How to check Subaru spark plugs?

In brief, you should inspect each plug for wear, verify the gap to the manufacturer’s specification, and replace worn or fouled plugs with the correct type and torque for your Subaru engine.

What you need to know before you start

Subaru engines often use a coil-on-plug ignition layout, with each cylinder having its own coil or boot. Accessing the plugs typically requires removing the engine cover and possibly the coil packs. Always consult your owner's manual or a factory service manual for your specific model to confirm plug type, gap, and torque specs.

Tools and model-specific considerations

Gathering the right tools and knowing your model’s specifications helps ensure a safe, accurate check. The following list covers common items you’ll need.

• Spark plug socket (usually 5/8 inch or 16mm) with a deep well and a rubber insert


• Ratchet, extension(s), and a swivel joint for access in tight engine bays


• Torque wrench capable of the engine’s specified range


• Spark plug gap gauge or feeler gauge


• Dielectric grease for spark plug boots/coils


• Replacement spark plugs of the correct type for your engine (OEM or equivalent platinum/iridium plugs as recommended)


• Optional: anti-seize compound (only if the manufacturer’s guidance allows it)


• Optional: spark plug boot puller or magnet for handling plugs


• Safety gear: gloves and eye protection

With the right tools, you can inspect each plug carefully and prepare for any needed maintenance. Always follow model-specific instructions to avoid damaging threads or coils.

Accessing Subaru spark plugs

Most modern Subarus use coil-on-plug ignition, so you’ll access the plugs by removing the coil boot assemblies rather than traditional spark plug wires. The steps below assume a typical boxer-engine layout common to many Subaru models, but variations exist between EJ, FA, FB, and EZ-series engines. If you’re in doubt, refer to the service manual for your exact model and year.

1. Ensure the engine is off and cool. Disconnect the negative battery terminal to prevent any accidental short or shock.


2. If present, remove the engine cover to expose the ignition coils and spark plugs.


3. Identify the coil-on-plug assemblies for each cylinder. Gently disconnect the wiring harness from each coil and remove the coil mounting bolts. Lift the coil away from the spark plug, taking care not to damage the boot.


4. Attach a spark plug socket to your ratchet, using an extension as needed. Carefully loosen and remove the spark plug from its well. Use a magnetic pick or a gentle wiggle to retrieve the plug.


5. Inspect the removed plug or set it aside for later comparison with other cylinders. If you’re planning to replace, keep track of which plug came from which cylinder.


6. Before reinstalling, clean the plug well area to remove any loose debris. If you’re replacing a plug, install the new one following the steps in reverse order (coil, boot, and eventually the cover).

Accessing the plugs can be the trickiest part of the procedure on some Subarus due to tight engine bays. If you encounter stubborn bolts or limited space, a small-angle swivel or another extension can help, but avoid forcing parts to prevent damage.

What to look for when inspecting spark plugs

Visual inspection of each plug gives you clues about engine health and fuel mixture. Here’s what to watch for on Subaru plugs.

• Normal wear: a light tan or grayish color on the insulator and electrode, with a slightly worn but intact ground electrode


• Fouling from oil: oily or wet dark deposits around the insulator or electrode surfaces


• Carbon fouling: dry, black, sooty deposits indicating a lean or rich-running condition or faulty ignition


• Fuel fouling or overheating: white or blistered insulator, melted electrodes, or excessive electrode wear


• Damage: cracked porcelain, melted insulator, bent or broken electrodes, or thread damage


• Cross-threading or loose fit: visibly damaged threads or plugs that won’t seat firmly

Document any unusual findings and compare plugs across cylinders. Significant differences can indicate a failing injector, coil, or vacuum issue that warrants further diagnosis.

Gap checking, testing, and reinstallation

After removing and inspecting, the next steps are to verify the gap and, if needed, test spark, then reinstall with proper torque. Note that many modern Subaru plugs are pre-gapped; if you do re-gap, handle the plug carefully to avoid damaging the electrode.

