#diagnostics

Insulation resistance, polarisation index, tan delta, VLF, partial discharge, contact resistance and thermography. Each one answers a narrow question, most of them are blind to where the defect is, and the order they are done in is set by cost rather than by diagnostic power.

Why two twisted wires replaced a loom, how dominant and recessive bits let modules arbitrate without a master, what the 120 ohm terminators are for, and the handful of measurements that tell you whether a network is healthy.

What a diagnostic trouble code actually tells you and what it does not, why freeze frame is usually worth more than the code itself, and what readiness monitors are really testing.

The request and response structure of ISO 14229, the services worth knowing by number, why a negative response is often more useful than a positive one, and what seed and key does and does not protect.

What a scope shows that a scan tool and a multimeter cannot, how a healthy CAN pair should look, the four faults that account for most bus problems, and why voltage drop beats resistance every time.