That’s definitely what a dealer or regular mechanic would do. The light will likely go off, at least for a while, but that’s an expensive tossing of parts at a car.

It may be an exhaust leak or other things that happen to throw that code.

The code is thrown when the reading from the first over the reading from the second O2 sensor does not meet some ratio. It could be that one of the o2 sensors is lazy, not giving the right reading.
With the right reader, one can actually read those values, seeing what the computer sees.
From there one can determine whether the cat is bad or an o2 sensor is not working.
There could even be an underlying problem that causes this to be thrown, unrelated to a bad sensors or CAT.

It would be difficult to help you without you having that capability in your hands.
This may be obvious but you need a mechanic that’s capable of properly diagnosing issues like this.
Perhaps people on the following forum can steer you towards a mechanic in your area that knows the intricacies of these sorts of problems, along with Subarus: http://www.ultimatesubaru.org/forum/

That is about the hardest code to troubleshoot that you can get. Most mechanics jump on the cat and that usually does not fix the problem. The first thing to look for is a leak in the exhaust system anywhere before the second O2 sensor. I have successfully fixed one of these by changing the second O2 sensor, but you have to get one from the dealer and they have to make sure it matches your VIN number.

Note: Premium gas is higher octane (which means it burns slower) and will function better for high compression engines. It will not function better in lower compression engines where you don’t need it.

Thanks everyone. I had the exhaust system replaced less than a year ago - just dug up the receipt because I couldn’t remember how long ago the work was done. I also found the warranty for the catalytic converter and it looks like it is still covered. As far as I can tell, the O2 sensors weren’t replaced -is that not a standard thing to replace along with muffler/cc etc?

Usually when they replace the CAT they replace the O2 sensors as well but more often than not, this is out of convenience to get the code to go away. That, and they can charge more.
Again, it could be that you’re having some underlying problem that happens to throw this code. It takes a bit more investigating to be sure.
If you do end up having some other problem, you could replace the O2 sensors and have the same problem pop up a couple of months from now if the underlying problem isn’t fixed.

The code means too much raw gas is getting to the cat, and perhaps past the cat. The air/fuel mixture is too rich. This could be an ignition system (plugs, wires, timing, computer control), low compression, poorly adjusted valves all will contribute to less than complete combustion.

Or, the cat could be worn out. Since it is a new cat, likely not the cat. How much to diagnose and fix? Seems like those that have tried previously have taken the “replace parts” and hope route.

Are you confident in your mechanic, or shop? Perhaps a new independent shop with extensive Subaru experience? Or, a different Subaru dealer than the one used if you like dealer service depts.

A subtle misfire for whatever reason on even one cylinder could cause that code to appear and the symptoms that you list.

Does this car have the original spark plugs in it?

The car had a 60K tune up when the head gasket was replaced just this past Feb. Spark plugs were changed at the time. Interestingly enough, while digging through my receipts I found one receipt from 2006 that mentioned the engine light being on and the gas cap being replaced as a fix. That receipt mentions both code 0442 and 0440.

I might start with the dealer although the catalytic converter was replaced by the local mechanic. Not sure who I trust the most, or the least. I just don’t want an endless series of repairs that don’t fix the problem, but this looks like it may be one of those. Still wish I had my Toyota!