When the check engine light comes on in your car, it means the onboard diagnostics system (OBD) has detected a problem. The check engine light can indicate issues with the engine, transmission, emissions control system, sensors or other systems in your car. Troubleshooting the electrical system is one way to diagnose the underlying problem. Here is a step-by-step guide to troubleshooting your car's electrical system when the check engine light turns on.
Gather Diagnostic Tools
Before diving into troubleshooting the electrical system, gather some basic diagnostic tools:
OBD code reader - This plugs into the OBD port under the dashboard to read trouble codes from the ECU. Knowing the codes points to the source of the problem.
Multimeter - An automotive multimeter allows you to check voltages and test electrical circuits and components. This is essential for electrical troubleshooting.
Service manual - Get the factory service manual for your specific car make and model. This details how all the electrical systems are wired and how to test them.
Socket set - You'll need various sockets to remove components and gain access for testing. Having a range of metric and SAE sockets is recommended.
Wires - Jumper wires allow you to bypass components for testing. Have several different gauge wires on hand.
Check for Trouble Codes
The first step is to plug in the OBD code reader and pull the diagnostic trouble codes (DTCs). This will point to the system(s) where the ECU is detecting issues.
Write down all the trouble codes so you have a reference. Then research what each code means for your vehicle by checking sites like AutoZone or searching online forums. This gives you an idea of which components could be malfunctioning.
Knowing the trouble codes sets the direction for your electrical troubleshooting.
Check Battery and Charging System
Electrical issues often stem from the battery or charging system, so this is one of the first places to check.
Follow these steps:
Check battery voltage - Use the multimeter to test battery voltage. It should be 12.4-12.6V with the engine off. If it's low, charge the battery and retest.
Check terminals and connections - Make sure the battery terminals are clean and tight. Loose or corroded connections can cause power issues.
Do a charging system test - Use the multimeter to test battery voltage while the engine is running. It should be 13.5-14.5V if the alternator is charging properly.
Check drive belts - Visually inspect the alternator belt for excessive wear or cracks. Replace if needed.
Load test the battery - Use a battery tester to do a load test. This checks if the battery can maintain voltage under an electrical load indicating its condition.
If you find problems with the battery or charging system, fix these first before moving to other electrical troubleshooting.
The next step is to check all the fuses in your vehicle. Fuses protect the electrical circuits and components.
Locate fuse boxes - There are usually multiple fuse boxes. Check under the hood, under the dash and in the trunk. Refer to the service manual.
Pull fuses and inspect - One by one, pull out each fuse and look for a broken wire inside indicating a blown fuse.
Check circuits with multimeter - For suspected blown fuses, use the multimeter to check for power on both sides of the fuse. No power on one side points to a blown fuse.
Replace blown fuses - Always replace blown fuses with the same amperage fuse. If it blows again, there's an underlying problem with the circuit.
Checking and replacing any blown fuses can quickly resolve electrical gremlins. Make sure you check all related fuses for a trouble code.
Visually Inspect Wiring Harnesses
Issues in wiring harnesses and connectors are common sources of electrical problems.
Look for chafed or cut wires - Visually inspect wires and harnesses near hot or moving components for worn or cut insulation. Repair or wrap any suspect wires.
Check for loose connectors - Make sure all connectors are fully plugged in. Jiggle them to see if there are any loose connections that may spark intermittent problems.
Look for corrosion - Check for corroded or oxidized pins and terminals in connectors. Clean or replace corroded connectors.
Inspect grounds - Verify all ground connections are clean and tight. Faulty grounds can cause power issues.
Review service manual - Use the wiring diagrams to locate harnesses and connectors related to trouble codes. Inspect these closely.
Carefully looking over wiring harnesses and connectors can reveal issues that create electrical faults.
Use a Multimeter for Diagnostics
A digital multimeter (DMM) is the most useful tool for troubleshooting electrical problems. You can test circuits, components, sensors and more.
Some key troubleshooting tests include:
Voltage drop - Measure voltage drop across fuses, wires and components. Excessive drop indicates issues.
Resistance - Check resistance specs of components like resistors and motors against factory values.
Continuity - Test for continuity to check if wires or circuits are intact. Breaks mean an open circuit.
Power and ground - Verify power and ground circuits have proper voltages at different points.
Sensor inputs/outputs - Check sensor readings and signal voltages against factory specs.
Use the service manual diagrams and specs to determine what to test with the multimeter. This provides the quickest path to isolating electrical faults.
Fix Any Identified Problems
After troubleshooting and testing, you may have found issues like:
- Bad sensor causing incorrect inputs
- Blown fuse interrupting power
- Shorted or broken wire in a circuit
- Loose ground or bad connection
Now you can replace or repair the problematic components and clear the codes. Retest everything to confirm normal operation, and the check engine light should stay off.
Thoroughly troubleshooting your car's electrical system when the check engine light comes on takes time and patience. Following these steps can help isolate the cause so you can make the needed repairs. Always refer to the factory service manual for proper testing procedures. Fixing codes promptly prevents further damage and keeps your car running smoothly.