Can a puncture be repaired? The rules a fitter actually works to

A car tyre can only be repaired within the central portion of the tread - broadly the middle three quarters of the tread width. Outside that band, in the shoulder and the sidewall, no repair is permitted at any price.

The reason is mechanical rather than bureaucratic. The tread band is supported by the belts underneath it and deflects relatively little. The sidewall and shoulder flex continuously through every revolution of the wheel, hundreds of times a minute, and no patch will stay bonded to a surface doing that. A sidewall repair does not fail gradually - it fails suddenly, at whatever speed you happen to be doing.

For a passenger car tyre the maximum permitted injury is 6mm across. A screw or a nail is typically well inside that. A bolt head, a large stone or anything that has torn rather than punctured usually is not.

Overlapping repairs are also excluded. If the tyre already carries a repair and the new damage sits in or near it, the tyre is finished.

A tyre cannot be repaired if the cords or ply are exposed anywhere on it, if there is a bulge or lump indicating the internal structure has separated, or if it is already at or below the legal tread limit - repairing a tyre you would have to replace next month is money thrown away.

The one that catches most people out is running flat. If the tyre has been driven on while deflated or seriously underinflated, even for a short distance, the sidewall has been flexing far beyond its design range and generating enough heat to destroy the inner liner. From the outside the tyre can look untouched. From the inside, the liner shows a distinctive scoring and crumbling pattern, and once a fitter sees that the tyre is scrap regardless of where the original puncture was.

Run-flats are their own category. Several manufacturers do not permit them to be repaired at all, and where a repair is permitted it hinges entirely on whether the tyre was driven while deflated - which, given that is the whole purpose of a run-flat, it usually has been.

The repair kits sold for pushing a sticky rope into a puncture from the outside, without removing the tyre, are not a compliant repair. They are a temporary get-you-home measure at best.

The problem is not really the plug. It is that nobody has looked inside the tyre. An object that goes through the tread does not always go straight - it can travel at an angle, and it damages the inner liner and the belt edges on the way. A proper repair means the tyre comes off the rim, the inside is inspected under light, the injury is reamed out to clean material, and a combined plug-and-patch unit is applied from the inside so that both the hole and the liner are sealed. That is the only method that meets the standard, and it is a permanent repair rather than a temporary one.

Leave the object where it is. It is holding pressure in, and its position and angle tell a fitter most of what they need to know before they set off.

Photograph the tyre showing where the object has gone in, with something in frame for scale. If the object is in the middle band of the tread and it is small, a repair is likely. If it is in the shoulder or the sidewall, it is not, and knowing that in advance means the right tyre arrives on the first visit rather than the second.

And do not keep driving on it to get somewhere convenient. That is the decision that turns a repairable tyre into a replacement.