How can I tell if my ignition coil is bad?

A faulty ignition coil typically shows misfires, rough idle, hard starting, reduced power, and poorer fuel economy, often with the check-engine light on. A reliable diagnosis combines visual checks, spark testing, resistance measurements, and scanning for trouble codes.

Ignition coils are a core part of modern ignition systems, delivering the high voltage needed to ignite the air-fuel mixture in each cylinder. When a coil or coil pack deteriorates, it can affect one cylinder or multiple cylinders, causing performance issues. This article explains the common signs, how to test them, and what to consider before replacing parts.

Common symptoms of a failing ignition coil

The following signals are frequently reported by drivers when a coil is on the fritz. They can also be caused by other ignition or fuel-system problems, so follow up with proper testing.

• Intermittent or persistent engine misfires in one or more cylinders


• Rough idle or noticeable engine vibrations when the car is stopped


• Hard starting or extended cranking to start the engine


• Loss of horsepower or sluggish acceleration


• Decreased fuel economy and higher emissions


• Check Engine light illuminated, often with misfire codes (P0300 and P0301–P0308)


• Backfiring, especially under load or high RPM


• Engine stalls or dies at idle

Interpreting these signals requires confirming spark delivery to the affected cylinder and ruling out plugs, wires, and fuel delivery problems.

How to diagnose: steps and tests

Use these steps to verify whether the ignition coil is the culprit. The exact method can vary depending on whether your vehicle uses coil-on-plug units or a distributor/coil-pack arrangement.

1. Visual inspection of the coil and surrounding area for cracks, oil leaks, burns, or corrosion on connectors.


2. Retrieve and review diagnostic trouble codes with an OBD-II scanner to identify misfire-related codes (such as P0300-P0308).


3. Perform a spark test on the affected cylinder with a spark tester or a known-good plug connected to the coil circuit, observing spark quality while the engine is cranking (exercise caution due to high voltage).


4. Measure coil resistance with a multimeter:
   
   
   
   • Primary winding resistance (low-voltage side): compare to the vehicle’s service manual. Typical values are low ohms, but exact specs vary by design.
   
   
   • Secondary winding resistance (high-voltage side): also compare to spec; ranges commonly fall in the kilo-ohm range and vary by model.
   
   
   
   


5. Conduct a cylinder-to-cylinder swap test (if feasible): move the suspected coil to a different cylinder and see if the misfire follows the coil or stays with the cylinder.


6. If your vehicle has coil-on-plug modules, consider a more advanced test such as using an oscilloscope to observe the coil’s primary/secondary waveform or using a professional ignition tester.


7. Assess related components: worn spark plugs, damaged wires, dirty or clogged injectors, and ignition control modules can mimic coil problems.

Tests should be performed with the engine off and safety precautions observed. If you are unsure or lack the right equipment, a professional inspection is advisable.

Replacement considerations and next steps

When to replace and how to plan repair

Replacement decisions depend on the test results and the vehicle’s design. Coil-on-plug systems imply individual coils per cylinder, while coil packs may serve multiple cylinders. Consider replacing faulty coils and, if needed, related components that wear out with them.

• Replace only the failing coil(s) if tests show isolated faults.


• Consider replacing all coils in a set if multiple are near failure or if the vehicle has high mileage to ensure uniform performance.


• Inspect and replace spark plugs if they are worn or fouled, as bad plugs can stress coils and lead to missed faults.


• Check for root causes such as oil leaks that can contaminate coils and cause premature failure; address leaks to protect new parts.

Costs vary by vehicle and design, with coil units ranging in price from moderate to premium, and labor depending on access and whether the job involves a full coil-pack or individual coils. Replacing coils in sets can save time and ensure consistent ignition performance across cylinders.

Bottom line

Ignition coils are a common source of performance problems in modern engines, but symptoms overlap with spark plugs, wiring, and fuel-delivery issues. A methodical approach—visual checks, code reading, spark testing, resistance measurements, and swap testing—helps isolate the fault. When in doubt, consult a qualified technician to avoid misdiagnosis and unnecessary part replacements.

Summary

In short, signs of a bad ignition coil include misfires, rough idle, hard starts, and reduced power, often accompanied by a check-engine light. Diagnose with a combination of visual inspection, code reading, spark testing, and resistance measurements, keeping in mind that exact specs vary by vehicle. If tests point to a faulty coil, replace it promptly and address any underlying causes to prevent future failures. A professional assessment can provide precision and peace of mind, especially for complex coil-on-plug systems.

How do I know if my ignition coil needs changing?

You can tell your ignition coil needs changing if you experience symptoms like a check engine light, rough idling, engine misfires, difficulty starting, poor acceleration, or a decrease in fuel efficiency. A flashing check engine light requires immediate attention to prevent further engine damage. 
You can watch this video to learn about the symptoms of a bad ignition coil: 53sHonest Mechanic ColoradoYouTube · Dec 27, 2023
Common symptoms of a bad ignition coil

• Check Engine Light: This is a common indicator. Your car's computer may also store a diagnostic trouble code (e.g., P0300 for a general misfire) that a mechanic can read with a scanner. 
• Engine Misfires and Rough Idling: You may feel a shudder, jerk, or vibration, especially when accelerating or at idle, as one or more cylinders are not firing correctly. 
• Difficulty Starting: The engine may crank for a longer time before starting, or it might not start at all if the coil isn't providing enough voltage to ignite the fuel. 
• Poor Acceleration: Your car may feel sluggish or hesitate when you press the gas pedal, and it may struggle to accelerate or climb hills. 
• Reduced Fuel Efficiency: Inefficient combustion due to a weak spark will cause your fuel economy to drop. 
• Backfiring: Unburned fuel can exit the engine and ignite in the exhaust system, creating a popping or banging sound. 
• Engine Stalling: A complete failure of the ignition coil can cause the engine to stall suddenly. 

