What happens when the ignition coil is bad?
A bad ignition coil typically shows up as engine misfires, rough idling, and reduced power. It can also lead to hard starts, higher fuel consumption, and, if left untreated, potential damage to the catalytic converter. In modern vehicles, diagnosis usually starts with an OBD-II readout and a coil-by-coil test.
The ignition coil’s role in the engine
The ignition coil is a small transformer that turns the car’s 12-volt electrical system into the high voltage needed to create a spark at the spark plug. In most newer cars you’ll find coil-on-plug units or coil packs rather than a single distributor. A healthy coil delivers a strong, consistent spark to ignite the air–fuel mixture in each cylinder; a failing coil disrupts that spark, leading to misfires and drivability problems.
Common symptoms of a failing ignition coil
The following signs can indicate a coil is wearing out or has failed. Symptoms can appear gradually or suddenly, and they may vary by vehicle.
- Engine misfires or rough idle, especially at low speeds or under load
- Hard starting or a no-start condition
- Loss of power or reduced acceleration
- Increased fuel consumption and poorer overall fuel economy
- Check Engine Light with misfire-related codes (for example P0300 and cylinder-specific codes like P0301, P0302, etc.)
- Engine shaking or surging, particularly at idle
- Potential damage to the catalytic converter if unburned fuel reaches the exhaust
If you notice these signs, it’s wise to have the ignition system tested promptly to avoid further damage and to improve safety and performance.
What happens if you drive with a bad ignition coil
Continuing to drive with a faulty coil can cause unburned fuel to enter the exhaust system, leading to overheating and damage to the catalytic converter. Prolonged misfires can also foul spark plugs, wear ignition wires or boots, and put extra stress on the engine management system. In some cases, a persistent misfire can cause the engine to stall unexpectedly, creating a safety risk.
Diagnosing and repairing a bad ignition coil
Diagnosing a faulty coil involves a combination of code reading, visual inspection, and electrical testing. The goal is to determine whether the coil itself is at fault or if another component is causing the symptoms.
- Use an OBD-II scanner to read trouble codes and check live data for misfire activity and fuel trim abnormalities.
- Visually inspect the ignition system: spark plugs, coil boots/connectors, and any signs of oil intrusion or damaged wiring.
- Test the coil’s primary and secondary resistance with a multimeter and compare results to the manufacturer’s specifications.
- If feasible, perform a swap test by swapping the suspected coil with a known-good coil (or swapping cylinders) to see if the misfire follows the coil.
- Check the ignition wiring and power supply for voltage drops, bad grounds, or damaged connectors.
- Consider replacing the coil(s) if tests indicate an internal short, open circuit, or degraded insulation; also replace related components (spark plugs, boots) as preventive maintenance.
- Recheck for codes and road-test the vehicle to confirm the issue is resolved and no new codes appear.
Note: Some vehicles rely on a single coil pack for multiple cylinders or a distributor-based system. Always follow the manufacturer’s service information for testing procedures and specifications.
Replacement options and cost considerations
When a coil is determined to be faulty, replacement choices vary by vehicle design and market. Costs include parts and labor, and can differ widely by region and whether OEM or aftermarket parts are used.
Typical ranges to guide expectations (manufacturer specs and local labor rates apply):
- Individual ignition coil (coil-on-plug or COP) per cylinder: roughly $50–$150 for parts
- Coil packs (for 4-cylinder or 6-cylinder engines): roughly $150–$600 for parts
- Labor for replacement: typically $80–$200 per hour, with total labor time ranging from 0.5 to 2 hours per coil pack depending on access
- Additional costs: spark plugs, boot replacements, wiring harness inspections, and potential sensor or PCM checks if trouble persists
Prices vary by vehicle make/model, part quality, and the need to replace multiple coils at once. It’s often prudent to replace related components (spark plugs, boots) during coil service to maximize reliability and prevent future failures.
