How to Tell If Your Ignition Coil Is Going Bad

If your ignition coil is failing, you’ll usually notice symptoms like rough running, misfires, hard starting, loss of power, poor fuel economy, or an illuminated Check Engine Light; a proper diagnosis often involves scanning for trouble codes, inspecting the coil and wiring, and sometimes using specialized tools to measure spark or coil resistance.

What the Ignition Coil Does and Why It Matters

The ignition coil is a critical part of your vehicle’s ignition system. Its job is to take the car’s relatively low battery voltage (typically around 12 volts) and transform it into tens of thousands of volts—enough to jump the gap at the spark plugs and ignite the air–fuel mixture inside the engine cylinders. When the coil starts to fail, the engine may still run, but it will typically run poorly, waste fuel, and may eventually stall or refuse to start at all.

Common Symptoms of a Bad Ignition Coil

Most drivers first suspect an ignition coil problem when they experience noticeable changes in how their vehicle drives. The symptoms often overlap with other ignition or fuel issues, but patterns in how the engine behaves can strongly point to a coil fault.

Engine Performance and Driveability Symptoms

The following list highlights the most common performance-related warning signs that may indicate a failing ignition coil and how they typically appear while driving.

• Rough idle: The engine may shake or vibrate noticeably when stopped, with the tachometer needle fluctuating more than usual.
• Intermittent or constant misfires: You may feel the car “jerk” or hesitate, especially under load (like going uphill or accelerating quickly).
• Loss of power and sluggish acceleration: The vehicle may feel weak, slow to respond, or unable to reach usual speeds easily.
• Poor fuel economy: You may find yourself refueling more often as unburned fuel passes through the engine due to incomplete combustion.
• Difficulty starting: The engine may crank longer before starting, or it may fail to start at all if multiple coils or a coil pack are failing.
• Engine stalling: The engine might suddenly shut off when idling at a stoplight or even while driving, then sometimes restart after a short wait.
• Backfiring or popping from the exhaust: Unburned fuel igniting in the exhaust can create pops or bangs, sometimes accompanied by a fuel smell.

While these symptoms can also be caused by bad spark plugs, plug wires, or fuel system issues, their presence—especially in combination—should prompt an inspection of the ignition coil or coils.

Dashboard Warnings and Diagnostic Trouble Codes

Modern vehicles use onboard diagnostics to monitor misfires and ignition problems, often giving you early clues that something is wrong with a coil before the problem worsens.

• Check Engine Light (CEL): A failing coil frequently triggers the CEL. It may flash during heavy misfiring (which can damage the catalytic converter) or stay on steadily.
• Misfire codes (P0300–P030X):
  

  • P0300: Random/multiple cylinder misfire, which can be due to one or more weak coils or other issues.
  • P0301–P0308 (or higher on some engines): Misfire on a specific cylinder. If your car uses one coil per cylinder (coil-on-plug), a misfire on a single cylinder strongly suggests a coil, plug, or injector issue on that cylinder.

• Specific ignition coil codes (e.g., P0351–P0362): These often point to “Ignition Coil A/B/C, etc. – Primary/Secondary Circuit” faults, indicating the computer is seeing an electrical problem in the coil or its wiring.

Reading these codes with an OBD-II scanner gives a more precise starting point for diagnosis, narrowing down which cylinder or coil circuit may be at fault.

Smells, Sounds, and Other Clues

Beyond how the vehicle drives and what the dashboard shows, your senses can reveal additional hints that the ignition coil may be compromised.

• Strong fuel smell from exhaust: Persistent misfires mean fuel isn’t burning fully, leading to a noticeable gasoline odor at the tailpipe.
• Unusual engine noise: You may hear a “puttering” or irregular exhaust note, or sharp pops from the exhaust under load.
• Occasional visible exhaust smoke: Excess unburned fuel can create darker exhaust (often alongside other issues).
• Electrical arcing sounds (rare but possible): In quiet conditions with the hood open, you might hear faint clicking or snapping if a coil or wire is leaking spark to ground.

These sensory cues don’t confirm a bad coil by themselves, but when combined with performance issues and warning lights, they raise the likelihood that the ignition system—and possibly the coil—is at fault.

How to Diagnose a Bad Ignition Coil

Determining whether the ignition coil is really the culprit involves a mix of basic checks and, in some cases, more advanced testing. Many vehicle owners can perform initial inspections and simple tests, while more definitive testing is often done by a professional technician.

Start with a Visual Inspection

Before using tools, looking closely at the ignition coil and related components can reveal obvious signs of damage, contamination, or poor connections that lead to coil failure.