This video explains the impact of a bad ignition coil on engine performance: 48sEasyAutoFixYouTube · Mar 25, 2023
What to do if you suspect a bad ignition coil

1. Check the Check Engine Light: If the light is solid, you can schedule an inspection. If it is flashing, pull over as soon as it is safe to do so and turn off the engine to prevent further damage. 
2. Scan for Codes: Use an OBD-II scanner to check for specific misfire codes (like P0301, P0302, etc.) or other diagnostic trouble codes that can pinpoint the problem. 
3. Inspect the Coils: Visually inspect the ignition coils for any signs of damage, such as cracks in the housing or oily residue. 
4. Consult a Professional: If you are unsure, have a qualified mechanic inspect the ignition system to accurately diagnose the problem and replace the faulty coil(s). 

You can watch this video for a demonstration of how to inspect ignition coils: 1m1ROADYouTube · Aug 7, 2023

What is a common symptom of a failing ignition coil?

Symptoms of a bad ignition coil include engine misfires, rough idling, and a check engine light. Other signs are sluggish acceleration, poor fuel economy, difficulty starting, and engine stalling. You might also notice sputtering, backfiring, or a strong smell of gas.
 
Performance and starting issues

• Engine misfires: The engine may feel like it's coughing, sputtering, or shaking because a spark plug isn't firing correctly. 
• Rough idling: Your car may shake or vibrate noticeably when it's stopped, such as at a traffic light. 
• Poor acceleration: You may experience sluggishness or a noticeable loss of power when trying to accelerate. 
• Difficulty starting: The engine may struggle to start, or may not start at all if multiple coils are bad. 
• Stalling: The engine may stall or hesitate while driving. 

Efficiency and other indicators

• Decreased fuel economy: A bad coil can cause the engine to burn more fuel, so you'll get fewer miles per gallon. 
• Backfiring: You might hear loud banging noises from the exhaust, which occurs when unburned fuel ignites in the exhaust system. 
• Strong smell of gas: This can happen when unburned fuel escapes through the exhaust system. 
• Check engine light: The vehicle's computer will detect the misfire and turn on the check engine light. A mechanic can use an OBD2 scanner to read the specific error codes. 

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.

How do you test the ignition coil?

You can check an ignition coil using a spark tester or a multimeter. To use a spark tester, connect it to the coil and look for a strong, blue spark when the engine cranks; a weak or absent spark indicates a faulty coil. To use a multimeter, test the primary and secondary windings for resistance within the expected range for your vehicle (e.g., 0.50.50.5 to 2.52.52.5 ohms for the primary and 2,0002 comma 0002,000 to 5,0005 comma 0005,000 ohms for a four-stroke engine's secondary). 
Method 1: Use a spark tester 
This video demonstrates how to check the ignition coil with a spark plug tester: 58sRatchets And WrenchesYouTube · Sep 1, 2015

1. Start the engine or crank it: Get someone to help you by cranking the engine. 
2. Connect the tester: Attach a spark tester between the coil and the spark plug or a known good spark plug. 
3. Observe the spark: Look for a strong, blue spark jumping the gap in the tester. 
   • Good coil: A strong, blue spark. 
   • Bad coil: No spark, or a weak, orange, or zigzagging spark. 

Method 2: Use a multimeter 
This video demonstrates how to test an ignition coil with a multimeter: 56sVintage Engine RepairsYouTube · Oct 10, 2023

1. Disconnect the battery: Always disconnect the negative battery terminal first to prevent short circuits. 
2. Disconnect the coil: Pull the high-tension lead (the thick wire) from the ignition coil. 
3. Check the secondary winding: 
   • Set the multimeter to the 202020k Ohm setting. 
   • Touch one probe to the coil's center point (where the high-tension lead connects) and the other to the negative point. 
   • The reading should be between 666k and 161616k Ohms, though this can vary by vehicle. Check your service manual for specific values. 
4. Check the primary winding: 
   • Set the multimeter to the 200200200 Ohm setting. 
   • Touch the probes to the primary winding terminals (consult your service manual for their location). 
   • The reading should be a low value, typically between 0.50.50.5 and 2.52.52.5 ohms for a four-stroke engine. 
5. Interpret the results:
   • An open circuit (no reading) or a very high resistance reading indicates a bad coil. 
   • If the multimeter readings are outside the specifications in your service manual, the coil is likely faulty. 

What to do next

• If the coil is faulty, replace it with a high-quality part. 
• If the coil tests good but the problem continues, the issue may be with the spark plugs, wiring, or other components in the ignition system.