Diagnosis is a controlled search, not a guess. Start by verifying the customer complaint, then gather information from the scan tool, wiring diagram and service data. A diagnostic trouble code identifies the circuit and the condition the module detected; it does not prove which component failed.
A repeatable sequence
- Verify the complaint and reproduce the condition.
- Read codes and freeze-frame data before clearing anything.
- Study the wiring diagram and identify supply, ground, signal and load.
- Measure the circuit under the conditions that produce the fault.
- Use voltage-drop testing to find resistance rather than guessing at parts.
- Repair, then re-test the original complaint and confirm no new codes.
Voltage drop localises the fault
Resistance from a voltage drop
Problem. A ground circuit carries 10 A and the voltage across the ground connection measures 0.8 V. What is the effective resistance of that connection?
Solution. R = 0.8 ÷ 10 = 0.08 Ω. The connection also dissipates P = 0.8 × 10 = 8 W, which explains the heat and the electrical symptoms.
Codes and data
Freeze-frame data records the conditions when a fault was set. Live data shows whether a signal is plausible. A scope shows the shape and timing that a meter averages away. Choose the tool that answers the question instead of collecting data without a hypothesis.
- A code describes a detected condition, not a failed part.
- Freeze-frame data is evidence; record it before clearing codes.
- Measure under load to reveal resistance faults.
- Confirm the repair against the original complaint.
- Verify the complaint before diagnosing.
- Codes identify circuits and conditions, not parts.
- Voltage-drop testing finds resistance faults under load.
- Always confirm the repair and re-check for codes.
Worked example
A lamp circuit shows 1.2 V across its ground connection at 8 A. R = 1.2 ÷ 8 = 0.15 Ω and P = 1.2 × 8 = 9.6 W at the joint — a clear high-resistance ground, not a failed lamp.