Ignition System

4 Lessons

From spark plugs to coil-on-plug — master ignition diagnosis and repair.

Overview

The ignition system creates the spark that ignites the air-fuel mixture. This module covers ignition coils, spark plugs, triggering mechanisms, ignition timing control, and how the PCM determines the optimal spark event for every combustion cycle.

Lessons

LESSON 01

Ignition System — How Your Engine Makes Fire

The ignition system has one job — deliver a spark to the right cylinder at exactly the right instant. That spark has to be hot enough and last long enough to reliably ignite a compressed air-fuel mixture under all operating conditions. Cold starts, wide open throttle, idle, high altitude — the ignition system has to perform every time. Miss once and you feel it as a misfire. Miss repeatedly and you damage the catalytic converter.

The basic ignition chain

Battery provides 12 volts. The ignition coil transforms that 12 volts into 20,000 to 100,000 volts. That high voltage travels to the spark plug. At the spark plug tip, the voltage jumps across a small air gap between the center electrode and the ground electrode. That arc is the spark. The spark ignites the compressed air-fuel mixture and the power stroke begins. Every component in this chain must work correctly or that cylinder does not fire.

Coil-on-plug — the modern standard

Most modern engines use coil-on-plug — COP — an individual ignition coil mounted directly on each spark plug. This eliminates high-voltage spark plug wires and the old distributor. Each coil fires only its own plug on command from the PCM. This gives the PCM individual control of ignition timing and spark energy on each cylinder. If one coil fails, only that cylinder misfires. The others keep firing normally.

Ignition timing

The PCM decides exactly when to fire each spark plug based on inputs from the crankshaft position sensor, camshaft position sensor, engine load, RPM, coolant temperature, knock sensor, and other inputs. At low RPM and light load, the spark fires earlier in the compression stroke — called advanced timing — to give the flame front more time to burn the mixture. At high RPM, the spark must fire even earlier because the piston is moving faster. Under heavy load and high cylinder pressure, the PCM retards timing to prevent detonation — also called knock. The knock sensor listens for knock and the PCM pulls timing back if it hears it.

Diagnosing a misfire — the swap test

The misfire monitor in the PCM identifies which cylinder is misfiring and stores a specific code — P0301 for cylinder 1, P0302 for cylinder 2, and so on. When you have a cylinder-specific misfire code, swap the coil from that cylinder with the coil from an adjacent cylinder. Clear codes and retest. If the misfire code moves to the new cylinder — the coil is the fault. Replace it. If the misfire code stays on the original cylinder — the coil is not the problem. Now swap the spark plug the same way. If it still stays — check injector, then compression. Always swap before you spend.

Crankshaft position sensor

Without a crankshaft position sensor signal, the PCM cannot determine where the pistons are in their cycle — no spark, no fuel injection, no start. A failing crankshaft position sensor often fails intermittently — drops the signal when hot, restores when it cools. The vehicle cranks normally but will not fire, then starts normally an hour later after sitting. Always test the crank sensor signal waveform during a confirmed no-start event before condemning any other component.

LESSON 02

Spark Plugs — How They Work

The spark plug is the component that actually ignites the air-fuel mixture inside the combustion chamber. It threads into the cylinder head with its electrode tip extending into the chamber. When the PCM commands the ignition coil to fire, a high-voltage surge — 20,000 to 100,000 volts — jumps across the small gap between the center electrode and the ground electrode at the tip of the plug. That spark ignites the compressed mixture and the power stroke begins.

Reading spark plugs

A spark plug that has been in the engine tells a story about what is happening in that cylinder. Light tan or gray deposits on the tip — normal combustion, everything is working correctly. Black sooty deposits — rich mixture. That cylinder is getting too much fuel or not enough air. Wet oily deposits — oil is getting into the combustion chamber. Worn rings, worn valve seals, or a failed PCV system. White blistered electrodes — the plug is running too hot. Lean condition, incorrect plug heat range, or ignition timing too advanced. Learning to read spark plugs is learning to read the engine.

