How do I know if my Subaru coil pack is bad?

Common signs point to a faulty coil pack: misfires, rough idle, hesitation or loss of power, and a check-engine light often accompanied by misfire codes (P0301–P0306). A definitive diagnosis usually involves reading codes with an OBD-II scanner and testing or swapping coils to see whether the problem follows the coil.

What a Subaru ignition coil pack does

Subaru engines typically use coil-on-plug (COP) or per-cylinder coil assemblies. Each coil delivers the high voltage needed to spark a plug in its own cylinder. When a coil or its boot fails, spark delivery can weaken or stop, leading to misfires and related drivability problems. This design makes diagnosing coil problems both straightforward and tricky, because symptoms can overlap with issues from spark plugs, injectors, or vacuum leaks.

Common signs of a failing coil pack

Look for a cluster of symptoms rather than a single clue. The most frequent indicators are listed below.

• Engine misfires or rough running, especially under load or during acceleration


• Check Engine Light with misfire-related codes (P0301–P0306) or a generic P0300 (random/multiple misfires)


• Hard starting, misfiring after a warm start, or stalling


• Noticeable loss of power or hesitation when pulling away from a stop


• Reduced fuel efficiency and elevated emissions due to incomplete combustion


• Visible damage to coil boots, moisture intrusion, or corrosion on connectors

If you notice several of these symptoms together, particularly a persistent misfire code, coil testing should be on the short list of likely causes.

Diagnostic steps to confirm a bad coil pack

Before replacing parts, run through a practical set of checks to determine whether the coil pack is the culprit.

• Check for trouble codes with an OBD-II scanner and note cylinder-specific misfire codes (P0301–P0306) and any P0300 patterns


• Inspect spark plugs and boots for wear, fouling, cracks, or oil contamination; replace plugs if they’re worn or fouled


• Perform a coil swap test: move the suspect coil from a misfiring cylinder to a known-good cylinder (or swap two coils between cylinders). If the misfire follows the coil, the coil is likely bad; if it stays with the cylinder, the issue may lie with the injector, wiring, or cylinder


• test coil resistance with a multimeter (primary and secondary windings) and compare to the service manual specifications for your exact engine code


• Inspect wiring harnesses and connectors for moisture, corrosion, or loose pins

After performing these checks, you’ll have a clearer picture: isolated coil failures often point to a single bad coil, while widespread misfires may indicate multiple failed coils or another system issue.

Replacement options and cost considerations

When a coil pack is confirmed faulty, you’ll weigh replacement strategies and costs. Here is how to think about it.

• Replace only the faulty coil if misfire is isolated to one cylinder and the coil test confirms failure


• Replace all coils if several cylinders show misfires, especially on high-mileage engines or if reliability is a priority


• Pair coil replacement with new spark plugs to ensure consistent ignition and to avoid premature fouling


• Choose between OEM Subaru coils and reputable aftermarket options; OEM parts often carry longer warranty coverage but can be pricier


• Consider labor costs and whether a shop diagnostic fee is applied; factory-trained technicians can also check related components (injectors, wiring) for a comprehensive repair

In practice, budgets vary by model and shop, but a single replacement coil can be modest in price, while a full set for a six-cylinder Subaru can represent a larger investment. Always confirm what’s included in the repair quote and whether plugs are replaced as part of the service.

Model-specific notes for Subaru engines

Ignition layouts in Subarus have evolved across generations. Most modern Subarus use coil-on-plug per cylinder, with the exact part numbers and installation procedures varying by engine code (for example, EJ, FA, or FB series) and by model year. While the basic diagnostic approach—reading codes, inspecting plugs/boots, and performing swap tests—remains the same, consult your vehicle’s service manual or a Subaru-qualified technician for model-specific guidance and torque specs.

• Understanding your engine code helps determine whether you’re dealing with COP hardware or a different ignition arrangement


• Turbocharged models (e.g., WRX/STi) may experience higher coil temperatures and different failure patterns; heat management and coil construction can influence life span


• Because ignition issues can mimic fuel, injector, or vacuum problems, a thorough diagnostic run is essential before replacing parts

Model-specific nuances matter for correct diagnosis and replacement, so reference your exact engine code and year when planning work.

Safety and maintenance reminders

Ignition coil work involves high-voltage components and electrical systems. Follow these safety reminders to minimize risk and avoid damage.

• Disconnect the negative battery terminal before handling ignition components to reduce shock risk


• Avoid touching coil boots or wiring while the engine is running; high voltage can cause injury even on modern cars


• Work in a dry, well-ventilated area; inspect boots and connectors for moisture or corrosion


• Use the correct torque specifications when reinstalling coils and ignition components


• If you’re not confident performing electrical diagnostics, seek a trained technician

With ignition work, safety comes first. When in doubt, a professional diagnosis is the prudent path.

Summary

In a Subaru, a failing coil pack typically shows up as misfires, rough idle, reduced power, and a check-engine light with cylinder-specific misfire codes. Diagnosis combines code reading, visual inspection of plugs and boots, and a swap or resistance test to confirm whether the coil or another system component is at fault. Replacement decisions hinge on whether the problem is isolated to one cylinder or affects multiple cylinders, with options ranging from a single coil to the entire set, often paired with new plugs. Always consider model-specific wiring and ignition layouts, and prioritize safety and professional guidance when needed. If you suspect a coil pack problem, a careful diagnostic followed by targeted replacement usually restores performance and efficiency.

