Ignition Coil-On-Plug (COP) Test (GM 3.5L, 4.2L)

Ignition Coil-On-Plug (COP) Test (GM 3.5L, 4.2L)

Testing the ignition coils on your 3.5L 5 cylinder or 4.2L 6 cylinder is not that hard to do and I'll show you how to do it in this tutorial.

As you may already be aware, this type of ignition system is known as COP coil system. The acronym COP stands for: Coil On Plug, and refers to the fact that there's an ignition coil sitting on top of each spark plug.

In Spanish You can find this tutorial in Spanish here: Cómo Probar Las Bobinas De Encendido (3.5L GM) (at: autotecnico-online.com).

Symptoms Of A Bad COP Ignition Coil

The most obvious symptom, you'll see when a COP ignition coil goes bad, is a miss at idle or when you accelerate. This miss being called a misfire in modern tech lingo.

You're also gonna' see the check engine light (CEL) shining nice and bright on your instrument cluster.

You'll also see one or several of the following symptoms of a bad ignition coil (COP coil):

  1. Diagnostic Trouble Codes:
    • P0300: Random Cylinder Misfire.
    • P0301: Cylinder #1 Misfire.
    • P0302: Cylinder #2 Misfire.
    • P0303: Cylinder #3 Misfire.
    • P0304: Cylinder #4 Misfire.
    • P0305: Cylinder #5 Misfire.
    • P0306: Cylinder #6 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.

What Tools Do I Need To Test The COP Coils

The most important tool that you're gonna' need is a spark tester. I'm gonna' recommend one that is the most effective (and the most inexpensive) out there: the HEI spark tester (OTC 6589).

From personal experience (I work full-time as an automotive tech) I can tell you that 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 then you can conclude that the ignition coil is good.

  1. An HEI spark tester.
  2. Scan tool.
    • A scan tool is not needed to check the COP coils with the info I'm presenting in this tutorial but it does come in handy to retrieve the misfire DTC (Diagnostic Trouble Code).
    • Don't have one? Check out my recommendation: Abe's Scan Tool Recommendation.
  3. Wire piercing probe.
    • 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.
    • If you need to see what this tool looks like, you can see it here: Wire Piercing Probe.

What Does The COP Coil Do/Work?

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

Its design 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:

  • A mechanical distributor assembly.
  • Distributor cap.
  • Distributor rotor.
  • Spark plug wires.

Now, in case you're really curious about how it works then below is a very brief description of how it works:

  1. When you turn the key and crank the engine:
    • Power is supplied to circuit A.
    • Ground is supplied to circuit C.
  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.
    • This activation signal (Triggering Signal) is supplied to circuit B.
    • This Triggering Signal activates an ignition module (also known as the power transistor) within the ignition coil
    • The ignition module, in turn, starts to switch the ignition coil 12 Volts ON and OFF (by interrupting the coil's Ground circuit).
    • Its the switching ON and OFF of the primary current (the fancy name for 12 Volts) that causes the COP coil to spark.
  4. With all 3 signals supplied, the ignition coil now starts to spark.

Where To Buy The COP Ignition Coil

You could run down to your local auto parts store and shell out about $75.00 (US) for a Chinese knock-off ignition coil (you know which Auto Parts Store I'm talking about) or you could buy the Delphi COP coil which is close to original equipment as you can get!

The Standard brand has a long tradition of quality and durability. Whether you choose the Standard brand or Delphi brand, you can't go too wrong.

You can find out more by clicking on the links on the boxes on the right (of the COP coils).

Circuit Descriptions Of The COP Coils

Ignition Coil-On-Plug (COP) Test (GM 3.5L, 4.2L)

Each COP ignition coil on your engine has 3 wires going to it (or coming out of it).

Each wire, independent of its specific color, can be identified with a letter.

These letters are A, B, and C.

Each one has a specific job to do. Below are their job descriptions.

IMPORTANT: The color of the middle wire will be different for each COP ignition coil and this is normal. What will be the same, even if the color of the wire is different, is its job description.

#1 Ignition Coil Circuits
Pin Wire Color Description
A Pink 12 Volt Ignition Power
B Purple Triggering Signal from PCM
C Black Ground

Buick Vehicles:

  • Rainier 4.2L
    • 2004, 2005

Chevrolet Vehicles:

  • Colorado 3.5L
    • 2004, 2005, 2006
  • Trailblazer
    • 2002, 2003, 2004, 2005

GMC Vehicles:

  • Canyon 3.5L
    • 2004, 2005, 2006
  • Envoy
    • 2002, 2003, 2004, 2005

Hummer Vehicles:

  • H3
    • 2006

Isuzu Vehicles:

  • Ascender
    • 2004, 2005

Isuzu Vehicles:

  • I-280
    • 2006
  • I-290
    • 2007, 2008
  • I-350
    • 2006
  • I-370
    • 2007, 2008

Oldsmobile Vehicles:

  • Bravada
    • 2004