Is your engine stuttering, losing power, or is the engine light on? A faulty ignition coil could be the cause. The ignition coil is crucial for combustion in your engine – without a working coil, your car won't start or the engine will run very erratically. In this comprehensive guide, you'll learn to recognize the symptoms of a faulty ignition coil, how to diagnose it, and what replacement costs are at FAAY Auto Groep.
What is a coil?
The ignition coil (or simply "coil") is a transformer that converts the 12 volts from your battery to 15,000-40,000 volts. This incredibly high voltage is necessary to create a spark between the spark plug electrodes. Without this spark, the air-fuel mixture can't ignite, and your engine won't run.
How Does a Coil Work?
- Low voltage in: The engine ECU sends 12V to the primary winding
- Magnetic field: Current through primary winding creates magnetic field
- Power outage: ECU cuts power at exactly the right time
- Induction: Collapsing magnetic field induces high voltage in secondary winding
- High voltage off: 15,000-40,000V goes to spark plug
- Spark: High voltage causes spark to jump, mixture ignites
This process happens thousands of times per minute – at 3,000 rpm, 1,500 sparks are generated per minute in a 4-cylinder engine.
Function of Coil in Ignition System
The coil is the heart of the ignition system. Without proper coil function, combustion is impossible. The ignition system consists of:
- Engine ECU: Determines ignition timing
- Coil: Generates high voltage
- Spark plug wires (older cars): Conduct high voltage
- Spark plugs: Create spark in cylinder
Modern cars have electronic ignition without a distributor – each cylinder has its own coil, or one coil serves two cylinders. This provides more precise timing and more reliable ignition.
Symptoms of a faulty coil
A faulty coil causes immediate engine problems. Do you recognize these symptoms?
1. Engine Runs Irregularly (Stutters)
The engine vibrates, jerks, and runs very erratically, especially at idle. This is because one or more cylinders aren't firing. With a 4-cylinder engine, your engine is effectively running on 3 cylinders—you can feel and hear this immediately.
The stuttering often becomes worse under load (acceleration, uphill) and may diminish at higher revs.
2. Misfires (Engine Stutters/Misfires)
“"Misfire" means a cylinder is not firing or is firing poorly. Symptoms:
- Vibrations in steering wheel and accelerator pedal
- Shocks when accelerating
- Sudden power interruptions
- Engine runs up and down in revs
Modern cars detect misfires and activate the check engine light. When diagnosing them, you'll see error codes like P0300 (random misfires) or P0301-P0304 (cylinders 1-4 misfires).
3. Loss of power
The car feels weak, especially when accelerating or driving uphill. A single faulty coil means one or two cylinders won't fire – you'll lose 25-50% engine power.
With gradual coil failure, it starts with a slight loss of power and becomes progressively worse until the engine barely responds to throttle.
4. Increased Fuel Consumption
A faulty ignition coil causes incomplete combustion. Unburned fuel leaves the cylinder through the exhaust. Result: 15-30% higher fuel consumption without driving faster.
The engine also has to work harder to deliver the same performance, which further increases consumption.
5. Engine Light On
The check engine light comes on when the engine ECU detects misfires. Error codes related to ignition coils:
- P0300: Random/multiple cylinder misfires
- P0301-P0308: Misfire specific cylinder (1-8)
- P0351-P0358: Coil primary/secondary circuit malfunction (cylinder 1-8)
- P0261-P0268: Injector circuit problems (can also be ignition related)
NB: Engine light can also have other causes. Read more in our guide Engine Light On: What Now?
6. Starting problems
If the ignition coil is faulty, the engine will be difficult to start or won't start at all. The engine will run (starter motor will work), but there's no spark, so no ignition. The problems are especially worse when cold or after a long period of inactivity.
With a completely defective coil (in single-coil systems), the engine will not start at all. With coil-per-cylinder systems, the engine will start but run very poorly.
