Mercedes D814 MOT failure rate

Lighting is the single most common MOT failure and the cheapest to put right. A blown headlight, brake-light or number-plate bulb, a cracked lens, or a headlamp beam that points too high or low will all fail. Most are a two-minute bulb swap.

This covers a range of smaller items, from a non-working horn to a warning light left on.

Brake failures cover worn pads and discs, an imbalance between wheels, corroded or scored discs, or a binding caliper. Braking efficiency is measured on a rolling road, so a spongy or pulling brake shows up as a low reading.

Exhaust failures cover leaks, insecure mountings and emissions over the limit. A blowing exhaust, a failed lambda reading or a diesel that smokes beyond the limit will fail.

Fuel-system failures are leaks or an insecure filler cap.

Structural failures are corrosion in load-bearing areas, sharp edges, or insecure panels. These are inspection-and-welding jobs rather than a simple bolt-on part, so weigh them against the value of the car.