How to diagnose a bad circuit?

A systematic approach is required for diagnosing automotive circuit faults. First, visually inspect wiring harness connectors, wire insulation layers, and electrical components for burning or looseness; corrosion on battery terminals should be addressed promptly. Use the continuity mode of a digital multimeter to test circuit connectivity-if the resistance value deviates by more than 10% from the standard, focus on troubleshooting. Meanwhile, measure the alternator output voltage (normal range: 13.8-14.5V) and battery voltage (should be above 12.4V). For hidden faults, the test light method can be used to quickly verify power supply and grounding status, or short-circuit/open-circuit tests can locate open/short-circuit points. For modern vehicles, It is best to first connect an OBD diagnostic tool to read fault codes and analyze Pxxxx-type electrical faults in combination with circuit diagrams. During operation, the principle of static before dynamic should be followed: conduct power-off testing of basic circuits first, then observe changes in phenomena after power-on. The performance of suspected faulty components should be verified before replacement-for example, spark plugs can be tested for working status via the spark jump method, and the relay control principle can distinguish between power circuit and control circuit faults.