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. Check 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. Check for wires that are too close to the following devices:
   • Secondary ignition components.
   • Motors.
   • The generator.
2. Determine whether 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. Check for an open diode across the A/C compressor clutch and for other open diodes. Some relays may contain a clamping diode.
5. Check 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 tests in this section do not reveal a problem, see FREEZE FRAME/FAILURE RECORDS DATA feature using the scan tool. This information can be reviewed and compared to the typical Scan Tool Data List values or to data values taken from a similarly equipped known-good vehicle. See appropriate PID VALUE CHARTS in PID VALUE CHARTS - VIBE article.