What they do
Spark plugs make the spark that lights the fuel in each cylinder. Ignition coils turn 12 volts into the tens of thousands of volts the plugs need. Most newer engines have one coil for each cylinder.
Common symptoms
- Misfiring: the engine shakes, stumbles or hesitates, especially under load
- Rough idle
- Check engine light, flashing during a bad misfire
- Hard starting, especially when it's cold
- Loss of power and worse gas mileage
- Backfiring or stalling
What to look for
- Read the codes: P0300 means a random misfire. P0301 through P0308 point to a specific cylinder, where the last number is the cylinder.
- Swap test: move the suspect coil to a different cylinder, clear the codes and drive. If the misfire follows the coil, the coil is bad. If it stays put, look at that cylinder's plug, injector or compression.
- Look at the plug: a worn electrode, oil or heavy carbon on the tip, or cracked porcelain means it's time for new plugs.
- Look at the coil: cracks in the boot, burn marks, or oil in the spark plug well.
Could be something else
Misfires can also come from fuel injectors, vacuum leaks or low compression.
Is it safe to drive?
A misfire wastes gas and can damage your catalytic converter. If the check engine light is flashing, ease off and get it checked soon. When a coil fails, replace that cylinder's spark plug too.
Related: Check engine light: solid vs. flashing · Bad fuel pump symptoms
Think it's your coils or plugs? Northwoods can check it right where your vehicle sits, anywhere in the greater Twin Ports area.