Relative compression

Cranking only — do not drive, and stay clear of the fan and belts. This compares how much each compression event slows the starter. Slots are in detection order, not firing order. A low slot is the weak one; a gauge test still tells you the number.

OBD RPM is a filtered number. The dip that matters is in the starter current, and many ECUs have already smoothed it out before this test sees it. A low slot is worth a gauge test. An even result is encouraging, but it does not prove the compressions are good.