Code P0302 Misfire - new plugs - Subaru Outback - Subaru Outback Forums
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post #1 of 50 (permalink) Old 11-28-2017, 08:40 AM Thread Starter
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Code P0302 Misfire - new plugs

I started getting P0302 after a month or so since I replaced all 4 spark plugs.....I bought some cheaper brand from Advanced Auto. I also need to get the car inspected and CEL will fail me. My question is can new spark plug cause P0302 misfire code?

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post #2 of 50 (permalink) Old 11-28-2017, 09:28 AM
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yes,

typically the misfire is caused by a loose plug wire at the plug or on the coil, a loose plug, or oil getting on the plug from a bad plug oring.

reusing the old plug wire(s) if they were still good is a option also.

what cheap plugs do is fail early. (the $10 you saved costs time and more money).

the NGK that subarus like are not very expensive as they are so small on 2000-2009 plain H4 engines. (turbo and H6 engines have long $$ ones).

Advance/Autozone etc. sometimes gets into negotiating price pissing contests with NGK, and don't stock what you want vs. their neighbor.
...or amazon or rockauto. (online they are always there and probably cheaper). the last set of NGK plugs I bought for my H4 were NGK double plats...and they were still cheap,...like $18 for 4. I am running the same plug wires for 50,000 miles. (subaru OEM sold packards back in the day...packard wires were like limited production wires for GM things at the time)...I will change them at 60,000 for NGK.


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post #3 of 50 (permalink) Old 11-28-2017, 02:56 PM
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P0300 is a generic misfire code. P0302 means the cam/crank sensor rotational velocity measurement zeroed in on Cylinder #2 as the source of instability. It could be a bad plug, fouling of that plug, a bad wire, or a bad coil pack, partially clogged injector, a valve seal issues, etc.

Before you go crazy, start with the basics. Observe your running engine on a damp, dark night. When I was getting a misfire code, I could actually 'see' a corona glow from leakage at the wire boot to coil pack junction. Remove the wire from the coil pack and inspect both sides for any metal discoloration or carbon tracks. Cleaning it up 'fixed' it temporarily, but it would return every month or so. Eventually I just replaced the wires and coil pack.

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post #4 of 50 (permalink) Old 11-28-2017, 03:10 PM Thread Starter
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I forgot to mention that I do not notice any problem with the engine in any way. It sounds and drives like it always did.
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post #5 of 50 (permalink) Old 11-28-2017, 06:44 PM
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The On Board Diagnostics system is much more sensitive than your tush, and can 'feel' things that you often won't be aware of. If you read out not just the Pxxxx number but also download the full freezeframe data, you will see exactly how many misfires occurred in the string on that cylinder along with each of the other 3 at that time, plus ground speed, rpm, load, temp, fuel trim, etc. Combined, that data can help you plan your next step.

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post #6 of 50 (permalink) Old 11-28-2017, 06:54 PM
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Quote:
Originally Posted by Fibber2 View Post
P0300 is a generic misfire code. P0302 means the cam/crank sensor rotational velocity measurement zeroed in on Cylinder #2 as the source of instability. It could be a bad plug, fouling of that plug, a bad wire, or a bad coil pack, partially clogged injector, a valve seal issues, etc.

Before you go crazy, start with the basics. Observe your running engine on a damp, dark night. When I was getting a misfire code, I could actually 'see' a corona glow from leakage at the wire boot to coil pack junction. Remove the wire from the coil pack and inspect both sides for any metal discoloration or carbon tracks. Cleaning it up 'fixed' it temporarily, but it would return every month or so. Eventually I just replaced the wires and coil pack.
I think recently @plain OM said that he did not believe subaru used the P0300 generic code in any cars.
(edit; clarified / corrected in next posts).

I have seen it in other makes denoting multiple minor misfires that were not active enough to make individual misfire codes. I have seen subarus flash a check engine light and not store a code misfires though.

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post #7 of 50 (permalink) Old 11-29-2017, 07:55 AM
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My 2014 shop manual lists P0300 as a valid code (Random/Multiple cylinder Misfire Detected), but it's always possible that Subaru's software writers suppress it in favor of an outright cylinder designation (P0301 thru P0304). Consistent with what you mentioned, my 2002 manual does not include P0300! Either way, the OP's recovery of a P0302 code points him in a direction to start looking.

He'd get even more info if he can capture the full freezeframe data. Then he can see if other cylinders are getting into the act (a systemic issue) or if it's isolated to the one.

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post #8 of 50 (permalink) Old 11-29-2017, 08:32 AM
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Re: P0300
@Fibber2 is correct.

It's not in the Gen 2 or Gen 3 FSMs, as I had noted in this post.

At that time I hadn't looked beyond 2009; however, following up, it is indeed shown in the 2013 and 2014 FSMs, but not in the 2011 or 2010. (I don't have 2012. ) So, a different approach within the same generation.
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post #9 of 50 (permalink) Old 11-29-2017, 11:48 AM
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I'll bet that the 2012 is consistent with the prior EJ diagnostics. 2013/2014 is the FB engine. The new engine probably required complete certification which includes OBDII compliance testing.
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post #10 of 50 (permalink) Old 12-07-2017, 03:35 PM Thread Starter
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Replaced cylinder 2 spark plug. CEL went away after 30 miles of driving but then came back on shortly after. What do you recommend replacing/checking next?

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