How to Tell If Your Coil Is Bad

If your coil is failing, you’ll typically notice clear performance changes: in cars, expect misfires, rough idle, poor acceleration, and a check-engine light; in vapes, expect a burnt taste, muted flavor, and reduced vapor. The word “coil” can refer to different components across cars, vapes, appliances, and HVAC systems, so the exact signs and tests depend on context. Here’s how to recognize and confirm a bad coil—and what to do next.

First, clarify which coil you mean

“Coil” most commonly means an ignition coil in vehicles or a heating coil in vaping devices, but it can also refer to relay/solenoid coils, HVAC contactor coils, or electric range elements. The symptoms and tests vary, so identify the system before you proceed.

Signs your car’s ignition coil is bad

Ignition coils amplify battery voltage to create the spark that ignites fuel. When they degrade or fail, combustion suffers and drivability problems follow. Watch for these typical red flags in modern coil-on-plug and coil-pack systems:

• Check Engine Light with misfire codes (P0300–P0308) or coil circuit codes (P0351–P0358)
• Rough idle, shaking, or stalling—often worse under load or at stoplights
• Hesitation, loss of power, or surging during acceleration
• Poor fuel economy and increased tailpipe emissions
• Hard starting or no-start in severe cases
• Raw fuel smell from the exhaust or catalytic converter overheating (dangerous if driven long)
• Audible popping from the exhaust or intake during misfires

These symptoms can overlap with bad spark plugs, wires/boots, injectors, or vacuum leaks, so confirmation testing is essential before replacing parts.

DIY diagnostics you can do (ignition coils)

With basic tools and a scan tool, you can narrow down whether the coil is the culprit. Proceed methodically and follow safety precautions around high voltage.

1. Scan for trouble codes and data: Look for P030X (misfire on cylinder X) and P035X (coil circuit faults). Check misfire counters in live data and note which cylinder(s) are affected.
2. Inspect visually: With the engine off, remove the suspect coil. Look for cracked housings, melted plastic, oil intrusion, or carbon tracking on the coil or spark plug insulator. Check the coil boot for tears or carbon trails.
3. Swap test: Move the suspect coil to another cylinder, clear codes, and recheck. If the misfire follows the coil (e.g., from cylinder 3 to 5), the coil is likely bad.
4. Use a spark tester: A dedicated adjustable-gap tester can confirm weak or no spark. Avoid pulling coils or plugs on a running engine—this can damage electronics.
5. Measure resistance (engine off, connector unplugged): Primary resistance is commonly around 0.3–2.0 ohms; secondary is often 6,000–15,000 ohms, but specifications vary widely—consult service data for your vehicle. Readings far outside spec suggest a bad coil, though some failures only appear when hot or under load.
6. Check power, ground, and trigger: With a wiring diagram, verify coil connector power supply (usually battery voltage with key on), ground integrity, and PCM/igniter trigger signal. A bad power feed, ground, or driver can mimic a bad coil.
7. Consider related parts: Worn spark plugs increase coil stress and heat. If plugs are old or gapped incorrectly, replace them first; many coil failures are secondary to plug issues.

If testing implicates the coil, replacement is straightforward on most coil-on-plug engines. If not, broaden the diagnosis to plugs, injectors, compression, vacuum leaks, or wiring/PCM faults.

When to replace one coil versus all

Most technicians replace only the failed coil unless the vehicle has high mileage and a pattern of repeated coil failures. Always replace the spark plug on the affected cylinder if it’s worn, oil-fouled, or out of spec, and consider new coil boots. If multiple coils are the same age and near end-of-life, proactive replacement may prevent repeated visits.

Signs your vape coil is bad

Vape coils are consumables. As they degrade or burn, flavor and vapor quality drop quickly and can taste harsh or burnt.

• Burnt or acrid taste that persists even after a few puffs
• Muted flavor and noticeably reduced vapor production
• Gurgling, spitting, or leaking from the tank (wick not vaporizing properly)
• Dark, sludgy e-liquid; wick discoloration or visible charring
• Ohms reading drifting far from the coil’s rated resistance

Most prebuilt coils last anywhere from a few days to a couple of weeks depending on wattage, e-liquid sweetness/viscosity, and vaping frequency. If symptoms persist after priming and proper settings, replace the coil.

What to do about a bad vape coil

If you confirm the coil is burnt or spent, swap it and adjust habits to extend the new coil’s life.

