How To Test a 3 or 4-wire COP Ignition Coil

The Right Way to Diagnose a 3 or 4-Wire Ignition Coil

When troubleshooting ignition issues, knowing how to test a COP ignition coil (Coil-On-Plug) correctly is critical. I’ve seen plenty of DIYers, and even some pros make the mistake of assuming a coil is bad when it’s actually something else—like a wiring issue or a weak ground. That’s why I’m going to walk you through exactly how to test a 3 or 4-wire ignition coil, what the wires do, and how to avoid misdiagnosing ignition problems.

Understanding 3 and 4-Wire Ignition Coils

Most COP ignition coils have either three or four wires. The thick wires are easy to understand—they’re for power and ground. The real confusion starts with the smaller wires.

Why do some coils have two grounds? Why do some have an ignition feedback circuit? These are the questions you need answers to before you can accurately test a 3 or 4-wire ignition coil.

Here’s the key difference:

1) Three-wire COP Ignition Coils—  Power and Ground, and the third wire tells the coil when to fire.

2) Four-wire COP Ignition Coils— Power and Ground. A second ground wire for cleaner signal processing or an Ignition Confirmation Signal (IGF) that reports back to the engine control module (ECM) that the coil has actually fired. The ECM uses this signal to provide a fail-safe fuel cut-off if a coil has failed and isn’t firing the spark plug to protect the catalytic converter.

Three-wire coils are generally simpler and more universal across manufacturers. But when it comes to four-wire coils, each manufacturer has its own way of handling that extra wire.

The first variation (Toyota and Mercedes-Benz) uses three of the four wires in the connector for power (12V), ground, and an ECM-controlled trigger (very similar to the three-wire COP), but the addition of a fourth wire provides feedback to the ECM that the coil has actually done what the ECM asked it to do.

The Toyota version provides a 5V bias signal on the IGF circuit that runs parallel to all the ignition coils. Internal electronics in each COP monitor the coil operation and pulse the IGF to ground when the coil fires successfully. The PCM will recognize a lack of pulse and set a trouble code for the coil that failed to fire.

3 and 4-wire COP ignition coils

How an Ignition Coil Works (And Why It Fails)

Before we get into testing, let’s quickly cover how an ignition coil actually works. No matter if it’s a COP ignition coil with two, three, or four wires, the process is always the same:

Power flows to the coil’s primary winding.

The control module (PCM/ECU) switches the circuit off, collapsing the magnetic field.
This collapse induces a high-voltage spark in the secondary winding, which fires the spark plug. The process repeats rapidly to keep the engine running smoothly.

The ECM adjusts ignition timing based on multiple factors like:

• Crankshaft and camshaft position sensors (tell the PCM when to fire the spark)
• Intake air temperature
• Coolant temperature
• Atmospheric pressure and barometric pressure (BARO) sensor
• Knock sensor feedback (to prevent detonation)

If any of these inputs are off, it can throw off ignition timing and cause misfires—even if your coil is perfectly fine. That’s why it’s crucial to test a COP ignition coil correctly before replacing it.

How to Test a 3 or 4-Wire Ignition Coil the Right Way

1. Don’t Just Pull a Plug and Check for Spark— A common mistake is people pulling out a spark plug, grounding it, and checking for spark. This method is unreliable because it only takes about 2,000 to 4,000 volts (2-4 kV) to jump the gap outside the cylinder. Inside the engine, where compression is much higher, it takes 25,000 to 40,000 volts (25-40 kV).

A weak coil might look like it’s firing when tested outside the cylinder, but under real conditions, it could be failing.
2. Use a Proper Spark Tester— Instead, use a professional spark tester like the ST-125, which forces the coil to generate 25,000 to 28,000 volts (25-28 kV) to jump the test gap. If you want even more accuracy, use an expandable spark tester that lets you increase the test gap to 35,000 or 40,000 volts.

COP Coil tester

3. Check Power and Ground First— Before you assume the coil is bad, check for power and ground using a multimeter. Here’s how:

• Set your multimeter to DC voltage.
• Probe the power wire (usually the thickest wire) while the key is in the ON position. You should see battery voltage (12V+).
• Probe the ground wire to confirm it’s properly grounded. A bad ground can mimic a failed coil.

4. Scope the Trigger Wire— On a 3-wire ignition coil, the third wire is the trigger signal from the ECM. You can test this using an oscilloscope or a test light:

• With an oscilloscope, you should see a sharp voltage drop when the coil is triggered.
• With a test light, probe the trigger wire while cranking the engine. The light should blink, indicating the coil is receiving a trigger signal.

5. Testing a 4-Wire Ignition Coil— For 4-wire coils, you need to determine whether the fourth wire is a second ground or an ignition feedback (IGF) circuit.

• If it’s a second ground, test it like any other ground wire—using a multimeter to check for continuity.
• If it’s an IGF wire, it should pulse when the engine is running. If it doesn’t, the ECM may shut down the injectors and prevent the engine from running.

For example, Toyota’s IGF circuit sends a confirmation signal back to the ECM to let it know the coil fired. If the ECM doesn’t receive this signal, it may cut off fuel delivery, causing a no-start or stall condition.

©, 2025 Rick Muscoplat