Electromagnetic Interference & Electrical Noise

Some electrical components or circuits are sensitive to Electromagnetic Interference (EMI) or other types of electrical noise. Perform the following procedures:

1. Inspect for a misrouted harness that is too close to a high voltage or high current device. This condition may induce electrical noise on a circuit that could interfere with normal circuit operation. Inspect for wires that are too close to the following devices:
   • Secondary ignition components.
   • Motors.
   • The generator.
2. Determine whether the electrical system interference is caused by a malfunctioning relay, PCM driven solenoid or switch. These components may cause a sharp electrical surge. Normally, the problem will occur when the malfunctioning component is operating.
3. Determine whether non-factory or aftermarket add-on accessories are installed in the vehicle. These accessories may lead to an emission related OBD-II failure. Determine if any of the following non-factory or aftermarket add-on accessories is causing the intermittent:
   • Lights.
   • 2-way radios.
   • Amplifiers.
   • Electric motors.
   • Remote starters.
   • Alarm systems.
   • Cell phones.
4. Test for an open diode across the A/C compressor clutch and for other open diodes. Some relays may contain a clamping diode.
5. Test for proper performance of the generator. See ON-VEHICLE TESTING in appropriate GENERATORS & REGULATORS article in STARTING & CHARGING SYSTEMS.
6. If a DTC is determined to be intermittent, and the tests in this section do not reveal a problem, see VEHICLE DATA RECORDER  .