What Happens When a Coil Goes Bad on a Car

A failing ignition coil typically causes engine misfires, rough idle, loss of power, hard starts or no start, and a flashing or steady check-engine light; continued driving can overheat and ruin the catalytic converter, so the fix is usually to replace the bad coil (and often the spark plug) after confirming with a scan for misfire/coil codes like P030X and P035X.

What an Ignition Coil Does

An ignition coil transforms the car’s 12-14 volts into the tens of thousands of volts needed to jump the spark plug gap and ignite the air-fuel mixture. Most modern vehicles use one coil per cylinder (coil-on-plug), while some use coil packs that serve multiple cylinders through plug wires. When a coil weakens or fails, spark quality degrades or disappears, and the cylinder misfires.

Symptoms You’ll Notice on the Road

Drivers usually encounter a mix of drivability issues when a coil goes bad. The severity depends on whether the misfire is intermittent, heat-related, or a complete failure, and whether one or multiple cylinders are affected.

• Rough idle or shaking, especially at stops
• Stumbling, hesitation, or surging on acceleration
• Noticeable loss of power and poor fuel economy
• Hard starting or extended cranking; in severe cases, a no-start
• Check-engine light (steady for stored misfires, flashing during active misfire)
• Raw fuel smell from the exhaust and possible popping sounds
• In automatic cars, harsh shifts due to torque reduction strategies
• In hybrids, frequent engine start/stop cycling with vibration when the engine kicks on

These symptoms often worsen under load (hills, passing) or when the engine is hot. Heat can expose marginal coils that seem fine when cold.

What You’ll See on the Dash and on a Scanner

A failing coil almost always leaves a trail of diagnostic trouble codes. Reading these codes helps pinpoint which cylinder or circuit is involved and how urgent the issue is.

• P0300: Random/multiple misfire
• P0301–P0308 (and higher on some engines): Misfire on cylinder X
• P0351–P0358 (and higher): Ignition coil primary/secondary circuit fault for coil X
• P0316: Misfire detected on startup (first 1,000 revolutions)
• P0420/P0430: Catalyst efficiency below threshold (possible damage from prolonged misfire)
• Fuel-trim codes (P0171/P0174 lean, P0172/P0175 rich) may appear as side effects

A flashing MIL (check-engine light) indicates an active misfire likely to damage the catalytic converter; reduce load and address immediately.

Risks of Continuing to Drive

Driving with a bad coil is tempting if the car still moves, but it can trigger expensive consequences. Understanding these risks helps prioritize repairs.

• Catalytic converter damage from unburned fuel (overheats and melts the substrate)
• Contaminated engine oil due to fuel washdown on cylinder walls
• Oxygen sensor damage from thermal shock and contaminants
• Overheating of the coil driver circuit if shorted wiring is present
• Loss of power that can create hazardous merging or passing situations

Short, gentle drives to a repair facility are usually fine, but avoid highway loads or towing until the misfire is fixed.

How to Confirm a Bad Coil

Basic checks can isolate a faulty coil. If you’re comfortable with DIY diagnosis, simple swaps and scans can be conclusive; otherwise, a shop can test quickly.

• Scan for codes and note freeze-frame data (RPM, load, temperature when misfire occurred)
• Identify the suspect cylinder and swap its coil with another cylinder to see if the misfire follows
• Inspect coil connector and wiring for corrosion, oil intrusion, broken locks, or chafing
• Check the spark plug on the misfiring cylinder for fouling, cracks, or excessive gap
• On coil packs with plug wires, measure wire resistance and look for arcing marks
• Use a noid light or scope (if available) to verify coil command and primary waveform
• Perform a cylinder drop test or Mode $06 misfire counters (if your scanner supports it)

If the misfire follows the coil to a new cylinder, the coil is bad. If it stays, look at the plug, injector, compression, or wiring/ECU driver.

Fixes and Typical Costs in 2025

Repairs vary by engine layout and whether you replace a single coil or do proactive maintenance on nearby components. Costs below reflect common U.S. pricing in 2025.

• Coil-on-plug replacement (single): Parts $35–$120 aftermarket, $90–$250 OEM; labor 0.3–0.7 hr per accessible coil
• Coil pack (multi-cylinder): Parts $80–$300; labor 0.4–1.0 hr
• Spark plugs: $10–$25 each (iridium/platinum); labor 0.5–2.5 hr for a full set depending on access
• Diagnostic scan/verification: $0 at many parts stores; $80–$150 at shops with live data and scope
• Catalytic converter (if damaged): $800–$2,500+ parts and labor, depending on vehicle and emissions configuration

Best practice is to replace the spark plug on the affected cylinder and inspect the boot; on high-mileage engines or hard-to-reach banks, many shops recommend replacing all coils and plugs together to avoid repeat labor.

Why Coils Fail

Ignition coils endure heat, vibration, and high voltage. Several factors accelerate their wear or trigger sudden failure.

