What happens to a car when the ignition coil goes bad

When an ignition coil fails, the engine typically misfires, runs rough, loses power, and may be hard to start or not start at all; the check-engine light often comes on, fuel economy worsens, and driving it for long can overheat and damage the catalytic converter. Replacing the bad coil (and often the spark plug/boot) is the usual fix after confirming the fault with basic tests or a scan-tool diagnosis.

What the ignition coil does

The ignition coil transforms the car’s 12 volts into tens of thousands of volts needed to jump the spark plug gap and ignite the air-fuel mixture. Modern engines often use coil-on-plug systems (one coil per cylinder), while others use coil packs serving multiple cylinders. The engine computer commands each coil precisely; if a coil or its circuit fails, combustion in the affected cylinder(s) becomes erratic or stops, causing misfires and related drivability issues.

Typical symptoms on the road

Drivers usually notice a combination of drivability changes and dashboard warnings when a coil begins to fail. The severity can range from a slight hesitation to a no-start condition, depending on whether one coil or multiple outputs are affected.

• Rough idle and shaking: The engine may vibrate noticeably, especially at stoplights.
• Loss of power and sluggish acceleration: Misfires reduce torque, most obvious under load or climbing hills.
• Hesitation or jerking: Intermittent coil failure causes surging or bucking during steady throttle.
• Hard starting or no start: A single failed coil on a coil-on-plug engine may start but run rough; a failed single main coil or coil pack can prevent starting entirely.
• Poor fuel economy: Unburned fuel from misfires lowers miles per gallon.
• Engine stalling: The engine may stall at idle or when transitioning off throttle.
• Backfiring or exhaust pops: Raw fuel igniting in the exhaust can cause noise and odor.
• Fuel smell and high emissions: Misfires increase hydrocarbon emissions and can trigger inspection failures.
• Check-engine light (often flashing during active misfire): The ECM will usually log misfire and coil circuit codes.

If symptoms are intermittent, heat soak can be a clue—coils often act up when hot, then work again after cooling. A flashing MIL indicates active misfire severe enough to risk catalyst damage and warrants immediate attention.

Warning signs in diagnostics

Scan tools and simple tests can pinpoint coil problems. Codes and data patterns often separate a bad coil from other causes of misfire.

• OBD-II codes: P0300 (random misfire) or P0301–P0308 (cylinder-specific); coil/primary circuit codes P0350–P0362 depending on engine.
• Live data: Misfire counters rising on a specific cylinder, roughness indices, and fuel trims driven positive (ECM adding fuel) during misfire.
• Mode $06 data: Cylinder contribution/misfire counts on some vehicles can confirm intermittent issues without an active code.
• Coil command and dwell: Advanced scan tools show coil dwell time; abnormal patterns can suggest control or power issues.
• Basic spark test: An inline spark tester or noid-like spark tool checks for strong, regular spark under crank or idle.
• Visual inspection: Cracked housings, carbon tracking, oil in plug wells, or corroded connectors/grounds.
• Electrical measurements: Primary/secondary resistance checks (with specs), and ideally an oscilloscope to view coil waveform.

Note that wiring faults, poor grounds, and ECM driver failures can mimic a bad coil. Always verify power, ground, and signal before replacing parts—especially if multiple cylinders are affected in a pattern.

Consequences of driving with a bad coil

Continuing to drive with an active misfire sends raw fuel into the exhaust, overheating and melting the catalytic converter and potentially damaging oxygen sensors—repairs that can exceed $1,000–$2,500. It also compromises safety due to sudden loss of power or stalling in traffic. If the MIL is flashing or the car runs poorly, minimize driving and head directly to a shop; consider towing if the engine is shaking severely.

Common causes of coil failure

Ignition coils are stressed components. Environment and maintenance play major roles in their lifespan.

• Heat cycling and age: Repeated thermal stress breaks down insulation and epoxy potting.
• Oil intrusion: Valve cover gasket leaks fill spark plug wells, degrading boots and coil windings.
• Moisture and corrosion: Water intrusion or car-wash runoff can short boots and terminals.
• Vibration: Loose mounts or rough-running engines accelerate internal damage.
• Worn or incorrectly gapped spark plugs: Higher required voltage strains coils and drivers.
• Poor-quality aftermarket parts: Inconsistent windings or inferior insulation fail early.
• Harness/connectors: Broken wires or greened terminals reduce coil current and cause intermittent faults.

