How To Test Auto Ignition Coil

Is your car sputtering, misfiring, or struggling to accelerate? These are common signs of a failing ignition coil. Diagnosing and testing your ignition coils is a crucial step in resolving these issues and getting your vehicle back to running smoothly. Ignoring a faulty ignition coil can lead to more significant problems down the road, so it's essential to address it promptly. This article will guide you through the symptoms, causes, testing procedures, and potential fixes for a faulty auto ignition coil.

Recognizing the Symptoms of a Failing Ignition Coil

A failing ignition coil can manifest in a variety of ways, and recognizing these symptoms early can save you time and money. Here are some of the most common indicators that your ignition coil might be going bad:

Engine Misfires: This is perhaps the most telltale sign. A misfire occurs when the cylinder doesn't receive the spark it needs to ignite the air-fuel mixture. You'll often feel a jerking or stumbling sensation, especially during acceleration. The check engine light will likely illuminate.
Rough Idling: A shaky or unstable idle, particularly when the engine is cold, can indicate a faulty coil. The engine might struggle to maintain a consistent RPM, leading to vibrations and a generally unpleasant experience.
Loss of Power: A noticeable decrease in acceleration and overall engine power is a common symptom. The engine may feel sluggish, especially when climbing hills or carrying heavy loads.
Poor Fuel Economy: When an ignition coil fails, the engine may compensate by using more fuel to maintain performance. This leads to a decrease in your miles per gallon (MPG).
Check Engine Light: The check engine light will almost always illuminate when an ignition coil is failing. The diagnostic trouble code (DTC) will typically indicate a misfire in a specific cylinder (e.g., P0301 for misfire in cylinder #1). You'll need an OBD-II scanner to read these codes.
Engine Stalling: In severe cases, a failing ignition coil can cause the engine to stall completely, especially at low speeds or during idling.
Hesitation During Acceleration: The engine might hesitate or stumble when you press the accelerator pedal, particularly from a stop.

Understanding the Root Cause

The ignition coil's primary function is to transform the low voltage from the car's battery (typically 12 volts) into the high voltage (20,000-40,000 volts) required to create a spark at the spark plug. This spark ignites the air-fuel mixture in the combustion chamber, initiating the engine's power cycle. Several factors can contribute to ignition coil failure:

Heat: Excessive heat, often caused by prolonged engine operation or poor cooling, is a major enemy of ignition coils. High temperatures can degrade the coil's internal insulation and windings, leading to shorts and failures.
Vibration: Constant engine vibrations can also take a toll on the coil, causing internal components to loosen or break down over time.
Moisture: Exposure to moisture or humidity can corrode the coil's terminals and internal wiring, leading to poor conductivity and eventual failure.
Age and Wear: Like any other automotive component, ignition coils have a limited lifespan. Over time, the internal components can simply wear out due to normal use.
Spark Plug Issues: Worn or improperly gapped spark plugs can place excessive stress on the ignition coil. A larger spark plug gap requires the coil to work harder to generate a spark, potentially leading to premature failure.
Voltage Spikes: Fluctuations in the vehicle's electrical system, such as voltage spikes, can damage the coil's internal circuitry.
Poor Installation: Incorrect installation of the ignition coil can also lead to premature failure.

Consequences of Ignoring a Faulty Ignition Coil

Ignoring a failing ignition coil might seem like a minor issue at first, but it can lead to more serious and costly problems if left unaddressed:

Catalytic Converter Damage: A misfiring cylinder dumps unburnt fuel into the exhaust system. This unburnt fuel can overheat and damage the catalytic converter, a crucial emissions control device. Replacing a catalytic converter is a significantly more expensive repair than replacing an ignition coil.
Increased Fuel Consumption: As mentioned earlier, the engine will compensate for the misfire by using more fuel, leading to decreased fuel economy.
Engine Damage: Severe misfires can cause engine knocking or pinging, which can damage pistons, connecting rods, and other internal engine components.
Stranded Vehicle: In the worst-case scenario, a completely failed ignition coil can leave you stranded on the side of the road.
Reduced Engine Lifespan: Continual misfires and rough running conditions put extra stress on the engine, potentially shortening its overall lifespan.