Spark plug replacement

Replace spark plugs at the manufacturer's recommended interval. Copper plugs typically last 30,000 miles. Platinum plugs last 60,000 to 100,000 miles. Iridium plugs can last 100,000 miles or more. Always install the exact plug type and heat range the manufacturer specifies. The wrong heat range causes either fouling from running too cold or pre-ignition damage from running too hot. When installing plugs, apply anti-seize to the threads — especially in aluminum heads — and torque to the manufacturer's specification. Overtightening strips the threads in the head. Undertightening allows combustion gases to leak past the plug seat.

LESSON 03

Ignition Coils and Coil-on-Plug

The ignition coil transforms the 12-volt battery voltage into the 20,000 to 100,000 volts needed to jump the spark plug gap. It does this through electromagnetic induction — the same principle that makes a transformer work. A primary winding of a few hundred turns of wire is wound around a core. A secondary winding of thousands of turns wraps around the primary. When the PCM interrupts current flow through the primary winding, the magnetic field collapses and induces a massive voltage spike in the secondary winding. That spike fires the spark plug.

Coil-on-plug — COP

Most modern engines mount an individual ignition coil directly on top of each spark plug. No spark plug wires. No distributor. Each coil fires only its own spark plug on command from the PCM. This gives the PCM individual control of ignition timing and energy for each cylinder. When a COP coil fails, only that cylinder misfires. The misfire code identifies the cylinder and the swap test confirms whether the coil is the fault.

Coil failure symptoms

A single-cylinder misfire at all engine speeds — the coil has failed completely on that cylinder. A misfire only under load or at high RPM — the coil is breaking down under demand. It may have a cracked housing that allows the spark to arc to ground instead of jumping the plug gap. Intermittent misfires that come and go — the coil may have an internal connection that opens when hot and reconnects when cool. Heat-related coil failures are extremely common. If one coil fails on a high-mileage engine, the others are likely not far behind.

LESSON 04

Crankshaft and Camshaft Position Sensors

These two sensors work together to tell the PCM exactly where every piston and valve is at every instant. Without this information, the PCM cannot fire the spark plugs at the right time or inject fuel into the right cylinder at the right moment. These sensors are the heartbeat of the engine management system.

Crankshaft position sensor — CKP

The crank sensor reads a toothed reluctor ring on the crankshaft or harmonic balancer. As each tooth passes the sensor, it generates a signal pulse. The PCM counts the pulses to determine crankshaft speed and position. A missing tooth or specially shaped tooth on the ring tells the PCM where top dead center is for cylinder number one. Without a crank sensor signal — no spark, no fuel injection, no start. The engine cranks normally but will not fire at all.

Camshaft position sensor — CMP

The cam sensor reads a reluctor ring or target on the camshaft. It tells the PCM which stroke each cylinder is on — because the crankshaft rotates twice per four-stroke cycle, the PCM needs the cam signal to distinguish between the compression stroke and the exhaust stroke on each cylinder. Without the cam signal, some engines default to batch fire mode — firing all injectors and coils in a less efficient pattern. Others will not start at all without the cam signal.

Intermittent failure pattern

Both crank and cam sensors are notorious for heat-related intermittent failure. The sensor works fine when cold. The engine heats up. The sensor fails — engine dies. The engine sits for an hour, cools down, the sensor recovers, and the engine starts again. The customer says the car randomly stalls and then starts fine later. If you cannot duplicate the fault, monitor the sensor signal with a scope while heating the sensor area with a heat gun to simulate operating temperature. The signal drops out — sensor confirmed bad.

Key Components

Ignition coils (COP and coil pack)
Spark plugs and gaps
Crankshaft and camshaft position sensors
Ignition control module/PCM
Knock sensor

How It Works

The PCM uses crankshaft and camshaft position sensor signals to determine engine position and speed. It calculates the optimal ignition timing based on load, RPM, temperature, and knock sensor input, then triggers the ignition coil to fire the spark plug at precisely the right moment.

Common Problems

Worn spark plugs causing misfire
Ignition coil failure (often heat-related)
Carbon tracking on coil boots
Crankshaft position sensor failure (no start)
Knock sensor false readings from carbon buildup

Diagnostic Tips

Swap coils between cylinders to confirm coil vs. cylinder issue
Check spark plug condition — it tells a story
Scope the crank and cam signals for pattern issues
Misfire counters in Mode $06 show which cylinders are struggling

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Ignition System

Overview

Lessons

Key Components

How It Works

Common Problems

Diagnostic Tips

Want to Dig Deeper?

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