• Age and heat cycles causing insulation breakdown and internal cracking
• Oil or coolant intrusion into the plug well (leaky valve cover or plug tube seals)
• Excessive spark plug gap or worn plugs forcing the coil to work harder
• Moisture and corrosion in connectors or boots
• Secondary arcing from damaged boots or improper dielectric grease use
• Overvoltage events or wiring harness damage
• Manufacturing defects or counterfeit/low-quality replacement parts

Addressing root causes—like oil leaks and overdue spark plugs—prevents repeat failures.

Prevention and Maintenance Tips

Simple maintenance goes a long way toward coil longevity and consistent ignition performance.

• Replace spark plugs at the intervals specified by the manufacturer
• Fix valve cover and plug tube seal leaks promptly
• Use quality coils matched to your vehicle; avoid mixing different brands on the same bank
• Apply dielectric grease to boots sparingly, only where specified
• Keep battery and charging system healthy; weak systems stress ignition components
• After washing the engine bay, avoid soaking coil areas; ensure connectors are dry

Following the service schedule and using correct parts reduces the odds of sudden misfires and collateral damage.

Frequently Asked Questions

Do I have to replace all coils at once?

No. You can replace just the failed coil. However, on high-mileage engines—especially when access is labor-intensive—replacing coils and plugs as a set can be cost-effective insurance against near-term repeat failures.

Can a bad coil damage the ECU?

It’s uncommon. Most failures are open-circuit or high-resistance. A shorted primary can stress the driver circuit, but modern ECUs usually have protection. Inspect wiring and fix shorts promptly.

What if the misfire happens only when hot?

Heat-related coil breakdown is common. Use a scan tool to watch misfire counters as the engine warms, or perform a heat soak test. Swapping coils hot can confirm the culprit.

Summary

A bad ignition coil causes misfires, rough running, power loss, and a check-engine light, and it can quickly damage the catalytic converter if ignored. Confirm with OBD-II codes and coil swaps, inspect the plug and wiring, and replace the faulty coil—often with a new plug—before driving far. With timely diagnosis and quality parts, the fix is straightforward and prevents much costlier repairs.

How to tell if a 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. 

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. 

Can you drive with a bad engine coil?

No, you should not drive with a bad ignition coil, as it can lead to further damage to your engine and exhaust system, potentially causing costly repairs. Symptoms like a rough engine, misfires, poor acceleration, and a lit check engine light are indicators of a faulty coil, and driving with a bad one can result in unburned fuel damaging the catalytic converter and oxygen sensors, diluting engine oil, and causing premature engine wear. 
Why you shouldn’t drive with a bad ignition coil:

• Damaged Catalytic Converter and Oxygen Sensors: Opens in new tabUnburned fuel from a misfiring cylinder can enter the exhaust system and ignite in the hot catalytic converter, leading to its failure and damage to oxygen sensors. 
• Engine Oil Contamination: Opens in new tabRaw fuel can bypass the piston rings and enter the engine oil, diluting it and severely compromising its lubricating properties. 
• Premature Engine Wear: Opens in new tabThe lack of proper lubrication from contaminated oil can lead to increased wear on cylinder walls, piston rings, and crankshaft bearings, potentially causing major engine failure. 
• Engine Instability: Opens in new tabA misfiring cylinder can act like a brake, causing the engine to vibrate, run unevenly, and put excessive stress on internal components. 

What to do if you suspect a bad ignition coil:

• Get it Diagnosed: Have the ignition coils and the entire ignition system checked by a mechanic to confirm the issue. 
• Replace Promptly: Replace the faulty ignition coil as soon as possible to prevent further, more expensive damage to your engine and exhaust components. 

How much does it cost to replace a coil in a car?

Replacing a car ignition coil typically costs between $230 and $330 for parts and labor, though the final price varies significantly based on the vehicle’s year, make, and model, as well as the type of ignition coil (conventional, coil-on-plug, or distributorless ignition system). The cost of the part itself can range from about $50 to $600, with labor costs making up the rest of the total. 
Factors influencing cost

• Vehicle Type: Luxury or high-performance vehicles often have more expensive parts and more complex systems, leading to higher costs. 
• Coil Type: Conventional coils are generally cheaper than coil-on-plug or distributorless ignition system coils, which are more advanced and cost more. 
• Part Quality: OEM (Original Equipment Manufacturer) parts are typically more expensive than aftermarket parts. 
• Labor Rates: Labor costs vary by shop and location. 

Cost breakdown (average)

• Parts: Can range from approximately $155 to $230. 
• Labor: Estimated between $75 to $100, according to some sources. 

Is it worth replacing?

• Yes, especially as a preventative measure. A faulty ignition coil can lead to poor fuel efficiency, engine misfires, rough idling, and potential damage to other components. 
• Replacing a single coil can be an option for DIYers with the right skills and tools.