How To Test The Toyota 1.8L COP Coils.

In this tutorial, I'm gonna' show you just how easy it is to test the ignition coils on your 1.8L Toyota and find out if one of them is bad or not. These ignition coils are known technically as Coil-on-Plug (COP) ignition coils.

If you suspect that your Toyota has a bad COP ignition coil that's causing a misfire condition (and setting a misfire code that is lighting the check engine light (CEL)), then this is the tutorial you need.

Symptoms Of A Bad COP Ignition Coil

Each cylinder of your Toyota's 1.8L engine needs three things to produce power. These are fuel, air (compression), and spark.

The ignition coils, that sit on top of the valve cover, are the ones that supply the spark and when one or several fail... those cylinders will go ‘dead’. When a cylinder goes ‘dead’, the PCM sets a misfire diagnostic trouble code (DTC).

Your Toyota will also show one or more of the following symptoms:

  1. Diagnostic Trouble Codes lighting up the check engine light (CEL):
    1. P0300: Random Cylinder Misfire.
    2. P0301: Cylinder #1 Misfire.
    3. P0302: Cylinder #2 Misfire.
    4. P0303: Cylinder #3 Misfire.
    5. P0304: Cylinder #4 Misfire.
  2. Rough idle.
  3. Smell of raw gasoline coming out of the tailpipe.
  4. Bad gas mileage.
  5. Won't pass the emissions test.
  6. Extended cranking time (hard start).
  7. If more than one ignition coil is bad, your Toyota won't start.

If your Toyota is experiencing any of the above symptoms, chances are that you do have a bad COP coil on your hands. Let's take a look at some of the basic tools you'll need to diagnose and repair this yourself.

What Tools Do I Need To Test The COP Coils?

The most important tool that you're gonna' need, to test the ignition coils for spark, is a spark tester.

I'm gonna' recommend one that is the most effective (and the most inexpensive) out there: the HEI spark tester (KD Tools 2756).

From personal experience (I work full-time as an automotive tech)... the HEI spark tester is a must have tool. You don't need to interpret the color of the spark or the weakness of it. With the HEI spark tester... if it sparks... the ignition coil is good.

  1. An HEI spark tester.
  2. Scan tool.
    1. A scan tool is a great tool to have, but you won't need one to test the ignition coils on your Toyota.
    2. Don't have one?... check out my recommendation: Abe's Scan Tool Recommendation.
  3. Wire piercing probe.
    1. Although this tool is not an absolute must, if you do buy one, you'll realize just how easy it makes testing the voltages inside the wires.
    2. If you need to see what this tool looks like, you can see it here: Wire Piercing Probe.

IMPORTANT: Testing the ignition coil by pulling the ignition coil off of the spark plug while the car is running (to listen for the audible clicking sound the spark makes) is not a good idea. Why? Because you run the risk of damaging an otherwise good ignition coil. Also, this way of checking the COP ignition coil for spark doesn't produce an accurate test result. This is why a spark tester becomes a must have tool.

How Does The COP Coil Work?

Although it's not absolutely necessary to know how the coil on Plug COP ignition coils work on your Toyota vehicle... I've included a brief description of how they work (for those of you who love to read the how and why of stuff).

In a nutshell, the COP ignition coil's job is to create and deliver spark to the spark plug.

Having an individual ignition coil per cylinder allows for a more maintenance free ignition system... since you have less moving parts that may wear out and that eventually need to be replaced... like:

  1. A mechanical distributor assembly.
  2. Distributor cap.
  3. Distributor rotor.
  4. Spark plug wires.

When you turn the key and crank the engine:


  1. Power and ground are supplied to 2 of the 3 wires sticking out of the COP ignition coil's electrical connector.
  2. The PCM (Powertrain Control Module = Fuel Injection Computer) gets the crankshaft position signal from the crank sensor.
  3. The PCM now activates each COP ignition coil.
    1. This activation signal (Triggering Signal) is supplied to the third remaining wire of the COP ignition coil's electrical connector.
    2. This Triggering Signal activates an ignition module (also known as the power transistor) within the ignition coil
    3. The ignition module, in turn, starts to switch the ignition coil 12 Volts On and Off (by interrupting the coil's ground circuit). Remember, all this is happening inside the ignition coil.
    4. It's the switching On and Off of the primary current (the fancy name for 12 Volts) that causes the COP coil to spark.
    5. The actual spark event is measured, by the ignition module, and corresponding signal is sent to the PCM (on the fourth wire of the coil's connector).
  4. With all 4 signals supplied or sent (power, ground, and the triggering signal, spark event signal), the ignition coil now starts to spark.
  5. With fuel being injected, the engine in your Toyota comes to life.

OK, with this brief overview of how the COP ignition coil works on your 1.8L equipped Toyota, let's turn the page and get testing...