A tyre that keeps going soft while the other three hold has a leak, and leaks come from a short list of places. A little loss over a month is normal; the same tyre needing air every few days is not. Finding it is mostly a process of elimination.
Where the air goes
Five causes cover almost every case:
- A slow puncture in the tread, often an embedded nail or screw, covered under finding a slow puncture
- A leaking valve, perished with age or poorly seated, per tyre valves and valve caps
- A corroded rim seal, where the tyre bead meets an oxidised alloy, common on older wheels
- A weeping previous repair that has begun to fail
- Cold weather, which is not a leak at all but explains a tyre that only drops in winter, as under pressure in hot and cold weather
Narrowing it down
The soapy-water test finds most leaks in minutes: set the tyre to its correct pressure, brush a soapy solution over the tread, valve and rim edge, and watch for bubbles. Where they grow tells you the cause:
- Bubbles in the tread, a puncture, usually repairable
- Bubbles at the valve, a valve replacement, cheap and quick
- Bubbles at the rim, corrosion, needing the tyre off to clean and reseal
Which need a garage
A small object in the central tread can be repaired; a leak at the rim, valve or sidewall needs the tyre broken down on a machine, so those are workshop jobs. Whether the tyre is worth fixing at all is the wider repair-or-replace decision, and a tyre that is also cracking with age has bigger problems, covered under sidewall cracking and dry rot.
Give the fitter a time-based record
Note the cold pressure, date and how much it changes before the next check. Explain whether a repair, new tyre or wheel refurbishment preceded the loss. The location of a leak determines the appropriate work, so do not assume every case needs a new tyre or every case can be plugged. If the tyre has been run deflated, tell the fitter: restoring air pressure does not establish that the internal structure escaped damage.
Common questions
Why does one tyre keep losing air?+
Air escapes in only a few places: a slow puncture in the tread, a leaking or perished valve, a corroded seal where the tyre meets an alloy rim, or a previous repair that has started to weep. A soapy-water test over the tread, valve and rim edge usually shows which by bubbling at the leak.
Is it normal for tyres to lose pressure over time?+
A small, gradual loss of one or two PSI a month is normal, and tyres also read lower as the weather turns cold. A tyre that needs topping up every few days, while the others hold, is leaking and needs finding rather than just refilling.
Can a tyre lose pressure without a puncture?+
Yes. A perished valve, a corroded alloy rim, or simply cold weather can all drop the pressure with no puncture at all. Rim corrosion is a common hidden cause on older wheels and needs the tyre taken off to clean and reseal.
Should I keep topping up a leaking tyre?+
Only as a stopgap while you get it checked, within a day or two. Running a tyre repeatedly soft overheats it, wears the edges and can damage the casing, so the leak should be found and fixed rather than managed indefinitely with the airline.