7. Rough Running and Vibrations
The engine feels rougher than normal. Especially noticeable:
- When idling (traffic light)
- Vibrations in steering wheel
- Shaking of entire car
- Rattle from engine compartment
8. Black Smoke or Gasoline Smell From Exhaust
Unburned fuel leaves the engine through the exhaust. This causes:
- Black smoke (especially when accelerating)
- Strong gasoline smell
- Backfiring from exhaust (popping sound)
Danger: Unburned fuel in the exhaust system can damage the catalytic converter (€400-€1,500 replacement). Troubleshoot the coil problem immediately.
Difference: Single Coil vs. Coil-Per-Cylinder
Single Coil + Distributor (Old System)
Older cars (before 2000) often have a single ignition coil for all cylinders. The distributor distributes the high voltage to the correct spark plug via spark plug wires.
Advantages:
- Simple system
- Cheap replacement (1 coil: €30-€80)
Disadvantages:
- Less accurate
- If coil fails, none of the cylinders will work
- Distributor and cables also wear out
Coil-Per-Cylinder (Modern System)
Modern cars (after about 2000) have a separate ignition coil for each cylinder, mounted directly on the spark plug. No distributor or spark plug wires are required.
Advantages:
- More precise ignition
- More reliable (no distributor/cables)
- If one coil fails, other cylinders will operate normally
- Better performance and lower consumption
Disadvantages:
- More expensive if defective (4 coils: €200-€500)
- More complex system
Double Coil (2 Cylinders Per Coil)
Some cars use two ignition coils for four cylinders (a waste spark system). Each coil serves two cylinders simultaneously.
In case of failure: two cylinders fail instead of one.
Diagnosis of a defective coil
1. Reading OBD Error Codes
Modern cars store error codes in the engine ECU. At FAAY, we can read the codes within 5 minutes using professional diagnostic equipment. Ignition coil-related codes:
- P0350-P0362: Coil circuit malfunctions
- P0300-P0308: Misfires (often indicate coil failure)
Codes indicate which cylinder has the problem, so we can immediately test the correct coil.
2. Visual Inspection
Our mechanics check:
- Cracks in coil: Moisture can enter and cause a short circuit
- Fire damage: Black spots or melted plastic
- Corrosion on connectors: Poor connection prevents proper operation
- Oil leak: Oil on coil can lead to failure
- Loose cables: Poor connection causes intermittent problems
3. Electrical Measurement (Resistance Test)
We use a multimeter to measure the resistance of the primary and secondary windings. Typical values:
- Primary winding: 0.5-2 Ohm
- Secondary winding: 6,000-15,000 Ohms
Deviating values indicate internal damage. Exact values vary by make/model – we use manufacturer's specifications.
4. Changing Coils (Swap Test)
If the error code indicates cylinder 2, swap the coil from cylinder 2 with cylinder 4. If the error code continues to cylinder 4, the coil is faulty. If the error code remains on cylinder 2, the coil is OK and the problem is the spark plug or injector.
5. Oscilloscope Test (Professional)
At FAAY, we use oscilloscopes to test coils under operating conditions. We see exactly how the coil performs while the engine is running – this detects intermittent problems that other tests miss.
Coil Replacement: When Is It Necessary?
Replace coil when:
- OBD error code direct ignition coil (P0350-P0362)
- Resistance measurement is abnormal
- Visual damage visible (cracks, fire damage)
- Swap test confirms defective coil
- Misfires keep coming back after replacing spark plugs
Replace One Coil or All?
For cars with coil per cylinder:
Only replace a defective coil if:
- Car less than 5 years old
- Mileage under 100,000 km
- Other coils test fine
Replace all coils at once if:
- Car older than 8-10 years
- Mileage over 150,000 km
- Budget available (saves future labor)
- Preventive maintenance
Ignition coils wear out evenly. If one fails, others often follow within one to two years. Replacing a set saves you multiple workshop visits.
Coil Replacement Costs
Costs vary greatly depending on system type and number of coils:
Single Coil (Older Cars)
- Element: €40-€120
- Labor: €40-€60 (30-45 minutes)
- Total: €80-€180
Simplest and cheapest option. The coil is usually easily accessible in the engine compartment.
Replacing One Coil (Coil-Per-Cylinder)
- Element: €50-€150 per coil
- Labor: €60-€100 (45-60 minutes)
- Total: €110-€250 per coil
Modern coils are often located deep in the engine (under the intake manifold in V-engines), which increases working time.