What are the symptoms of a faulty coil pack?

Symptoms of a faulty coil pack include a misfiring engine (causing sputtering, rough idling, and vibration), difficulty starting, poor acceleration and power loss, worse fuel economy, and a check engine light. You may also notice loud noises, such as backfiring, or the smell of gas from the exhaust. 
Engine performance

• Misfiring engine: The engine may sputter, cough, or jerk, especially at low speeds or when idle. 
• Rough idle: The engine may shake or vibrate noticeably, particularly when stopped, as one or more cylinders fail to fire consistently. 
• Loss of power and acceleration: The vehicle may feel sluggish when you try to accelerate. 
• Difficulty starting: The engine may take longer to start or crank but fail to turn over. 
• Engine stalling: The car may stall, particularly while idling at traffic lights. 

Other indicators

• Check engine light: This is a common indicator that a diagnostic system has detected a problem, often related to engine misfires. 
• Poor fuel economy: The engine burns more fuel than usual due to incomplete combustion. 
• Loud noises or smell: You might hear a loud popping or banging noise from backfiring, or smell gas from the exhaust. This is caused by unburned fuel entering the exhaust system and igniting. 
• Thick or black smoke: The presence of black smoke from the tailpipe can indicate unburned fuel. 

How do I check if my coil packs are bad?

A bad coil pack can be identified by symptoms like an illuminated check engine light, a misfiring engine, rough idling, poor acceleration, and difficulty starting. Other signs include a stuttering or jolting sensation, backfiring, and a noticeable drop in fuel economy. You can also perform a quick test by unplugging each coil's electrical connector while the engine is running; a bad coil will cause no change in the engine's performance when unplugged.
 
This video demonstrates how to check for a bad ignition coil pack: 57sDirt Road Auto RepairYouTube · Feb 15, 2023
Common symptoms

• Check Engine Light: The vehicle's computer will likely detect a misfire and illuminate the check engine light. Diagnostic trouble codes, like P0300 for a general misfire or specific cylinder misfire codes (P0301, P0302, etc.), can confirm the issue. 
• Misfiring Engine: You may feel a coughing or sputtering, a jolting sensation, or a loss of power as one or more cylinders fail to fire correctly. 
• Rough Idle: The engine may shake, vibrate, or stumble while at a stop, often because of the uneven power pulses from a misfiring cylinder. 
• Difficulty Starting: The engine may crank for a longer time before starting or fail to start at all, especially noticeable after the car has been sitting. 
• Poor Acceleration and Loss of Power: The car may feel sluggish and hesitant during acceleration due to reduced engine power. 
• Backfiring: If a coil is misfiring, unburned fuel can exit the engine and ignite in the exhaust, causing popping or banging sounds. 
• Decreased Fuel Economy: Inefficient combustion from a weak spark can lead to a noticeable drop in gas mileage. 

This video explains the symptoms of a faulty ignition coil: 59sHonest Mechanic ColoradoYouTube · Dec 27, 2023
How to perform a quick test

1. Ensure the engine is running: and at idle speed. 
2. Locate the electrical connector: for each coil pack. 
3. Disconnect the connector: from one coil pack at a time. 
4. Listen carefully to the engine's sound. The engine should run noticeably worse or stumble when you disconnect a good coil pack. 
5. If the engine's performance does not change: after disconnecting a coil, that specific coil is likely the one that is bad. 
6. For confirmation, you can swap the suspected bad coil with a coil from another cylinder and repeat the test. The misfire or lack of change should now follow the coil to the new cylinder, indicating a bad coil. If the problem remains with the original cylinder, the issue may be elsewhere, such as the spark plug. 

What is a common symptom of a failing ignition coil?

Symptoms of a bad ignition coil include engine misfires, rough idling, and a check engine light. Other signs are sluggish acceleration, poor fuel economy, difficulty starting, and engine stalling. You might also notice sputtering, backfiring, or a strong smell of gas.
 
Performance and starting issues

• Engine misfires: The engine may feel like it's coughing, sputtering, or shaking because a spark plug isn't firing correctly. 
• Rough idling: Your car may shake or vibrate noticeably when it's stopped, such as at a traffic light. 
• Poor acceleration: You may experience sluggishness or a noticeable loss of power when trying to accelerate. 
• Difficulty starting: The engine may struggle to start, or may not start at all if multiple coils are bad. 
• Stalling: The engine may stall or hesitate while driving. 

Efficiency and other indicators

• Decreased fuel economy: A bad coil can cause the engine to burn more fuel, so you'll get fewer miles per gallon. 
• Backfiring: You might hear loud banging noises from the exhaust, which occurs when unburned fuel ignites in the exhaust system. 
• Strong smell of gas: This can happen when unburned fuel escapes through the exhaust system. 
• Check engine light: The vehicle's computer will detect the misfire and turn on the check engine light. A mechanic can use an OBD2 scanner to read the specific error codes. 

How to test a Subaru coil pack?

How To Test Subaru Ignition Coils

1. Scan for Trouble Codes. Use an OBD-II scanner.
2. Inspect Visually. Look for cracks or damage on the coil body, corrosion on the connections, loose wires, or oil contamination.
3. Swap Coils.
4. Check Resistance.
5. Load Test.