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Subaru Outback Spark Plugs Replacement Guide
How to check or change the spark plugs in the FB25 2.5L engine of a 6th generation 2015 to 2018 Subaru Outback.

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2017 Outback FB25 Engine
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Spark Plugs - Sides of Engine
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Two Plugs - Passenger Side

This automotive maintenance tutorial was specifically written to assist owners of the sixth generation (2015, 2016, 2017, 2018 and maybe also the updated 2019 and 2020 model years) Subaru Outback station wagon in checking or changing the spark plugs in the FB25 2.5 liter four cylinder Boxer engine.

Owners of other Subaru vehicles such as the Forester, Impreza, Legacy, XV Crosstrek, WRX, BRZ, SVX, Tribeca, Exiga, Trezia, Levorg and Baja may also find these DIY instructions to be helpful.

The OEM spark plugs in this 2017 Subaru Outback were the laser iridium NGK SILZKAR7B11.

A few other compatible replacement spark plugs with their part numbers are as follows: Autolite XP5683, Denso (4712) IXEH22TT, Subaru 22401AA781 and Denso FXE22HR11.

The tools needed to remove and replace the spark plugs include a 10mm socket with a 1/4" drive ratchet, a 14mm spark plug socket or a 9/16" spark plug socket, a short extension bar, a 3/8" drive ratchet and a tube of dielectric grease.

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Two Ignition Coils
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Single Bolt - Ignition Coil
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Loosen Counterclockwise
The Subaru FB25 "Boxer" engine has two spark plugs mounted horizontally on both the driver and passenger sides of the engine.

For easier access to the ignition coils and the spark plugs, you may choose to remove the engine air filter box on the passenger side and the 12V automotive battery on the driver side.

I chose to start on the passenger side of the engine. The procedure is the same for either side. If you don't remove the 12V battery, place a thick towel or mat over the top of the battery to help prevent accidental electrical contact.

Remove the single bolt that secures the ignition coil to the engine by turning it in the counterclockwise direction with a 10mm socket and a 1/4" drive ratchet.

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Single Bolt Removed
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Ignition Coil Bolt Removed
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Remove Bolt - 2nd Coil
Then remove the 10mm bolt on the second ignition coil housing by turning it in the counterclockwise direction.
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Second Bolt Removed
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Both Coil Bolts Removed
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Open Air Box - Easier Access
Set the two 10mm bolts aside in a safe place.

For easier access to the ignition coils, I released the two metal latches on the air box to separate the two halves.

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Pull Out Ignition Coil
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Ignition Coil Removed
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9/16" Spark Plug Socket
Carefully pull the ignition coil housing straight out of the side of the engine.

If you have trouble removing the ignition coil, try gently wiggling or rotating it back and forth while pulling it straight out.

Set the ignition coil aside in a safe place.

To remove the spark plugs, you can use either a 14mm spark plug socket or a 9/16" spark plug socket.



 

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Insert Spark Plug Socket
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Push On To Old Plug
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Loosen Counterclockwise
To prevent from having the spark plug socket pop off and become stuck in the spark plug well, I highly recommend attaching it to the extension bar with some packing tape or something stronger like Gorilla Tape.

Attach the spark plug socket to a short extension bar.

Push the socket and extension bar into the spark plug well and securely attach it to the top of the spark plug.

Loosen the old spark plug by turning the ratchet in the counterclockwise direction.

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Old Spark Plug Removed
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Inspect Old Spark Plug
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Spin In New Spark Plug
Once the old spark plug is loose, detach the ratchet and spin out the old spark plug the rest of the way by hand in the counterclockwise direction.

Detach the old spark plug from the socket.

Inspect both sides of the old spark plug.

If the electrode end of the old spark plug appears to be ashy white, the plugs might have been exposed to high temperatures such as overheating or they might be the incorrect heat range for your driving conditions or environment.

If the electrode tip of the old spark plug looks grey or is covered in dark black soot, the engine might be burning oil and should be inspected by a professional mechanic.

I recommend buying the genuine OEM NGK SILZKAR7B11 laser iridium spark plugs that are installed in the Outback at the factory.

Push the new spark plug into the socket and insert it into the spark plug well.

Carefully spin the new spark plug in the clockwise direction by hand to prevent it from becoming cross threaded.

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Tighten Clockwise
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Dielectric Grease
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Re-Insert Ignition Coil
Make sure your spark plug socket is securely attached to the extension bar with some tape to prevent from having it pop off and become stuck in the well.

(If you do get your socket stuck in the spark plug well, just re-insert the extension bar, loosen the spark plug and fully remove it to recover the socket.)

Tighten the new spark plug in the clockwise direction.

If you are replacing the old spark plug after checking it, you only need to tighten it to a small fraction of a turn past finger tight.

If you are installing a new spark plug, tighten it to just past the point when you feel the new crush washer collapse.

Try to avoid over tightening the new spark plug to prevent from cracking it or damaging the threads.

Apply a small amount of dielectric grease to the opening at the bottom of the rubber dust boot on the ignition coil housing.

The dielectric grease will help keep out moisture and debris to ensure a reliable electrical connection.

Push the ignition coil straight back into the spark plug well.

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Ignition Coil In Place
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Spin In 10mm Bolt
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Tighten Bolt Clockwise
Re-insert the 10mm bolt and spin it a few turns by hand in the clockwise direction to prevent it from becoming cross threaded.

Tighten the ignition coil bolt in the clockwise direction with the 10mm socket and a 1/4" drive ratchet until it is snug.

Try to avoid over tightening the bolt to prevent from cracking the plastic ignition coil assembly.

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Re-Assemble Air Box
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2 Spark Plugs - Driver Side
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Driver Side Ignition Coils
If you opened the engine air filter box, close it and secure the two metal clips.

Repeat the process to replace the two spark plugs on the driver side of the engine bay. Be sure to cover the 12 volt car battery terminals with a thick towel or a rubber mat to prevent yourself from being electrocuted.

Record the spark plug change in your vehicle's service records.

For more, please check out all of my 2015-2018 Subaru Outback DIY Repair & Maintenance Guides.
 

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