Chapter 6 — Section 5: Seat belts and supplementary restraints

Section 5 checks that every seating position has a compliant restraint and that the restraint system as a whole would still do its job in a collision. Requirements vary by the age and class of vehicle; most cars first used after 1 April 1987 need three-point lap-and-diagonal belts for the driver and front passenger, and at least lap belts in the rear. The tester works without dismantling anything, relying on visual checks and testing the mechanisms by hand.

01Structural integrity and anchorages

A seat belt is only as strong as its mounting point. The tester checks the condition of the vehicle structure within 30cm of any anchorage, covering the floor pan, inner sills and B-pillars. Trim isn't removed, but the tester applies pressure and looks for corrosion, cracking or a poor repair. Patch repairs at an anchorage point must be fully seam-welded; spot or tack welding there is grounds for rejection.

The physical attachment of the belt to its anchorage is checked too. Loose mounting bolts fail the car, though it's worth knowing that many inertia reel mounts are deliberately designed with some float so the belt pulls at the right angle, and the tester has to tell that apart from genuine structural failure. If a mounting point is corroded enough to deform by hand, or crumbles under light pressure from the corrosion tool, the car fails.

02Condition of webbing and buckles

The webbing carries the load, so cuts, fraying or heavy fluffing that could weaken it are checked closely. A minor snag usually isn't enough to fail, but a cut that goes through the edge or into the weave is a Major defect. Chemical contamination, from battery acid or some cleaning products, can weaken the fibres without obvious fraying; stiff or discoloured webbing is likely to fail for that reason.

Buckles are tested by hand. The tongue should lock in first time and stay locked under a sharp pull, and the release has to work with one hand. A buckle that's cracked, deformed or needs jiggling to lock is treated as a serious risk. On modern cars the tester also looks for the ECE 'E' mark or British Standard kite mark, though older cars with a belt of an equivalent standard are usually accepted.

03Retraction and locking mechanisms

Inertia reel belts should retract smoothly and fully. A belt that hangs slack once unbuckled fails, since it risks getting trapped in a door or not taking up slack quickly enough in a crash. The tester checks the locking mechanism with a sharp pull on the webbing; the reel should lock immediately. This sharp-pull test is the practical way the g-sensor inside the reel is assessed during an MOT.

• The belt should extend fully and not bind on the casing.
• Webbing shouldn't twist within its guide loops.
• Height adjusters or anti-cinch mechanisms, where fitted, should sit securely in their detente positions.
• The belt shouldn't be fitted twisted at the mounting or buckle.

04Supplementary restraint systems and airbags

The SRS check is largely electronic, centred on the dashboard warning light. Since 2012 the MOT has included a check that this light comes on at ignition and then goes out once the self-test completes. A light that stays on, or never comes on, fails the car; the latter often means a removed bulb or a tampered circuit hiding a fault.

Testers also check that airbags fitted as original equipment are still physically present. An obviously deployed airbag that hasn't been replaced, or an aftermarket steering wheel fitted without an airbag where one was originally present, fails the car. The tester can't inspect the airbag canisters themselves, so this relies on the warning light and visible trim condition. A passenger airbag off switch is fine provided it looks like a deliberate override rather than a fault indication.

05Pre-tensioners and load limiters

Pyrotechnic pre-tensioners pull the belt tight in the moments after an impact is detected, and are monitored by the same SRS control unit, so a faulty pre-tensioner nearly always triggers the warning light. There's a visual check too: many pre-tensioners have a mechanical tell, a plastic tab or concertina sleeve on the buckle stalk, that changes appearance once triggered. A fired pre-tensioner means the belt is no longer serviceable and must be replaced. Any modification that bypasses these systems, such as a dummy buckle clip fitted to silence the seat belt reminder, is an immediate fail.

06Special cases and exemptions

Classic vehicles built before certain dates, typically pre-1965, may not require seat belts at all if unmodified. But the DVSA rule is 'once fitted, must work': if belts have been retrofitted to a car that didn't legally need them, they still have to meet the standard checks. Disabled person's belts and specialist harnesses have their own criteria that allow modifications suited to the occupant's needs.

Section 5 doesn't touch the car's trim, but it gives a reasonable degree of confidence that the primary and secondary restraints will work when needed. It's worth checking webbing condition and warning lights yourself between tests.