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I have a Subaru Outback 2005 2.5L and my engine light came

Hi Pearl, I have a...
Hi Pearl, I have a Subaru Outback 2005 2.5L and my engine light came on some time ago. I took the car to a trusted mechanic who found the diagnostic code for the fault was P0304 (Cylinder 4 misfire). He cleared the code, replaced the spark plugs and spark plugs leads and the engine light went off. A few days later the engine light came on again. I returned the car to the mechanic who checked the code again and found P0304 again. He switched the injector, spark plug and spark plug between cylinder 3 and 4. Car tested OK. A few day later the engine light came on again. Through all this the car drives perfectly, no misfiring, no trouble a slow or high speed but the light continues to come on and the go off repeatedly. Is there anything else you know of that would cause the light to come on like this?
JA: Do you have a scan tool to read the codes?
Customer: I don't but the mechanic does.
JA: Are you fixing your Outback yourself? What have you tried so far?
Customer: I've detailed that in the message, I have also taken the car to my local Subaru dealership who couldn't fix the issue either.
JA: Anything else you want the mechanic to know before I connect you?
Customer: No, thats all thanks
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Answered in 1 minute by:
11/18/2017
Mike V.
Mike V., Subaru Technician
Category: Subaru
Satisfied Customers: 26,051
Experience: Certified Subaru Technician, factory trained by Subaru.
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First, the ignition coil could be causing this. Sometimes the coils fail but only under certain conditions, so it might be misfiring but you don't feel it. Like if it's misfiring at higher speed for example, it's not going to be hugely noticeable.

If it persists with a new coil, then he's probably going to need to do a leak down test. If there is any carbon buildup on the valves it can cause this problem as well.

Please let me know if there is anything else I can do to help.
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Mike V.
Mike V., Subaru Technician
Category: Subaru
Satisfied Customers: 26,051
Experience: Certified Subaru Technician, factory trained by Subaru.
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Customer reply replied 28 days ago
Hi Mike
Thanks for your response. If it was a faulty coil, I'd expect to be getting different codes (P0301,P0302,etc) but the issue seems to always occur on cylinder 4 (P0304). Is my understanding correct?
Thanks

Well, it depends on the if the coil is failing at more then one post to be honest.

Sometimes they fail across from each other so you would see #3 and #4 misfiring, but not always. If there is any carbon track on #4's post, then it can be causing the issue.

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Mike V.
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