Car systems
Engine ignition: how it works and what fails
On a gasoline engine, the ignition fires a spark at each spark plug at exactly the right moment to burn the mixture; a diesel has no spark, and its glow plugs only help it start when cold. When a cylinder does not fire properly, the engine shakes and loses power, and if the check engine light flashes, the catalytic converter can be damaged.
Reviewed by Daniel Morales, founder of Reprorace SL, on
How it works
The engine computer works out the timing from the crankshaft, camshaft and knock sensor signals and tells each coil when to charge and discharge. The coil raises the battery's 12 V to tens of thousands of volts and the spark plug fires the spark across its electrodes.
To detect misfires, the computer measures small changes in the crankshaft's speed: a cylinder that does not burn slows the rotation for an instant. That is how it knows which cylinder is misfiring and records it (P0301 for cylinder 1, P0302 for cylinder 2…). On diesels, the glow plug controller heats the glow plugs before and after starting and reports any that has burned out.
The most common faults
- Worn spark plugs: with use, the electrode gap grows and the spark weakens; they are replaced on the maintenance schedule.
- Ignition coils: a failing coil leaves a cylinder without a spark: jerking, a rough idle and the check engine light, sometimes flashing.
- Wires and connectors: moisture or oil in the spark plug well cause shorts and faults that come and go.
- Diesel glow plugs: a burned-out one is barely noticeable when warm, but in the cold the engine starts badly and puffs white smoke.
- Knock sensor: when it fails, the computer retards the timing to be safe and the engine delivers less power.
A guide, most likely first: a repair shop confirms the diagnosis of each car with its own data.
Symptoms that can come from this system
Flashing engine warning light: what it means and what to do
Stop as soon as it is safe
Car jerks when accelerating: causes and what to do
See a garage soon
Car slow to start: causes and what to do
Check without hurry
Glow plug light on or flashing: what to do
Check without hurry
Rough idle: why the engine shakes or hunts when stationary
Check without hurry
Car vibrates when accelerating: what it could be
See a garage soon
The most relevant trouble codes
- P0300Random or multiple cylinder misfire detected
- P0301Cylinder 1: misfire detected
- P0302Cylinder 2: misfire detected
- P0303Cylinder 3: misfire detected
- P0304Cylinder 4: misfire detected
- P0316Misfire detected at start-up (first 1000 revolutions)
- P0351Ignition coil A, primary control circuit open
- P0352Ignition coil B, primary control circuit open
- P0325Knock/combustion vibration sensor A, circuit: malfunction
- P0327Knock/combustion vibration sensor A, circuit: signal low
- P0380Glow plug/heater, circuit A: malfunction
- P0381Glow plug/heater indicator, control circuit open
- P0670Glow plug control module 1, control circuit open
- P0671Cylinder 1 glow plug, circuit open
- P0683Communication circuit between glow plug control module 1 and PCM: malfunction
Generic OBD-II/EOBD codes; each manufacturer adds its own. The same symptom can come from another system.
Code blocks of this system
Questions and articles about this system
- What are engine misfires (P0300)?
- What is a catalytic converter and why does it fail?
- What does a pending, stored or permanent code mean?
Learn more
See what your car is saying, system by system
The ECO-LITE plugs into the OBD port and shows the codes and live data your car publishes on your phone, explained; when needed, you share the session with your repair shop.
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Frequently asked questions
Can I drive with the check engine light flashing?
It is wiser not to: the flashing means misfires that send unburned fuel to the catalytic converter and can overheat it. Ease off, stop when it is safe and have the car checked.
How often should spark plugs be replaced?
It depends on the engine and the type of plug: the maintenance schedule says, usually between 30,000 and 100,000 miles. Diesels have no spark plugs.
What does an OBD reader say about misfires?
Codes P0300 to P0312 tell which cylinder is misfiring, and the freeze frame stores how the engine was running at that moment. Some cars also show a misfire counter for each cylinder.