Engine Misfire Guide (P0300-P0306): Is It Spark Plugs or Ignition Coils?

Few things are scarier than a flashing Check Engine Light. Your car starts shaking at the stoplight, acceleration feels sluggish, and it sounds like a tractor.
You scan the code and get P0300 (Random Misfire) or P0301-P0306 (Cylinder Specific Misfire).

The mechanic says it's an "Ignition Issue." But does that mean you need new Spark Plugs (

$10fix)ornew\ast \ast IgnitionCoils\ast \ast ($100+ fix)?

At Automotive-leader, we want you to fix it right the first time. Here is how to diagnose the root cause and stop the shaking.

1. The Symptoms: Coil vs. Plug

While both cause misfires, there are subtle differences:

• Bad Spark Plugs: usually fail gradually. You might notice slightly worse MPG or a little hesitation over months.

• Bad Ignition Coils: often fail suddenly. One day the car runs fine; the next day it's shaking violently.

  • Hot Tip: If the misfire gets worse as the engine warms up (heat soak), it is highly likely a failing Ignition Coil (internal resistance increases with heat).

For a deeper list of signs, read our symptom guide:
👉 [Read More: Ignition Coil Failure Symptoms: A Comprehensive Diagnostic Guide] 

2. The Easy DIY Test: "The Swap Method"

You don't need expensive tools to diagnose this.
Let's say your scanner shows code P0302 (Misfire Cylinder 2).

1. Locate: Find the ignition coil on Cylinder 2 (usually labeled on the engine cover or count from the belt side).

2. Swap: Switch it with the coil from Cylinder 1.

3. Clear & Drive: Clear the codes and drive until the light comes back on.

4. Result:

   • If the code moves to P0301, the Coil is bad.

   • If the code stays at P0302, the Coil is fine (it's likely the Spark Plug or Injector).

(Note: Always inspect the old spark plug for oil or cracks while you have the coil out!)

3. Should I Replace All 4 Coils?

If one coil fails around 80,000-100,000 miles, the others are likely close to death.
Mechanics always recommend replacing them as a full set.

• Pro: Restores balanced engine power and MPG.

• Con: Costs more upfront.

However, buying a Set of 4 from Automotive-leader is often cheaper than buying just one from the dealership. Plus, you save labor by doing it all at once.

4. Common Problems by Car Brand

Different cars have different "weak spots" for coils.

• Honda (Fit / Civic): The Hitachi/Denso coils often fail due to internal short circuits. This is notorious on the Honda Fit/Jazz (GD/GE models).

  • Common Part: 30520-PWA-003

Own a Honda Fit or Jazz? The rear coils are a common failure point causing jerking issues. Read our specific guide here: [Honda Fit/Jazz Ignition Coil Problems]

• Toyota (Corolla / Camry): Generally reliable, but the rubber boots can crack over time, causing "arcing" (sparks jumping to the engine block).

• Volkswagen / Audi: Notorious for coil pack failures. Always keep a spare in the trunk!

5. Conclusion

Don't ignore a flashing engine light—unburnt fuel can melt your catalytic converter (a $1,000 mistake).
If you've done the "Swap Test" and confirmed a bad coil, don't overpay at the parts store.

Find Your Ignition Coils:
We stock high-voltage coils that meet or exceed OEM specs.

👉 [Shop All Ignition Coils]