Summary
A failing ignition coil disrupts the spark that powers combustion, leading to misfires, rough running, hard starts, and lower fuel efficiency. If ignored, it can worsen engine performance and damage exhaust components like the catalytic converter. Diagnosis combines code reading with targeted electrical tests, and repairs typically involve replacing the faulty coil(s) and sometimes related parts. With modern ignition systems, timely diagnosis and replacement help restore smooth operation and protect the vehicle’s emissions system.
How do I know if my ignition coil is bad?
A bad ignition coil can be identified by a check engine light and symptoms like rough idling, engine misfires, poor acceleration, and difficulty starting the engine. Other signs include stalling, backfiring, decreased fuel economy, and a strong smell of gasoline.
Common symptoms
- Check Engine Light: The onboard computer will often detect ignition problems and illuminate the check engine light. A mechanic can use an OBD2 scanner to retrieve misfire codes, such as P0302, to pinpoint the specific cylinder.
- Engine misfires: You may notice the engine sputtering, shaking, or jerking, especially when accelerating.
- Rough idle: The engine may feel shaky or vibrate noticeably while the vehicle is idling.
- Poor acceleration: The vehicle may feel sluggish or hesitate when you press the gas pedal.
- Difficulty starting: The engine may crank for a longer time than usual before starting, or it may not start at all.
- Stalling: The engine may stall while driving or when stopped at a light.
- Backfiring: A sudden loud bang from the exhaust can occur when unburned fuel ignites.
- Reduced fuel economy: Incomplete combustion due to a weak spark means more fuel is wasted, leading to lower miles per gallon.
- Smell of gasoline: Unburned fuel can exit the exhaust, creating a strong gas smell.
How to test further (requires caution)
- Use an OBD2 scanner: This can help identify specific misfire codes that point to a faulty cylinder, as seen in this YouTube video.
- Swap ignition coils: A simple test is to swap the coil from the suspected bad cylinder with a known good cylinder. If the misfire code follows the coil to the new cylinder, then the coil is likely bad, notes this YouTube video.
- Check resistance (for advanced users): A multimeter can be used to measure the resistance of the coil windings. A well-functioning coil will have a specific resistance range, but this requires a repair manual for correct specifications and extreme caution due to high voltage.
How to tell if a coil needs replacing?
You can tell a coil needs replacing by observing symptoms like a check engine light, rough engine operation (sputtering, shaking, backfiring), and stalling or difficulty starting. For vapes, a coil needs replacing when there is a weak or burnt flavor, the device gurgles or leaks, or the e-liquid color changes.
For vehicle ignition coils
- Illuminated check engine light: The light will often come on to indicate a problem, which could be a coil-related misfire.
- Engine misfires: You may feel or hear the engine sputtering, shaking, or having a "coughing" sound, especially at idle.
- Difficulty starting: The engine may struggle to start, or take longer to crank.
- Stalling: The engine can stall while driving or when stopped, such as at a traffic light.
- Reduced power: The vehicle may feel sluggish, with poor acceleration and reduced power.
- Poor fuel economy: A failing coil can lead to decreased gas mileage.
- Backfiring: Unburned fuel can exit the exhaust, causing a loud "bang".
For vape coils
- Weak or burnt flavor: The taste will diminish and become less consistent, or develop a burnt taste.
- Gurgling sounds: Excess e-liquid can flood the chamber and cause a gurgling noise when you take a puff.
- Leaking: E-liquid may leak from the tank or pod, often from the center of the coil.
- Flashing light: Many devices will flash or display an "atomiser error" message when there is a coil issue.
- Reduced vapor production: You may notice less vapor is produced with each puff.
- E-liquid color change: The color of the e-liquid in the tank may change or darken.
What damage can a bad ignition coil cause?
Coughing, Sputtering Engine
When a failing ignition coil prevents a spark plug from sparking, the cylinder begins to misfire. This causes the engine to cough, sputter, and shake as it struggles to keep going. The slower you drive, the worse these symptoms of misfiring are likely to become.
Can a bad ignition coil cause rough idle?
A rough idle can also indicate the presence of a misfire due to a bad ignition coil. The engine idles roughly based on the varying voltage the spark plugs receive and deploy in combusting the air/fuel mixture.
