How do I diagnose an intermittent problem that will not set a code when the car is in the shop?
Short answer: Use freeze frame and the customer's description to recreate the conditions, record data during the event, and wiggle-test wiring while monitoring the circuit.
Intermittent problems are about conditions. Ask exactly when it happens: cold mornings, after rain, over bumps, at a certain speed, with the air conditioning on. Freeze frame and any history codes tell you temperature, load and speed at the moment of the fault.
Next, set up to catch it. Many scan tools can record live data during a road test and let you mark the moment the symptom happens, then replay it. A scope with a long recording window can catch dropouts in sensor signals that a meter or scan tool refresh rate would miss.
In the bay, wiggle-test connectors and harnesses while watching the signal, and use gentle heat or cold where the customer's description suggests temperature matters. Check grounds and connectors for corrosion and spread pin terminals. Document what you tested so the next visit starts ahead, and set expectations with the customer that intermittent faults can take more than one visit.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.