A lawnmower condenser is a small capacitor that stores a high-voltage charge. When the magneto points close, the condenser releases its charge through the plug lead to the spark plug. The spark plug then ignites the compressed fuel/air mixture in the combustion chamber. The expanding gases force the piston downward, rotating the crankshaft and transmitting power to the driveshaft. If your mower stops working from an apparent ignition problem, the condenser may have failed. Fortunately, after removing the flywheel, testing this component with a multimeter is fairly straightforward.
Pull the rubber plug lead boot off the spark plug and remove the plug with a long socket wrench. Remove the cover from the engine by undoing the attaching screws with a Phillips screwdriver or nut driver. Insert a length of nylon starter rope into the plug hole until you can't push any more in, then rotate the flywheel counterclockwise until the engine stops turning.
Remove the flywheel nut by turning it counterclockwise off its left-hand thread with a socket wrench. Lift the large washer off the shaft.
Push the tips of two large flat-head screwdrivers between the opposing sides of the flywheel and the crankcase. Tap the screwdriver handles with a hammer to firmly wedge the screwdrivers in place; do not risk cracking the flywheel by using excessive force.
Place a copper drift tool on the end of the flywheel shaft and deliver one or two sharp blows to the end of the drift with a heavy hammer; the flywheel will pop right off the tapered shaft. Remove the flywheel to expose the magneto and the small tubular condenser wired to the magneto points.
Open the points with a pair of needle-nose pliers. Slide a thin strip of plastic between the points to insulate the condenser. Allow the points to close.
Adjust a multimeter to the ohms setting. Touch one of the multimeter probes to the metal side of the condenser and the other probe to the terminal connecting the condenser to the points. The multimeter should read several megohms, drop to zero and then start rising until the reading returns to several megohms. Reverse the probes and you should get the same results.
Replace the burned-out condenser if the multimeter either produces a steady low reading or no reading at all, indicating a shorted-out condenser.
Reassemble the mower engine by reversing the procedures carried out in Steps 1 through 4.