1. Check the gap with a gauge. If the plug is designed to be pre-gapped, avoid re-gapping unless you have a precise spec from Subaru for your engine. If you must adjust, set the gap to the OEM specification for the model and plug type.


2. If a plug is acceptable, clean the threads and seating surface. A light coating of dielectric grease on the inside of the coil boot can help with future removal, but avoid getting grease on the electrode.


3. Reinstall the plug using a proper torque. Use the torque wrench and tighten evenly to the manufacturer’s torque specification for your engine and plug type. Do not overtighten to avoid thread damage.


4. Reinstall the ignition coil/boot over the plug and secure with its mounting bolts. Reconnect the wiring harness. Repeat for each cylinder as needed.


5. Reconnect the negative battery terminal and start the engine to confirm proper operation. Monitor for misfires or check engine codes if you have a scan tool available.

For a definitive ignition check, you can perform a live spark test with a spark tester between the coil and plug, and watch for a bright, consistent spark across all cylinders. If you don’t see a strong spark in any cylinder, that coil, boot, or wiring may be failing and require replacement.

When to replace spark plugs in a Subaru

Spark plugs wear over time, and replacement schedules depend on plug type and engine. Subaru’s newer vehicles often use iridium or platinum plugs with longer service intervals, commonly ranging from 60,000 to 100,000 miles, depending on the exact engine and driving conditions. Copper plugs typically require earlier changes, around 30,000 miles. Always follow the maintenance schedule in your owner’s manual and use OEM-recommended plugs for your model.

Safety considerations and maintenance tips

Work in a well-ventilated area, secure the vehicle, and use proper lifting or supports if you’re jacking up the car. Disconnecting the battery is essential when working near electrical components. Never force plugs or coils into place, and keep track of all components as you remove them. If you’re unsure about your model’s layout or torque specs, consult a Subaru-certified technician or the official service manual for your year and engine code.

Summary

Checking Subaru spark plugs involves identifying and accessing the plugs, visually inspecting for wear or fouling, confirming the proper gap, and reinstalling or replacing with the correct plugs at the recommended interval. The steps vary slightly by engine type (J-series EJ, FA/FB, and newer EZ engines), but the core process—safety first, careful access, inspection, gap confirmation, and proper torque—remains consistent. Always reference your model’s service manual for exact specifications and procedures.

How do I know if my Subaru spark plugs are bad?

One of the first signs of spark plug wear is trouble starting your Subaru. If the engine cranks longer than usual before starting or if you experience intermittent starting issues, it's time to inspect your spark plugs.

How often should I change my Subaru spark plugs?

Subaru recommends spark plug replacement at 60,000 to 100,000 miles, depending on the model and type of spark plug, so check your owner's manual for the specific interval. For some models, the 60,000-mile mark is a key replacement point, while others have a longer interval up to 100,000 km or miles. 
You can watch this video to learn how to replace spark plugs in a Subaru Forester or Outback: 55sTDR AutoYouTube · Jan 17, 2021
Key factors

• Model and year: Different Subaru models have different maintenance schedules. 
• Spark plug type: The recommended interval depends on whether the car uses copper, platinum, or iridium spark plugs. 
• Owner's manual: The most accurate source is your car's owner's manual, which provides the precise mileage for your specific vehicle. 
• Symptoms: If you notice signs of worn-out spark plugs, such as hard starting, rough idling, or poor acceleration, you may need to replace them sooner than the recommended interval. 

You can watch this video to learn how to replace spark plugs in a Subaru Legacy 2.5i: 50sCool Stuff Guys LikeYouTube · Apr 15, 2018

How can I check to see if my spark plugs are bad?

