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Discussion Starter #1
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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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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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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Discussion Starter #4
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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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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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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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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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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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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Discussion Starter #10
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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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?
any oil or soot on the plug that came out?
 

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Did you inspect the boots (spark plug and coil end) for hardness, cracks, carbon tracks? How do the silver connector ends look? How about the coil conductors? Was there anything odd about the plug you pulled out?

Remember that this could also be a fuel or air issue.
 

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Coils are a weak point on Subarus. You can usually see carbon tracking around the terminal when it fails. I would suggest replacing both the coil and spark plug wires. I have had good results doing that.

Good luck!
 

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subaru coils rarely fail but occasionally do. (like from advanced age ).

my own 2002 H4 one is original and it says "subaru" etched on the front., and I wonder what is on the OP's car today.

I would put a used OEM one on before getting a new aftermarket one. (some people here have had to throw those aftermarket ones in the garbage, and sad they spent money on trying them).
 

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Discussion Starter #16
Did you inspect the boots (spark plug and coil end) for hardness, cracks, carbon tracks? How do the silver connector ends look? How about the coil conductors? Was there anything odd about the plug you pulled out?

Remember that this could also be a fuel or air issue.
I did not notice anything unusual
 

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Discussion Starter #17
I reset CEL. After 133 miles of driving, CEL came back.
Right now my concern is to pass state inspection. Do you think the inspection will somehow see reset CEL?
 

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I reset CEL. After 133 miles of driving, CEL came back.
Right now my concern is to pass state inspection. Do you think the inspection will somehow see reset CEL?
yes,

but whatever you have to reset the code probably can see what the readiness is and see if you have enough good data there without a reset to clear it out and loose that data.

I use the $5 torque pro app on my cheapo droid phone (just needs 4.0 or newer). seen the same screens on cheapo mechanics hand helds too.

but usually on any device or app under "real time data" is a
"ODBC2 emissions readiness" part that shows the individual sensors and if they are ready to be tested and pass. I live in NY state and the one they let you have still not ready here is the evap system. but all the rest need to be listed as passed before you take the test.

or they give you a 10 day paper to drive around and try to get enough to pass again.

I am trying to figure out right now how long my 4 cylinder subaru needs to run without clearing codes to get all those needed things to pass.
(I don't think it is very long on a otherwise good car, but winter time might make some items round up enough good data "slow" )
 

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I often don't clear the codes. Inactive code do not cause a failure. If the codes haven't been cleared, once the misfire is gone the CEL will turn off and you will be emissions ready. CEL off and all of the readiness tests done and you are good to go. Here in Washington 2001 and newer cars can have up to two readiness monitors incomplete and still pass.

Generally the catalyst can take a few drive cycles to reset. By far the Evap monitors is the one I most often have problems with. Some cars are more difficult than others.


Here is a good generic OBDII drive cycle that should allow the readiness monitors to run:

The OBDII drive cycle begins with a cold start (coolant temperature below 122 degrees F and the coolant and air temperature sensors within 11 degrees of one another).
NOTE: The ignition key must not be on prior to the cold start otherwise the heated oxygen sensor diagnostic may not run.
1. As soon as the engine starts, idle the engine in drive for two and a half minutes with the A/C and rear defrost on. OBDII checks oxygen sensor heater circuits, air pump and EVAP purge.
2. Turn the A/C and rear defrost off, and accelerate to 55 mph at half throttle. OBDII checks for ignition misfire, fuel trim and canister purge.
3. Hold at a steady state speed of 55 mph for three minutes.

OBDII monitors EGR, air pump, O2 sensors and canister purge.
4. Decelerate (coast down) to 20 mph without braking or depressing the clutch. OBDII checks EGR and purge functions.
5. Accelerate back to 55 to 60 mph at half throttle. OBDII checks misfire, fuel trim and purge again.
6. Hold at a steady speed of 55 to 60 mph for five minutes.

OBDII monitors catalytic converter efficiency, misfire, EGR, fuel trim, oxygen sensors and purge functions.
7. Decelerate (coast down) to a stop without braking. OBDII makes a final check of EGR and canister purge.

Evaporative leak test.
8. Ensure the fuel level in the fuel tank is between the ¼ and ¾ level. Park the car for 10½ hours with an ambient temperature from 40° — 113°F.
9. The vehicle should be driven at a steady-state of speed of 55mph, with minimal throttle angle change for more than 5 minutes.
 

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Passing the inspection is indeed important. But ignoring the problem will cost you greatly if you continue to ignore it. Misfires can lead to burned valves and damaged catalytic converters. Next you'll be telling us about the P0420 codes you've logged!

If you can't solve this yourself, seek out a trusted mechanic.
 
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