What live data should I look at first on a check engine light?
Short answer: Start with fuel trims, coolant temperature, engine load, MAF or MAP readings, oxygen sensor activity and RPM, then compare them to the freeze frame from when the code set.
Live data tells you how the engine computer sees the world. A small set of PIDs answers most first questions.
Short and long term fuel trims show whether the computer is adding or removing fuel. Large positive trims suggest a lean condition such as a vacuum leak or weak fuel delivery; large negative trims suggest rich. Engine coolant temperature confirms the engine is warm and the sensor is believable. Calculated load, MAF grams per second or MAP pressure show airflow; compare them to known-good values for the engine size. Upstream oxygen or air-fuel sensors should respond quickly to throttle changes.
Next, open the freeze frame. It records these values at the moment the code set, which tells you whether the fault happened cold, at idle, at cruise or under load. Recreate those conditions on a test drive while recording data. Graph a few PIDs at once rather than staring at a long list of numbers.
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.