• Cracks, burns, or melted plastic on the coil body: These may indicate overheating or internal short circuits.
• Oil or coolant contamination: Coils that sit in spark plug wells can be exposed to leaks from valve cover gaskets or other seals.
• Corroded or loose electrical connectors: Greenish corrosion, bent pins, or a connector that doesn’t snap in firmly can cut power or signal to the coil.
• Damaged coil boots or plug wires: Torn, brittle, or carbon-tracked boots and wires can cause misfires even if the coil itself is marginally functional.
• Moisture intrusion: Evidence of water in the coil area (from washing the engine or leaks) can temporarily or permanently damage coils.

If the visual inspection reveals clear damage or fluid intrusion, addressing these issues—often by replacing the coil and fixing leaks—may restore proper operation.

Use an OBD-II Scanner

An OBD-II scanner is one of the most effective tools for narrowing down ignition problems, especially on vehicles equipped with individual coil-on-plug systems.

• Read stored and pending trouble codes: These codes help identify which cylinder is misfiring or whether the engine computer detects an ignition circuit issue.
• Check live data (if supported): Some scanners show misfire counts per cylinder in real time, helping track which cylinder is acting up at idle, under load, or at certain RPMs.
• Monitor fuel trims and O2 sensor data: Chronic misfires can push fuel trims high and cause downstream oxygen sensors to report excess oxygen or unburned fuel.

While a scanner doesn’t confirm a coil failure outright, it significantly refines the search, making subsequent tests more targeted and efficient.

Swap Test on Coil-on-Plug Systems

On vehicles where each spark plug has its own ignition coil (coil-on-plug), a practical at-home diagnostic method is to swap coils between cylinders to see if the misfire follows the coil.

• Identify the misfiring cylinder: Use OBD-II codes (e.g., P0303 for cylinder 3 misfire).
• Swap coils: Move the coil from the misfiring cylinder to a cylinder that is not misfiring (e.g., swap coils from cylinder 3 to cylinder 1).
• Clear codes and test drive: After the swap, clear the codes with a scanner and drive the vehicle under similar conditions.
• Re-scan for codes:
  

  • If the misfire code moves to the new cylinder location (e.g., from P0303 to P0301), this strongly suggests that the coil is faulty.
  • If the misfire stays on the same cylinder, the problem may be the spark plug, injector, compression, or wiring—not the coil.

This method doesn’t require special tools beyond a scanner and basic hand tools, and it is one of the clearest indicators of a bad individual coil.

Testing with a Multimeter (Resistance Checks)

A digital multimeter can sometimes identify coils that are clearly out of specification, though it may not catch intermittent or heat-related failures.

• Check manufacturer specs: Service manuals list acceptable resistance ranges for primary and secondary windings of the coil.
• Primary resistance test: With the coil disconnected, measure resistance across the primary terminals (input side). Compare the reading to the spec.
• Secondary resistance test: Measure resistance between the coil output terminal (or boot contact) and one of the primary terminals.
• Interpret results carefully:
  

  • Significantly higher or lower resistance than specified suggests an internal fault.
  • A completely open circuit (infinite resistance) indicates a broken winding.

Because many modern coils can fail only under heat or load even while passing static resistance checks, passing this test doesn’t fully rule out a bad coil—but failing it is a strong reason to replace the coil.

Advanced Shop-Level Testing

Professional technicians often use more sophisticated equipment to confirm ignition coil problems with greater certainty, especially when issues are intermittent.

• Oscilloscope waveforms: An automotive oscilloscope can display coil primary and secondary patterns, revealing weak spark, breakdown under load, and internal shorts.
• Ignition analyzers: Some tools measure spark strength and duration, comparing coils across cylinders while the engine is running.
• Cylinder balance tests: Using scan tools, technicians can temporarily disable cylinders to see how engine RPM and smoothness change, helping isolate weak ignition components.

These advanced diagnostics are useful when basic tests are inconclusive, or when multiple systems (fuel, ignition, mechanical) could be at fault.

Distinguishing a Bad Ignition Coil from Other Problems

Because many symptoms of a bad ignition coil mimic other issues—such as failing spark plugs, plug wires, fuel injectors, or even mechanical engine problems—differentiating between them is crucial before replacing parts.

Ignition Coil vs. Spark Plug or Wire Issues

The spark plug and its associated wiring (or boots) work hand in hand with the coil, so failures often overlap. However, subtle differences can guide your diagnosis.

• Age and maintenance history: Neglected spark plugs often wear out before coils do; if plugs are overdue, start there.
• Plug inspection: Pulling and inspecting plugs can reveal fouling, excessive gap, or damage that clearly explains misfires.
• Effect of moisture: Older plug wires may misfire mainly in damp conditions or after washing the engine, while a failing coil may misfire regardless of moisture.
• Multi-cylinder vs. single-cylinder issues: On engines with a single coil pack feeding multiple cylinders via wires, multiple cylinders may misfire together when the pack fails, whereas a bad plug or wire usually affects only one cylinder.

