The names vary between cars and scanners, so look for these two:

Common labels you'll run into: charge air pressure, boost pressure, intake manifold pressure. Some cars show kPa absolute, some bar or mbar. The unit doesn't matter for this test, only whether the two lines move together.

Connect the scanner, go into the engine module, open live data, and find both boost values. Display them on the same graph if the tool allows it. Then test in two phases.

Idle check. A small difference at idle is normal, actual can even sit slightly above specified. Ignore it. Idle tells you almost nothing about a turbo, the load isn't there yet.

Driving test. This is the one that matters. Do a few firm accelerations and watch how the two lines behave as the revs climb. A turbo only shows its weakness under load.

On a healthy turbo, actual boost follows specified closely on every pull. There's a small lag while the turbo spools, then it catches the target and holds it.

On a failing turbo, the lines split. Specified climbs as you accelerate, but actual builds slowly and never reaches it. That growing gap under load is the symptom.

Here's the honest limit of this test: it tells you the turbo isn't making target boost. It doesn't tell you which part is at fault. A shortfall like the one above usually comes from one of these:

• Boost leak, split hose, loose clamp, cracked intercooler or pipe
• Faulty turbo actuator or wastegate
• Vacuum or control issue
• A genuinely worn or failing turbo

Treat the graph as confirmation that something is wrong, then chase the cause. Most of the time it's a leak or an actuator, not the turbo itself, and that's the cheap end to check first.

It's at its best in three situations: checking a used car before you buy it, diagnosing a car that feels down on power, and verifying that a turbo or boost repair actually worked. In all three you get a clear yes or no in one short drive.