Symptoms of a bad ignition module are sudden engine stalling or power loss while the car is in motion, engine stuttering accompanied by a smell of gasoline and an inability to restart the engine following a stall. Decreased gas mileage is also a sign of a compromised ignition module.
Ignition module failure can be caused when the unit overheats. To test for this, the car should be kept idle for 30 minutes, then the ignition module should be tapped with a screwdriver. If the car begins to stutter or stall following the tapping, the unit is overheated. Oxidized or dirty electrical connections can also cause failure. Cleaning and replacing corroded connections to the ignition module can rectify this.