You can check for bad spark plugs by observing engine performance issues like misfires, rough idling, poor fuel economy, and difficulty starting. You can also perform a physical and visual inspection of the removed spark plugs, looking for signs of wear, carbon buildup, or physical damage, and use a spark plug tester to check if they produce a strong, blue spark. 
This video demonstrates how to inspect spark plugs for wear and other issues: 1mEngineering ExplainedYouTube · Apr 20, 2016
Performance symptoms to watch for

• Engine misfires: The engine hesitates, stumbles, or jerks, especially when accelerating. 
• Rough idling: The engine shakes or vibrates noticeably when the car is stopped. 
• Poor fuel economy: You may notice a decrease in your car's gas mileage. 
• Difficulty starting: The engine cranks slowly or takes longer than usual to start. 
• Lack of acceleration: The car feels sluggish and doesn't accelerate as quickly as it should. 
• Check Engine Light: A faulty spark plug can trigger the check engine light. 

Physical inspection and testing 

1. Remove the spark plug: Carefully remove the spark plug from the engine. 
2. Visual inspection:
   • Look for cracks in the ceramic insulator. 
   • Check the electrode tip for wear or rounding. 
   • Inspect for deposits like carbon, oil, or fuel buildup. 
   • Ensure the gap between the center and ground electrode is within the manufacturer's specifications, using a spark plug gap tool if necessary. 
3. Spark test:
   • Reconnect the plug to its wire, then ground the plug's metal body against the engine block. 
   • Have someone turn the ignition to crank the engine. 
   • Observe the spark at the electrode tip. A strong, blue spark indicates the plug is likely good; a weak, yellow, or absent spark suggests it's bad. 
   • Safety note: Use insulated tools to perform this test and avoid contact with high-voltage parts. 

You can watch this video to learn how to perform a spark test: 59sehowautoYouTube · May 21, 2015

Can I check my spark plugs myself?

Yes, you can check spark plugs yourself by performing a visual inspection for wear, carbon buildup, or oil fouling, and by testing for spark using a simple tester or a multimeter to check resistance. For modern cars, visual inspection is often the most accessible method, while spark plug testers can confirm if the spark plug is functioning properly. 
Visual inspection

1. Disconnect: Carefully pull the spark plug wire or ignition coil off the spark plug. 
2. Remove: Use a spark plug socket and ratchet to unscrew and remove the spark plug. 
3. Inspect: Check the tip of the plug for signs of wear, carbon buildup, or oil fouling. 
4. Check gap: Use a spark plug gap gauge to measure the distance between the electrode and the ground strap, ensuring it's within the manufacturer's recommended range. 

This video demonstrates how to inspect a spark plug after removing it: 1mEngineering ExplainedYouTube · Apr 20, 2016
Testing for spark

1. Disconnect: Remove the spark plug from its wire or coil and place it in the boot. 
2. Ground: Touch the metal body of the spark plug to a clean, unpainted metal part of the engine block. 
3. Crank: Have someone briefly crank the engine. A bright, steady spark should jump across the gap at the tip of the spark plug. A weak, yellow, or orange spark indicates a problem. 
4. Safety first: Use electrically insulated pliers to hold the plug, and avoid touching any part of the ignition system while cranking the engine. 

You can watch this video to learn how to check for spark by yourself: 51sProblem Solver GarageYouTube · Feb 20, 2023
Multimeter test

1. Set to Ohms: Set your multimeter to the Ohms (Ωcap omegaΩ) setting.
2. Connect probes: Touch one probe to the metal threaded end of the spark plug and the other probe to the metal tip at the very end of the plug.
3. Check reading: An acceptable reading is typically between 4,0004 comma 0004,000 and 8,0008 comma 0008,000 Ohms, though this can vary. 

This video shows how to test a spark plug with a multimeter: 58sTightWadDIYYouTube · Apr 9, 2019
Spark plug tester

1. Connect: Attach the tester's clip to the spark plug wire or coil, and connect the other end to the spark plug itself.
2. Crank: Have someone crank the engine.
3. Observe: A light on the tester should flash, confirming that a spark is being generated.