Addressing the root cause—especially oil leaks and worn plugs—prevents repeat failures and protects the catalytic converter.

How to confirm and fix

Quick checks

DIYers and technicians can use targeted steps to confirm whether a coil is at fault before replacing components.

• Swap test: Move the suspect coil to another cylinder and see if the misfire code follows.
• Inspect and clean: Check plug wells for oil/water, look for cracks or carbon tracking, and clean/secure connectors.
• Spark test: Use an inline tester to confirm consistent, strong spark; avoid pulling coils while running.
• Measure spark plug gap and condition: Replace fouled, worn, or incorrectly gapped plugs.
• Electrical tests: Verify 12V supply, ground integrity, and control signal; measure coil resistances if specified.
• Scope the waveform (if available): Look for weak burn time, irregular firing lines, or secondary leakage.
• Check service bulletins (TSBs): Some vehicles have updated coils, boots, or ECM calibrations.

If the misfire does not move with the coil, investigate the injector, compression, wiring, or ECM drivers for that cylinder.

Repair and cost expectations

Most repairs are straightforward, but the final bill depends on engine layout and whether multiple parts need replacement.

• Replace the failed coil (or coil pack): Typical parts cost $30–$150 per coil; $150–$400 for multi-output packs.
• Replace spark plugs and boots/wires as needed: $50–$200 in parts; many shops recommend plugs with coil replacement.
• Fix contributing issues: Valve cover gasket leaks, corroded connectors, or water intrusion paths.
• Labor: Commonly 0.3–1.0 hours per coil; more for difficult access (e.g., transverse V6 rears).
• Post-repair steps: Clear codes, road test, and verify no misfire counts; update ECM if a TSB applies.

Neglecting a misfire can escalate costs dramatically if the catalytic converter or O2 sensors fail due to prolonged raw fuel exposure.

Prevention

Regular maintenance and a few protective measures extend coil life and reduce the chance of sudden misfires.

• Change spark plugs at the manufacturer’s interval and use the correct type and gap.
• Use quality OEM or proven aftermarket coils and boots; apply dielectric grease on boot interiors.
• Repair oil leaks promptly to keep plug wells dry and clean.
• Avoid pressure-washing the engine; if cleaning, cover coils and connectors.
• Keep battery and charging system healthy to ensure proper coil energy.

These steps reduce electrical stress on coils and prevent contamination that leads to early failure.

When it’s not the coil

Other faults can produce similar misfire symptoms and codes, especially if diagnostics are incomplete or intermittent.

• Fuel injector problems: Clogged, leaking, or electrically open/shorted injectors.
• Vacuum leaks or unmetered air: Intake gaskets, PCV hoses, or cracked lines.
• Low compression: Worn rings, burnt valves, or head gasket leaks.
• Air/fuel metering issues: Faulty MAF/MAP sensors or significant exhaust leaks ahead of O2 sensors.
• Fuel pressure problems: Weak pump, clogged filter, or failing regulator.
• ECM/driver or wiring faults: Broken grounds, chafed control wires, or water intrusion in harnesses.
• Crankshaft/camshaft sensor faults: Timing reference errors leading to misfire-like symptoms.

A disciplined diagnostic process—verifying spark, fuel, compression, and timing—prevents unnecessary parts replacement and ensures a reliable fix.

Summary

A failing ignition coil causes misfires that make the engine run rough, reduce power, and often trigger the check-engine light. Left unresolved, it can quickly damage the catalytic converter and O2 sensors. Confirm the issue with a scan tool and basic tests, then replace the faulty coil and any worn plugs or boots, addressing oil or moisture intrusion. With proper diagnosis and maintenance, coil-related misfires are typically quick and affordable to fix—and far cheaper than repairing a damaged exhaust after prolonged driving on a misfire.

What are the symptoms of a failing ignition coil?

