The ignition coil is the part that takes 12 volts from the magneto in the flywheel and turns it into the 20,000+ volt pulse that fires the spark plug. It's a small black or grey block sitting next to the flywheel, with the spark plug wire coming out of one end.

Coils fail for two main reasons. Heat (they cook themselves over the years) and vibration (internal windings break). When they fail, they often fail intermittently. Edger fires up cold, runs for 10 minutes, then dies and won't restart until it cools down. Or it fires up some days but not others. Frustrating to diagnose.

Quick test for spark. Pull the spark plug out, leave it connected to the plug wire. Hold the plug body against the metal cylinder fins (so the plug threads are touching metal). Pull the cord. You should see a clear blue spark across the plug gap. If there's no spark, or a weak orange spark, you've got an ignition problem.

Before condemning the coil, check:

• The kill switch. Faulty kill switches short the ignition to ground and prevent spark. Disconnect the kill switch wire from the coil and try again.
• The plug wire and boot. Cracks or deterioration can let the spark jump to ground instead of through the plug. Inspect carefully.
• The coil air gap. The coil sits a fraction of a millimetre from the flywheel. If it's drifted out, no spark. Set it to the thickness of a business card (about 0.3mm) using a feeler gauge or piece of card as a spacer.

If all that checks out and there's still no spark, the coil's done. New coils are $35 to $80 depending on the model. Fitting is two bolts and a wire. Set the air gap correctly when you fit the new one (engine cold, business card between coil and flywheel, tighten the bolts, remove the card).

One more thing. Don't bother with a multimeter test on a coil unless you've got a specialist tester. The ohms readings vary wildly between brands and don't reliably tell you whether the coil works under spark conditions. A spark test is the only reliable check.