1. Replace the coil and prime it thoroughly: Saturate the wick, reassemble, fill, and let it soak several minutes.
2. Start at the low end of the recommended wattage and step up slowly to avoid scorching.
3. Avoid chain hits; allow a few seconds between puffs for wick re-saturation.
4. Use appropriate e-liquid: Very sweet or high-VG juices gunk coils faster; match viscosity to your device.
5. Keep airflow and power settings within the coil’s specifications.

If new coils burn out rapidly, check for incorrect wattage, inadequate airflow, or wicking issues with your specific tank or pod.

Other coils you might mean (quick checks)

Coils also appear in home and industrial equipment. Here are fast ways to spot trouble in common systems.

• HVAC contactor or furnace igniter coil: Buzzing, chattering, or failure to pull in; 24V control signal present but no actuation; coil measures open or shorted with a multimeter.
• Relay/solenoid coil (automotive or appliances): Clicks weakly or not at all; device doesn’t actuate despite correct control voltage; coil has no continuity or shows a short.
• Electric stove/oven heating element: Visible breaks or hot spots; no heat even with power; continuity test fails.
• Guitar pickup coil: Unusually low output, excessive hum, or microphonic feedback; DC resistance reading far off the maker’s spec.

These checks typically require a multimeter and basic safety practices. Always isolate power before testing continuity or resistance.

Safety notes

Coils handle high voltage or heat; improper handling can cause injury or equipment damage. Keep these precautions in mind.

• Automotive ignition systems can exceed 20,000 volts—never touch coils or plugs with the engine running.
• Disconnect the battery and wait before unplugging coils; avoid open-circuiting high-voltage outputs.
• Unplug appliances/HVAC and discharge capacitors before testing.
• Use insulated tools and follow the manufacturer’s procedures and torque specs.
• If unsure, seek a qualified technician—misdiagnosis can be costly, especially with catalytic converters and control modules.

Taking a few minutes to work safely reduces the risk of shocks, burns, or collateral damage during diagnosis and repair.

Summary

You likely have a bad coil if you see context-specific warning signs: for cars, misfires, rough running, and codes like P030X or P035X; for vapes, burnt taste and weak vapor; for other devices, loss of actuation or continuity. Confirm with targeted tests—scan data, visual inspection, swap tests, spark testing, resistance checks, and power/ground verification—then replace the faulty part and address contributing causes (like worn spark plugs or incorrect settings). Accurate identification up front saves time, money, and prevents further damage.

How do I test if my coils are bad?

To test for a bad ignition coil, perform a power balance test by unplugging each coil and listening for a change in engine RPMs; a failing coil won’t cause the engine to change when disconnected. Alternatively, use a spark tester or multimeter to check for a strong, blue spark or correct resistance readings, respectively. A faulty coil will show no spark, a weak or orange spark, or incorrect resistance levels.
 
Method 1: Power Balance Test (Engine Running)
This is a simple method to test coils without special tools. 

1. Locate: the ignition coils on your engine. 
2. Start: the engine. 
3. Disconnect: the electrical connector and the coil/spark plug boot from one cylinder at a time. 
4. Listen: carefully for a change in the engine’s RPMs or sound. 
5. If the engine’s behavior doesn’t change: when you disconnect a specific coil, that coil is likely the problem. 
6. Repeat: the process for each coil to find the faulty one. 

Method 2: Spark Test (Engine Off)
This method visually confirms if the coil is producing a spark. 

1. Turn off: the engine and remove the spark plug wires or coils. 
2. Use a spark tester: or a screwdriver with an insulated handle to test the spark. 
3. Crank: the engine over. 
4. Observe the spark: at the end of the tester or screwdriver held near a ground. 
   • No spark: or a weak, orange spark indicates a bad coil. 
   • A strong, blue spark shows a good coil. 

Method 3: Multimeter Test (Engine Off)
This method checks the coil’s electrical resistance. 

1. Disconnect: the coil and set your multimeter to the appropriate resistance (ohms) setting. 
2. Consult your vehicle’s manual: to find the correct terminals to test. 
3. Place the multimeter probes: on the designated terminals to measure resistance. 
4. A stable and within-spec reading: indicates a good coil. 
5. A high resistance reading: or fluctuating values often point to a bad, internally broken coil. 

