إطار ناقص النفخ

A tyre below its specified pressure, which changes its rolling radius and therefore how fast that wheel turns for a given road speed. A soft tyre has a smaller effective radius and turns faster than its neighbours, and the stability and traction systems compare wheel speeds continuously.

The difference they detect is small, which is why this matters. A few tenths of a bar is enough to read as permanent low-level slip on one corner, so the system either intervenes when it should not or reports an implausible comparison. A stability or wheel speed code with no mechanical fault and no warning light for pressure is worth taking to a gauge, because a monitoring system detects a fast leak better than a slow one.

It is also the cheapest check available and is skipped constantly. Measure all four cold against the placard, which is on the door pillar or the filler flap rather than on the tyre. The tyre's own sidewall states a maximum, not a recommendation — inflating to it is a common error and produces the opposite error in rolling radius, along with worse grip and uneven wear.