Symptoms of a failing ignition coil include a Check Engine Light, engine misfires and rough idling, stalling, loss of power, poor fuel economy, difficulty starting the engine, and exhaust backfiring or a smell of gas. These issues occur because the coil fails to deliver enough voltage to the spark plugs, preventing proper combustion. 
Common Symptoms

• Check Engine Light: A blinking or steady Check Engine light often indicates a misfire, which is a common sign of a failing ignition coil. 
• Engine Misfires/Rough Idling: You may notice a jerky or stuttering motion while driving, or the engine may feel unstable and vibrate when idling. 
• Stalling: The engine might cut off or stall when you come to a stop. 
• Loss of Power: The vehicle may feel sluggish, especially during acceleration, due to incomplete combustion in the cylinders. 
• Poor Fuel Economy: With less efficient combustion, your car will consume more fuel, leading to lower gas mileage. 
• Difficulty Starting: The engine may crank longer or fail to start altogether. 
• Exhaust Backfiring: You might hear a sputtering or coughing sound from the engine or tailpipe. 
• Smell of Gas: An incomplete combustion process can send unignited fuel into the exhaust, creating a gas smell. 

What’s Happening Inside the Engine
A failing ignition coil cannot provide the proper voltage to the spark plug. This leads to: 

• Incomplete Combustion: The fuel-air mixture in the cylinder doesn’t ignite correctly. 
• Misfires: The spark plug fails to do its job, causing the engine to misfire. 
• Engine Performance Issues: The misfires disrupt the smooth operation of the engine, resulting in the symptoms listed above. 

Which of the following symptoms could be caused by a failing ignition coil?

The following are eight key symptoms of a faulty ignition coil to look out for.

• Your Engine Starts Misfiring.
• Your Car Idles Roughly.
• Your Engine Cuts Off.
• You Struggle to Gain Speed.
• Your Car Starts Backfiring.
• You See Smoke Behind You.
• Your Check Engine Light Comes to Life.
• Your Ford Dealer Identifies a Bad Coil.

What happens if you drive your car with a bad ignition coil?

If an ignition coil fails while driving, the engine will likely misfire, causing the vehicle to lose power, jerk, or vibrate, and the Check Engine light may illuminate. Prolonged driving can lead to severe engine damage, including catalytic converter failure from unburnt fuel, cylinder wall damage, piston ring problems, and bearing failure due to oil dilution. It is best to pull over to a safe location as soon as possible.
 
Immediate Symptoms

• Engine Misfires: The cylinder won’t receive the spark it needs to ignite the fuel and air, causing the engine to misfire. 
• Loss of Power: The vehicle will feel sluggish or lose power, especially when accelerating. 
• Rough Engine Operation: You may notice jerking, vibrations, or rough idling. 
• Check Engine Light: A faulty ignition coil will trigger the Check Engine light, and a diagnostic code related to the malfunction will be stored. 
• Stalling: In extreme cases or if the last working coil fails in a single-coil vehicle, the engine could shut down entirely. 

Potential Long-Term Damage

• Catalytic Converter Failure: Opens in new tabUnburnt fuel can enter the exhaust system and ignite in the catalytic converter, causing extremely high temperatures and damaging it, according to Firestone Complete Auto Care. 
• Cylinder Damage: Opens in new tabUnburnt fuel can wash oil from the cylinder walls, leading to excessive wear on the cylinder and piston rings. 
• Oil Dilution: Opens in new tabFuel can mix with the engine oil, diluting it and reducing its lubricating properties, which can lead to bearing failure. 
• Engine Bearing and Crankshaft Failure: Opens in new tabThe increased stress on the engine from the non-firing cylinder and the lack of proper lubrication can cause catastrophic damage to bearings and the crankshaft. 

What to Do

1. Pull Over Safely: As soon as you notice symptoms, find a safe spot to pull over. 
2. Do Not Delay Repairs: Driving with a failing ignition coil can quickly escalate from a minor inconvenience to an expensive repair. 
3. Consult a Mechanic: Have the vehicle inspected by a professional to diagnose the issue and replace the faulty ignition coil promptly. 

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.