How does a car act when the coil is going bad?

Symptoms of a bad ignition coil include the illuminated check engine light, rough or erratic engine idling, engine misfires, backfiring, difficulty starting the engine, reduced power and poor fuel economy, and even engine stalling. A foul smell of gas from the exhaust can also be a sign of unburned fuel due to a misfiring cylinder. These symptoms indicate the ignition system is not consistently creating the spark needed for combustion, which can lead to more severe engine damage if left unaddressed. 
Here are the common symptoms of a faulty ignition coil:

• Check Engine Light: The most common initial sign of an ignition coil problem is a glowing check engine light on your dashboard. 
• Engine Misfiring: The engine may cough, sputter, or vibrate due to a cylinder not firing correctly because the ignition coil isn’t providing enough voltage for the spark plug. 
• Rough Idling: The engine’s smooth rumble at idle will turn into a juddering, faltering, and noisy experience, especially when the car is stationary. 
• Difficulty Starting: A failing ignition coil can make it harder for the engine to start, requiring the ignition system to work overtime. 
• Engine Stalling: In severe cases, a misfire from a faulty ignition coil can cause the engine to stall while driving. 
• Reduced Power and Fuel Economy: You might notice a significant loss of power and slower acceleration because the engine isn’t combusting fuel efficiently. 
• Backfiring and Gas Smell: When fuel doesn’t ignite in a cylinder, it can exit the exhaust and ignite, causing loud pops (backfiring) and a strong smell of gas. 

How do I know if my ignition coil needs replacing?

You can tell your ignition coil needs replacing if you notice symptoms like a check engine light, a rough or shaky idle, engine misfires, stalling, backfiring, poor acceleration, reduced fuel economy, and a difficulty starting the engine. These symptoms often indicate the coil isn’t delivering enough voltage to the spark plugs, leading to incomplete combustion. 
Common Symptoms

• Check Engine Light: This is a common indicator that the vehicle’s computer has detected an ignition issue. 
• Rough Idle or Shaking: You may feel the engine vibrating or shaking when the car is idling, like at a stoplight. 
• Engine Misfires or Stuttering: The engine may feel like it’s jerking, sputtering, or coughing, especially when you accelerate. 
• Difficulty Starting: The engine might take longer to crank or fail to start altogether. 
• Stalling: The vehicle may stall or shut off suddenly. 
• Reduced Power and Sluggish Acceleration: The car may feel slow to respond when you press the gas pedal. 
• Poor Fuel Economy: Less efficient combustion due to a faulty coil can lead to increased fuel consumption. 
• Backfiring: Unburned fuel that exits through the exhaust can ignite and cause a popping or backfiring sound. 

What to Do
If you notice these symptoms, have your car inspected by a mechanic. They can use a scan tool to read the check engine light codes (often P0300-P0308) and perform diagnostic tests to pinpoint the problem, ensuring a timely replacement and preventing more costly damage to other engine components.

How do I know if my coil needs replacing?

To determine if an automotive ignition coil needs replacement, look for symptoms like a rough idle, engine misfires, poor fuel economy, a Check Engine light, or a stalling/sputtering engine. For a vape coil, signs include a burnt taste, a bubbling noise, weak vapor production, a leaking tank, or a diminished e-liquid flavor. 
For an automotive ignition coil:

• Misfires and rough idle: The engine may shake, stumble, or feel jerky, especially during acceleration. 
• Stalling or poor starting: The engine may stall or crank for a long time before starting. 
• Reduced power: You might notice a decrease in the vehicle’s power, making acceleration sluggish. 
• Poor fuel economy: Inefficient combustion due to weak spark can lead to lower miles per gallon. 
• Check Engine light: A fault in the ignition system often triggers the Check Engine light. 
• Backfiring: Unburned fuel may exit the exhaust and ignite, causing a loud popping or backfiring sound. 

For a vape coil:

• Burnt taste: A common sign of an aging coil is an unpleasant, burnt taste when inhaling. 
• Weak vapor or flavor: A diminished flavor and reduced production of vapor can indicate a worn-out coil. 
• Tank leaking or gurgling: A faulty coil can lead to e-liquid leaks or a hollow gurgling sound from the device. 
• Harsh hits: The device may produce harsher, less pleasant hits. 
• Bubbling sounds: A hollow bubbling noise while using the device can also signal a coil needs replacing.