Complete Set Replacement (4 Cylinders)
- Parts: €200-€500 (set of 4)
- Labor: €80-€150 (all at once is more efficient)
- Total: €280-€650
Premium Brands (BMW, Mercedes, Audi)
Premium cars often have more expensive, more powerful coils:
- Per coil: €80-€200
- Set of 4: €300-€700
- Set of 6 (6-cylinder): €450-€1.000
- Labor: €100-€200 (more complex installation)
Cost Comparison Per Scenario
| Scenario | Element | Labor | Total |
|---|---|---|---|
| Single coil (old system) | €40-€120 | €40-€60 | €80-€180 |
| 1 coil replaced (modern) | €50-€150 | €60-€100 | €110-€250 |
| Set of 4 coils (budget brand) | €200-€350 | €80-€120 | €280-€470 |
| Set of 4 coils (premium brand) | €300-€700 | €100-€200 | €400-€900 |
| Set of 6 coils (6-cylinder) | €450-€1.000 | €120-€250 | €570-€1.250 |
What is included with FAAY?
- Professional diagnosis with OBD scanner
- High quality coils (OEM quality)
- Test all coils
- Check spark plugs and cables
- Clear error codes
- Test drive after installation
- 2 year warranty on parts and labor
- Free 32-point safety check
Can You Replace a Coil Yourself?
Technically it is possible, but we advise against it without experience:
Challenges DIY Replacement
- Diagnosis difficult: Without an OBD scanner you cannot be sure which coil is defective
- Accessibility: Coils are often located deep in the engine, requiring disassembly of other parts
- High voltage dangerous: Even after switching off, the coil can retain voltage
- Clear error codes: Without scanner the engine light stays on
- Risk of misdiagnosis: Replacing the coil while the problem is the spark plug
If You Do It Yourself
- Read error codes with OBD scanner (€20-€50)
- Disconnect battery (safety)
- Remove necessary parts for access
- Disconnect the coil electrical connector
- Remove coil (often 1 bolt)
- Install new coil with correct torque
- Connect everything and test
- Clear error codes
Our recommendation: Have your coil replaced at FAAY from €110. You'll receive a correct diagnosis, professional installation, a warranty, and the assurance that the problem has been resolved.
Frequently Asked Questions: Replacing a Coil
How long does a coil last?
Average lifespan: 100,000-200,000 km or 10-15 years. Quality varies greatly by brand. Premium coils (Bosch, NGK, Delphi) last longer than budget options. Extreme conditions (hot/cold climates, frequent short trips) shorten their lifespan.
What causes coil failure?
Most common causes:
- Normal wear and tear (heat, vibration)
- Moisture/water in engine compartment (corrosion)
- Overheating due to bad spark plugs
- Voltage spikes in electrical system
- Oil leak on coil
- Outdated insulation (cracks due to age)
Can I drive with a broken coil?
Not recommended. With a coil-per-cylinder system, you can drive technically (the engine runs on 3 of the 4 cylinders), but:
- Unburned fuel damages catalytic converter (€400-€1,500 repair)
- Engine runs very irregularly and has little power
- Increased consumption (15-30%)
- Possible damage to other cylinders due to vibrations
With a single-coil system, the car won't start at all. Schedule repairs immediately at FAAY.
Is the coil part of the MOT inspection?
Not directly, but a defective coil can lead to MOT rejection due to:
- Excessive emissions (incomplete combustion)
- Irregular engine running
- Engine light that stays on
Solve coil problems for you APK inspection plant.
Do I also need to replace spark plugs when coil is running?
Highly recommended. Bad spark plugs put extra strain on the coil and can quickly damage new ones. Bad spark plugs can also cause coil-like symptoms. At FAAY, we always check spark plugs when replacing coils and recommend replacement if necessary (adding €40-€120 to the cost).
Is your engine running irregularly or is the engine light on? Have your car checked immediately at FAAY Auto Groep. We diagnose your problem within 30 minutes and replace faulty ignition coils starting from €110 with a 2-year warranty. Call 085-0606274 or schedule an appointment online.
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