Recommended Fixes and Testing Procedures

The primary fix for a faulty ignition coil is, of course, replacement. Before replacing the coil, it's crucial to properly diagnose the issue to ensure you're not addressing a symptom rather than the root cause. Here's a breakdown of testing procedures:

Testing Procedures

Safety First: Before performing any electrical testing on your vehicle, disconnect the negative battery cable to prevent accidental shocks or damage to the electrical system.

Visual Inspection: Carefully inspect the ignition coil for any signs of physical damage, such as cracks, burns, or corrosion on the terminals.
OBD-II Scanner: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the vehicle's computer. Note any misfire codes (P0300-P030n, where n is the cylinder number) or ignition coil-related codes.
Coil Swap (For Multiple Coils): If you suspect a particular coil is failing, try swapping it with a known good coil from another cylinder. Clear the DTCs and start the engine. If the misfire follows the swapped coil, you've confirmed that the coil is the problem.
Multimeter Testing: You can use a multimeter to check the coil's resistance.
- Primary Resistance: Set the multimeter to ohms (Ω) and measure the resistance between the coil's primary terminals (typically the two smaller terminals). Compare the reading to the manufacturer's specifications (usually found in the vehicle's repair manual or online). A significantly different reading indicates a faulty coil.
- Secondary Resistance: Measure the resistance between the coil's primary terminal and the spark plug terminal. Again, compare the reading to the manufacturer's specifications.
Note: Some modern coils don't allow for reliable resistance testing, so consult your vehicle's service manual.
Spark Tester: A spark tester is a tool that allows you to visually check if the coil is producing a spark. Connect the tester between the coil and the spark plug. Crank the engine and observe the tester. A healthy coil will produce a strong, consistent spark. A weak or absent spark indicates a faulty coil.

Replacing the Ignition Coil

Disconnect the Negative Battery Cable: Again, safety first!
Locate the Faulty Coil: Identify the coil based on the OBD-II code or testing results.
Disconnect the Electrical Connector: Carefully disconnect the electrical connector from the coil. Some connectors may have a locking mechanism that needs to be released.
Remove the Coil: Remove any bolts or screws securing the coil to the engine. Gently pull the coil out.
Install the New Coil: Install the new coil in the reverse order of removal. Make sure the electrical connector is securely attached.
Reconnect the Negative Battery Cable:
Clear the DTCs: Use the OBD-II scanner to clear any stored DTCs.
Test Drive: Take the vehicle for a test drive to ensure the misfire is resolved and the engine is running smoothly.

Cost Estimates and Shop Advice

The cost of replacing an ignition coil can vary depending on the vehicle's make and model, the type of coil, and whether you choose to do the work yourself or have it done by a professional mechanic.

DIY Repair: The cost of a new ignition coil typically ranges from $30 to $100 per coil. If you're comfortable working on your car and have the necessary tools, you can save money by replacing the coil yourself.
Professional Repair: If you prefer to have a mechanic do the work, expect to pay for both parts and labor. Labor costs can vary depending on the shop's hourly rate, but you can typically expect to pay between $50 and $150 in labor per coil. Therefore, the total cost for a professional ignition coil replacement can range from $80 to $250 per coil.

Shop Advice: When choosing a repair shop, it's always a good idea to get multiple quotes from different shops. Ask about the brand of ignition coil they use and their warranty policy. A reputable shop will stand behind their work and provide a warranty on both parts and labor.

Credibility and Common Failure Points

Ignition coil failure is a common issue, and many manufacturers have issued Technical Service Bulletins (TSBs) addressing specific coil-related problems. For example, some vehicles are known to experience coil failures due to heat soak, while others have issues with coil connectors becoming brittle and cracking.

Data from online automotive communities and forums often indicates that ignition coils typically last between 60,000 and 100,000 miles. However, this can vary depending on driving conditions, maintenance practices, and the quality of the coil itself. Replacing all ignition coils at once when one fails is generally recommended, especially on older vehicles with higher mileage, to prevent future failures and ensure consistent performance.

By understanding the symptoms, causes, and testing procedures outlined in this article, you can effectively diagnose and address ignition coil issues, keeping your vehicle running smoothly and avoiding potentially costly repairs down the road.