If plugs and wires/boots are old or visibly compromised, replacing them first can often solve misfires without needing a new coil—though sometimes both need replacement.

Ignition Coil vs. Fuel and Mechanical Problems

Some symptoms of coil failure overlap with clogged injectors, fuel pump issues, or internal engine problems like low compression.

• Fuel-related issues: Often show up as hesitation, lean codes (P0171, P0174), or issues under sustained load, and may not trigger specific misfire codes tied to ignition.
• Mechanical issues (compression loss, valve problems): Typically create persistent misfires on one cylinder that do not move when swapping coils or plugs.
• Vacuum leaks: Can cause rough idle and lean codes, but the misfire may improve at higher RPM and is less likely to be isolated to coil-related codes.

Simple tests, such as a compression test or fuel pressure check, can help rule out non-ignition causes when coil-related diagnosis is uncertain.

Is It Safe to Keep Driving with a Bad Ignition Coil?

Continuing to drive with a failing coil is generally not recommended. While the car may remain drivable for a time, the risks and potential damage can become costly.

Potential Consequences of Ignoring Coil Problems

Allowing a known or suspected coil problem to persist can have both short-term and long-term effects on your vehicle’s reliability and components.

• Catalytic converter damage: Ongoing misfires dump unburned fuel into the exhaust, which can overheat and destroy the catalytic converter—an expensive repair.
• Engine damage risk: Severe, prolonged misfires can lead to washing away of cylinder lubrication and increased wear.
• Reduced safety and drivability: Loss of power or sudden stalling at intersections or during highway driving can be hazardous.
• Increased operating costs: Poor fuel economy and potential downstream repairs cost more than an early coil replacement.

Addressing coil-related misfires quickly—especially when the Check Engine Light is flashing—is critical to protect the engine and exhaust system.

When and How to Replace an Ignition Coil

If diagnosis points to a faulty ignition coil, replacement is often straightforward on many modern vehicles, though access varies by engine design. Planning the repair and related maintenance can save time and money.

Deciding Which Coils to Replace

Once you know at least one coil is failing, you’ll need to decide whether to replace just the bad one or multiple coils at once—this depends on vehicle age, cost, and access.

• Single-coil replacement:
  

  • Often reasonable on newer vehicles where only one coil has failed prematurely.
  • More cost-effective if coils are expensive or difficult to access.

• Multiple coil replacement:
  

  • Consider replacing all coils if the vehicle is high mileage and several coils are original, to avoid repeated failures.
  • Practical when access is difficult (for example, coils under an intake manifold) and labor is the major cost.

• Bundling related maintenance: Many technicians recommend replacing spark plugs (and boots where applicable) at the same time, if they are due, to ensure a fresh, reliable ignition system.

Your decision should balance current budget, vehicle age, and how hard the coils are to reach; a trusted mechanic can help weigh these factors.

DIY vs. Professional Replacement

Whether you replace the coil yourself depends heavily on your comfort with tools and how accessible the coils are on your specific engine.

• Good candidates for DIY:
  

  • Coil-on-plug systems with coils visible and reachable on top of the engine.
  • Vehicles where removal requires only basic tools and no major disassembly.

• Better suited for professionals:
  

  • Engines where intake manifolds, engine covers, or other major components must be removed.
  • Situations where advanced diagnostics are still needed to confirm the root cause.

• Important final steps: After replacement, clearing codes and test driving under similar conditions ensures the misfire is resolved and no new codes appear.

For many owners, an accessible coil-on-plug setup allows a manageable DIY project; for more complex layouts, professional service provides both proper diagnosis and warranty-backed repair.

Summary

A bad ignition coil typically reveals itself through rough idle, misfires, hard starting, reduced power, poor fuel economy, and a Check Engine Light—often accompanied by specific misfire or ignition circuit trouble codes. Visual inspection, OBD-II scanning, coil swapping (on coil-on-plug engines), and, where needed, multimeter or oscilloscope tests can help confirm the diagnosis and distinguish coil problems from spark plug, fuel, or mechanical issues. Because ongoing misfires can damage the catalytic converter and reduce safety, it’s wise to address suspected coil failures promptly, replacing the faulty coil (and often related components like spark plugs and boots) either as a targeted repair or as part of broader ignition system maintenance.

How do I check if my ignition coil is bad?

You can check if an ignition coil is bad by swapping it to a different cylinder and seeing if the misfire follows, performing a “power balance test” where you disconnect each coil and listen for an engine stumble, or using a multimeter to test the coil’s resistance against manufacturer specifications. A visual inspection for damage, along with signs like a hard start, engine misfiring, rough idling, or poor acceleration, can also indicate a faulty coil. 
Method 1: Swap the coils (power balance test) 

1. Start the engine and use an OBD-II scanner to identify which cylinder is misfiring or rough running (e.g., a P0302 code for cylinder 2).
2. Swap the ignition coil from that cylinder with a coil from another cylinder.
3. If the misfire code moves to the other cylinder, the coil is bad and needs replacement. 

