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Mike V.
Mike V., Subaru Technician
Category: Subaru
Satisfied Customers: 23519
Experience:  Certified Subaru Technician, factory trained by Subaru.
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Subaru: p0301..p0302,p0303,p0304 and p0172

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subaru code p0301,p0302,p0303,p0304 and p0172. can you tell me where to start eliminating problems please.
thank you
Hi. Thank you for you question.

What you have here is a misfire on all four cylinders and a lean fuel mixture.First and foremost you need to check for vacuum leaks. Very common with all Subarus.

I have dealt with several vacuum related issues with Subaru. From split vacuum hoses to blown intake gaskets.Check this first and let me know what you come up with.

Customer: replied 6 years ago.

Here is some things that were done to correct this already......could this have any affect on the problems?


The first thing they did was torque the plugs to spec (since I
apparently didn't tighten them down enough). Then they changed the
battery and battery cables because the excessive corrosion caused
current to run across the battery terminals.

Then they replaced the Idle Air Control Valve (IACV) because the old
one didn't open or close. This is a small part but cost me $458 plus
labor, taxes, and shipping.

They also did a compression test - all 4 cylinders checked out ok.

A week prior to taking the car to the shop, I changed the plugs and
air filter. I also replaced the front and rear catalytic converters -
$600 for the part. At the back end of the rear cat is a tapered donut
style gasket. The parts store gave me the wrong one so i had to reuse
the old one. I wonder if this is part of the problem. The cat also
contains two O2 sensors that I'll have to get them to check.

HI there! How does the car run? Does it feel like it is missing on all 4 cylinders? Let me know!
Customer: replied 6 years ago.

yes it runs rough and that is when we checked for error showed all 4 cylinder misfire codes and The last one is the fuel trim malfunction (A/F too rich).


thank you, XXXXX XXXXX helps.

How many miles do you have on the car? Also, has it had any timing belt work done to it?
Customer: replied 6 years ago.
130,000 documented history of timing belts.
OK, here is where I would start. Take a spray bottle and put some water in it. With the car running,spray the ignition coil and check for sparks arcing. If there are none, I think you should check the timing belt. It may have slipped a couple of teeth. This would cause the valves to open at the wrong time and give you the multiple cylinder misfire. It is rare to have these all together. I find it is normally a timing issue when it occurs. The 172 code is probably due to the fact that all of the fuel is not burning.
Customer: replied 6 years ago.
could it have anything to do with front oxygen sensor on front catalytic converter? i changed the catalytic converters after changing plugs and air filter.....back donut gasket was reused because parts sent had too small of gasket with .
This should not be a factor as even if you have an exhaust leak, it is after both sensors. According to the service manual, we need to check these other codes first, then go after the 172 code. Like I said, if the car is not burning all of the fuel, the front O2 will see to much fuel and throw this code
Mike V., Subaru Technician
Category: Subaru
Satisfied Customers: 23519
Experience: Certified Subaru Technician, factory trained by Subaru.
Mike V. and 4 other Subaru Specialists are ready to help you
Customer: replied 6 years ago.
I forgot to mention the car is almost undriveable when cold but
after about 10 or 15 minutes it smooths out. I think that may help
pinpoint it a little better.
So, the miss fires don't stay with the car? I mean it only misses when it is cold?
Customer: replied 6 years ago.
Hi. It misses constantly when cold. After about 15 minutes running it
smooths out but still misses periodically, I'd guess about every 2
minutes at the longest. If driven before completely warm, it's very
rough, slow to respond to throttle, with small backfires that sound like
they're coming from ahead of the exhaust.
Boy, you are making me think possible front O2 sensor or IATor coolant temp sensor. If the belt was off it would run the same hot or cold. The intake air tem sensor or the coolant temp sensor can cause problems with a hot or cold engine. Possibly the thing is getting the wrong info and doing improper adjustments with the fuel mixture. You really need a scan tool to look at these,but I actually think I might start there.
Might be worth it to pay for the diagnostics. You could do the work yourself, but I think a scan tool that can read real time data.
Customer: replied 6 years ago.
the misfire codes and the fuel mix code showed up on a code reader in the beginning. for some reason they just haven't been able to correct it..
Yeah, the code reader can't read the data streem. We need thos numbers to help check the other items. I know the book said to check the misfire codes first, but incorrect fuel mixture is going to cause the rough running and the misfires.
One other thing that just occured to me. Take a fuel sample. Diconnect it at the fuel filter and pump some into a clear glass jar. Let it sit for a while and see how much water is in it. Might just be bad gas

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