What does 4.10 mean on an MOT certificate?

It refers to sub-section 4.10 of the MOT inspection manual, "Trailer electrical socket", within section 4, lamps, reflectors and electrical equipment. The letter or letters after it — (a), (b)(i) and so on — identify the specific defect from the 6 listed here. 1 of them is graded Dangerous, 3 Major and 2 Minor.

Which trailer electrical socket defect is recorded most often?

"A 13-pin trailer socket not functioning correctly" (4.10 (c)(i)), recorded about 0.0 times per 100 failed MOT tests nationally.

Does a Minor defect fail the MOT?

No. a defect with no significant effect on safety or the environment. Recorded on the certificate; the vehicle passes. Major and Dangerous defects fail. A Dangerous defect also means the car must not be driven until it is repaired.

Section 4 · Lamps, reflectors and electrical equipment

4.10 Trailer electrical socket

Manual indexed 3 September 2026

Quick answer

Sub-section 4.10 of the MOT inspection manual covers trailer electrical socket and lists 6 defects, 1 of them Dangerous. A reference like "(4.10 (a)(i))" on a certificate points to the matching row below. The most recorded is "A 13-pin trailer socket not functioning correctly", about 0.0 times per 100 failed tests.

Official wording

The defect clauses

4.10 (a)(i)Minor
A trailer electrical socket insecure
4.10 (a)(ii)Major
A trailer electrical socket likely to become detached
4.10 (b)(i)Minor
Trailer electrical socket wiring insulation damaged or deteriorated
4.10 (b)(ii)Major
Trailer electrical socket wiring insulation damaged or deteriorated and likely to cause a short-circuit
4.10 (c)(i)Majorunder 0.1 per 100 fails
A 13-pin trailer socket not functioning correctly
4.10 (c)(ii)Dangerousunder 0.1 per 100 fails
A 13-pin trailer socket not functioning at all

Wording and categories reproduced from the DVSA inspection manual under the Open Government Licence. Frequencies are items per 100 failed class 4 tests across the DVSA anonymised MOT dataset; a clause with no figure was not recorded often enough to appear in any model’s top forty failure items. 0 of 6 clauses link to our own explanation.

Grading

What the categories mean

Dangerous: a direct and immediate risk to road safety or the environment. The vehicle fails and must not be driven until repaired, even to a garage.
Major: a defect that affects safety, the environment or other road users. The vehicle fails.
Minor: a defect with no significant effect on safety or the environment. Recorded on the certificate; the vehicle passes.

The full guide to defect grading

Section 4

Other sub-sections of lamps, reflectors and electrical equipment

All sections of the manual

Common questions

What does 4.10 mean on an MOT certificate?: It refers to sub-section 4.10 of the MOT inspection manual, "Trailer electrical socket", within section 4, lamps, reflectors and electrical equipment. The letter or letters after it — (a), (b)(i) and so on — identify the specific defect from the 6 listed here. 1 of them is graded Dangerous, 3 Major and 2 Minor.
Which trailer electrical socket defect is recorded most often?: "A 13-pin trailer socket not functioning correctly" (4.10 (c)(i)), recorded about 0.0 times per 100 failed MOT tests nationally.
Does a Minor defect fail the MOT?: No. a defect with no significant effect on safety or the environment. Recorded on the certificate; the vehicle passes. Major and Dangerous defects fail. A Dangerous defect also means the car must not be driven until it is repaired.