Can I Drive With Faulty Coil Pack? (Mechanic says…)


Just thirty minutes from our holiday location, car packed with kids, bags, toys you name it. The car suddenly started to shudder under acceleration. I’m a mechanic, I had a good idea what the problem was.

It is possible to drive with a faulty Coil On Plug (COP), but not advisable. Driving with a faulty waste spark ignition system won’t be possible. Driving with a faulty coil pack can damage other components of the engine.

In this post you’ll learn why diving with a faulty coil pack is likely not possible, and why you really shouldn’t. You’ll also learn how to diagnose and replace your faulty coil.

Coil Overview

Coil pack terminology often gets mixed up and as coils all do the same job, it’s understandable. All systems today use a distributor-less system (DIS). The DIS system doesn’t use a distributor cap, rotor, and in most cases plug wires to fire the coils.

Instead, it uses a crankshaft and camshaft sensor to locate and time the engine, and the PCM (Power-train Control Module) manages the process. This is a ton more reliable than the old mechanical system.

Two types of coils are common on cars today, the individual coil per cylinder known as Coil on Plug (COP) and the older (and becoming a little rarer) coil pack.

Coil On Plug (COP)

Plug Coil

The COP is by far the most common type. It’s referred to as the COP because it lives directly over or on the spark plug. Unlike the coil pack, the COP is located and each cylinder/plug has its own coil. The great advantage of this is, of course, only a single coil needs to be replaced when faulty.

Check out the Amazon coil link below for price and delivery to your door.

Amazon Auto Ignition Coil

Driving With A Faulty Coil

It is possible to drive with a single faulty COP, I drove a couple of hundred miles with a faulty one once and while it was a pain in the ass, no harm was caused to the car. On a recent family holiday, I had another failure, this time I was prepared. Ten minutes in a gas station and we were back on the road, took longer to find the spare coil in the trunk than it did to fit it.

That said, it is possible to damage your engine so unless it’s an emergency, I wouldn’t put undue stress on the engine. Running the engine on a cylinder less will mean the car is underpowered and off-balance.

Note also that most cars will register a faulty coil and immediately stop gas feed to that cylinder. However, damage may be caused if the fault isn’t registered by the PCM and the dead cylinder continues to receive gas. Your catalytic converter and oxygen sensors may become contaminated by raw gas and need to be replaced.

Raw gas will dilute the motor oil too and put major mechanical components at risk. Any misfire should be repaired ASAP.

Wasted Spark Coil Pack

The coil pack comes in a few different shapes, but all were bulky bits of kit. Early types were bolted to the engine with short regular plug wires to each cylinder. Others were assembled in a row which pushed down onto the spark plugs as one unit (without the need for plug wires).

Coil packs commonly used wasted technology which was first introduced in the mid-’80s. Wasted spark simply means it uses only two coils to fire a four-cylinder engine. A six-cylinder will have a coil pack containing three individual coils.

If your car is fitted with a coil pack that employs wasted spark technology, you likely won’t be able to drive it. A faulty coil in a wasted coil pack will kill spark to two cylinders, a four-cylinder car won’t run on two cylinders, a six possibly might, and an eight cylinder should.

Components Of DIS System

The ignition system consists of the following components:

  • Crankshaft sensor – Identifies crank movement and TDC (Top Dead Centre) and sends info to PCM
  • Camshaft sensor – Identifies engine cycle and sends info to the PCM
  • Knock sensors Identify detonation and the PCM uses this info to adjust timing
  • MAP (Mass Absolute Pressure sensor) – Measures the load on the engine by tracking engine vacuum. The info is used to prepare the engine for acel and decel.
  • Coils – Produces the voltage for spark. COP coil typically three wire – power, ground and signal wire. The PCM identifies the correct time to fire the plug and the PCM signals the coil to remove the ground. This forces the coil to dump the voltage through the spark plug, causing a spark in the process.
  • PCM – Manages the timing of spark, duty cycle of injectors and many other important functions including self diagnoses. Love PCM’s!

Diagnoses Of Faulty Pack

OBD 2 fault code reader

A misfiring engine is usually easy to identify, you’ll have a check engine light on, and the car won’t feel or sound just right – Rough idle, stalling at idle, Vibration on accel, Hard on gas, Sluggish.

A scan tool is the best place to start diagnosing. Reading the fault codes will likely show one of the following:

  • P0301 – Cylinder 1 misfire
  • P0302 – cylinder 2 misfire
  • P0303 – cylinder 3 misfire
  • P0304 – Cylinder 4 misfire
  • P0305 – Cylinder 5 misfire
  • p0306 – Cylinder 6 misfire
  • P0307 – Cylinder 7 misfire
  • P0308 – Cylinder 8 misfire
  • P0300 – Random cylinder misfire

You may also have a faulty coil code such as P0358 or P0351 but sometimes you may not. If the coil is intermittent it may only set a misfire code. Check out the fault code readers I use here on the Mechanics repair tools page.

A failed coil is the most likely cause of the misfire but it’s not the only possible cause. Other common causes include – fouled or faulty spark plug, fuel injector, vacuum leak or a compression issue may also cause a misfire condition.

There are a ton more reasons an engine misfires, you can check them out here – “Why is my car shaking”.

How To Check Coil

All these tests apply to the more common COP, only some of these tests will apply to the Wasted Coil Pack. A DVOM (voltmeter) and DTC code reader are required.

You can find these tools on both the Mechanics tools page and the Auto electrical repair tools page.

The tests are basic. To diagnose or confirm a coil pack fault code:

  1. Clear the codes using code reader
  2. Move the suspect coil to another cylinder
  3. Run the engine
  4. Read fault codes again
  • If the fault code follows the coil, go ahead and replace the coil.
  • If the fault stay’s with the cylinder, repeat the process with the spark plug.
  • If the fault persists with the cylinder, check resistance across the injector coil using the DVOM.
  • If the injector test between 12 – 16 ohm’s, run a compression test.
  • Compression test OK, check for a vacuum leak, spray WD40 around known vacuum weak spots and listen – the engine note changes.
Resistance testing injector
Resistance check on injector 12 – 16 ohm’s – Okay
Compression test infographic

Replacing A Faulty Coil Pack

Coil over plug wiring connector

Replacing a coil is a straightforward procedure, it’s plug-and-play. Removing the engine cover reveals the coils. Some large engines may require more components to be removed. The coils are secured to the cam cover with one or two bolts. Removing the plug connector and bolts releases the coil.

Coil fitting
Oil leak from a cam cover. into the plug well, may also cause a misfire fault.

When fitting a new coil put a small amount of di-electric grease to the rubber tip, helps installation, prevents moisture and corrosion.

Check out all the tools I used in this repair including workshop manuals and wiring diagrams, right here on the Mechanics tools page.

Related Questions

What does a misfire feel like? You may experience an engine misfire as roughness in the engine, and off-beat sound from the engine, the interior such as seats and headrests may vibrate especially under acceleration.

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of classic car ownership, from tires to roof aerials and everything in between.

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