Method 2: Disconnect each coil while the engine is running 

1. Start the engine and let it idle. 
2. Carefully disconnect the electrical connector from one ignition coil at a time. 
3. Listen for a noticeable drop in engine RPMs or a change in the engine’s sound. 
4. If the engine’s performance does not change after disconnecting a coil, that coil is likely bad. 
5. If disconnecting a coil causes a change, plug it back in and move to the next coil. 

Method 3: Test with a multimeter

1. Set your multimeter to the “ohms” or “resistance” setting. 
2. Remove the ignition coil. 
3. Consult your owner’s manual or search online for your specific vehicle’s specifications to find the correct resistance range and terminals to test. 
4. Touch the multimeter probes to the correct terminals on the coil. 
5. A reading outside the specified range indicates a faulty coil. 

Method 4: Visual inspection

1. Visually inspect the ignition coil for cracks, burn marks, or other damage. 
2. Check the electrical connector for any corrosion or frayed wires. 

What does a car do when it has a bad coil?

One of the most common symptoms associated with a faulty ignition coil is engine performance issues… Faulty coils may cause the vehicle to experience misfires, a rough idle, a loss in power and acceleration, and a reduction in gas mileage. In some cases the performance issues may even result in the vehicle stalling.

What is a common symptom of a failing ignition coil?

Symptoms of a bad ignition coil include a rough idle, engine misfires, poor acceleration, and reduced fuel economy. You may also notice a “check engine” light, difficulty starting the car, and a sputtering or shaking sensation while driving. Other signs include backfiring and the vehicle stalling.
 
Performance issues

• Engine misfires: The engine may hesitate, jerk, or feel like it is coughing and sputtering, especially when accelerating. 
• Rough idle: The car may shake or vibrate noticeably when idling at a stop. 
• Poor acceleration: The vehicle may feel sluggish and have less power when you press the gas pedal. 
• Difficulty starting: The engine may crank for a long time before starting, or it may not start at all. 

Warning signs

• Check Engine Light: The dashboard warning light will often illuminate. The specific error codes (like P0300-P0308) can help pinpoint which cylinder is misfiring. 
• Backfiring: You may hear loud bangs from the exhaust, sometimes accompanied by black smoke, when unburned fuel ignites in the exhaust system. 
• Engine stalling: The engine may shut off unexpectedly, particularly at low speeds or while idling. 

Other symptoms

• Reduced fuel economy: The engine becomes less efficient, leading to more frequent trips to the gas station. 
• Strange noises: You may hear unusual sounds from the engine, such as sputtering or a shaky, noisy idle. 

How to tell if a coil needs replacing?

You can tell a coil needs replacing by observing symptoms like poor performance, such as a burnt taste from a vape or engine misfires in a car. Other signs include a decrease in vapor or power, an illuminated “check engine” light, leaking, or unusual sounds like gurgling or backfiring. A visual inspection for physical damage like cracks, and using a diagnostic scanner to check for specific codes can also confirm a bad coil. 
For a vehicle ignition coil

• Check Engine Light: An illuminated light on the dashboard is a common indicator of an ignition system issue. 
• Engine Misfires: You may feel the engine shudder, jerk, or run unevenly, especially during acceleration. 
• Rough Idle: The engine may shake or vibrate noticeably when the car is stopped. 
• Difficulty Starting: The engine may take longer to crank before starting or might not start at all. 
• Reduced Power: You might notice a loss of power when accelerating or a general sluggishness. 
• Poor Fuel Economy: The engine may burn fuel inefficiently, leading to worse gas mileage. 
• Backfiring: A sudden backfire (a loud pop) can occur if unburnt fuel ignites in the exhaust system. 
• Visual Damage: Look for physical damage like cracks in the coil’s housing or oily residue around it. 
• Diagnostic Codes: An OBD-II scanner can read trouble codes (like P0300) that point to a specific cylinder misfire. 

For a vape coil

• Burnt Taste: This is the most common sign, as the cotton wicks and coil element wear out over time. 
• Diminished Flavor: Your e-liquid may not taste as good or the flavor profile becomes weaker. 
• Reduced Vapor: Your device produces less vapor than it used to. 
• Gurgling Sounds: A hollow bubbling sound can indicate the coil is having trouble and needs replacing. 
• Leaking: A worn-out coil can cause the e-liquid to leak from the device. 
• Darker E-juice: The color of your e-liquid might darken prematurely. 
• Airflow Clogging: You may notice the airflow feels restricted or clogged.