Written by the fitting team at 24/7 Mobile Tyre Services, Glasgow. Last reviewed: July 2026. General information, not legal advice.
You paid for this tyre last month. Now you are back at the air machine in Rutherglen for the third week running, feeding it 50p and wondering whether you were sold a dud.
The frustration is fair. The conclusion usually is not.
On a brand new tyre, the tyre itself is the least likely culprit. There are nine realistic causes of a slow leak, and eight of them are around the tyre rather than in it.
Quick answer: A new tyre losing air is usually the valve, the TPMS sensor seal, corrosion on the wheel’s bead seat, or an object picked up since fitting, not the tyre itself. Check pressures cold with one gauge before assuming a leak. Pressure falls roughly 1 PSI for every 5°C drop in temperature.
Step zero: make sure it is actually losing air
Every October we take a wave of these calls, and a good share of them turn out to be the weather.
Temperature. Tyre pressure drops by roughly 1 PSI for every 5°C fall in air temperature.
Set your pressures on a mild 15°C September afternoon. Check them again on a frosty 0°C November morning. That is around 3 PSI down, on every tyre, with nothing wrong at all.
It looks exactly like a slow leak. It is physics.
Check cold. Before driving, or after the car has sat for at least three hours. Driving warms the air inside and raises the reading, sometimes by several PSI.
Use one gauge and check your tyre pressures the same way every time. Forecourt machines vary enormously and many are simply out of calibration, a point TyreSafe’s own pressure guidance makes repeatedly. A decent pencil or digital gauge costs less than one top up.
Use the right target figure. The door pillar plate, fuel flap sticker or handbook. Not the number moulded into the sidewall, which is a maximum rating rather than a recommendation.
Then quantify it. Set to spec, write the number down, recheck cold at 24 hours and again at 7 days.
Now you have a rate, and a rate tells a fitter far more than “it keeps going down”.
| Loss | What it usually means |
|---|---|
| A couple of PSI a month | Normal permeation through the rubber |
| Several PSI a week | A genuine leak worth chasing |
| Flat overnight | Something significant. Do not drive on it |
The nine causes, most likely first
1. The valve. A new tyre should get a new valve, every time.
A reused rubber valve that has been on the car five years is stiff and cracks at the base. A loose or corroded valve core weeps constantly and silently.
The tell: bubbles at the valve, or the leak appearing to slow when the cap is tightened. A cap is not a seal, but a weeping core often reveals itself that way.
This is the cheapest possible fix and one of the most common causes.
2. The TPMS sensor seal. On clamp in aluminium sensors, the grommet, washer, nut and core form a service kit that should be renewed at every tyre change, with the nut torqued to specification.
Over tighten it and you crack the sensor. Under tighten it and it leaks.
The tell: bubbles right at the base of the sensor’s valve stem. See TPMS sensor replacement and when your tyre pressure light stays on.
3. Corrosion on the wheel’s bead seat. Very common in Scotland. Road salt attacks the aluminium exactly where the tyre bead has to seal, and a pitted surface will not hold air.
The tell: bubbles appearing in a line around where the tyre meets the rim.
This one has enough going on that we gave it its own page: an alloy wheel leaking around the bead.
4. An object picked up since fitting. A screw or a nail typically causes a slow, clean leak with no dramatic flat.
The tell: bubbles from a single point in the tread.
Worth saying plainly: a new tyre picks up screws just as readily as an old one. Newness is not armour.
5. A damaged or cracked wheel. A hairline crack from a pothole strike, or porosity in the original casting.
The tell: bubbles from the rim face or the inside of the barrel, often only visible in a water bath. More in what to do when a pothole buckles an alloy wheel.
6. Sealant residue from a puncture kit. If a sealant kit was used before the tyre was replaced, residue can prevent a clean seal and can foul the sensor.
The tell: you know you used the kit. The wheel should have been cleaned thoroughly at fitting.
7. A previous repair failing. Relevant where a repair was done rather than a replacement.
A proper repair to BS AU 159 is a permanent patch plug fitted from the inside of the tyre, sealing the injury through the casing. An externally inserted rope plug is not a permanent repair and can weep for months. Our puncture repair page explains the difference.
8. The tyre was fitted with a damaged bead. Rare, but a bead nicked by a poorly set machine will never seal properly.
The tell: it never held pressure correctly from day one.
9. A faulty tyre. Genuinely uncommon, but manufacturing faults in the inner liner do exist.
This is a warranty matter, and it should be your last conclusion, not your first — much like the conicity issues covered in our guide to cars pulling after new tyres, it’s rare but real.
How to find it yourself in ten minutes
The soapy water method, done the way it actually works.
What you need: warm water, a good squirt of washing up liquid, a spray bottle or a sponge. Nothing caustic anywhere near an alloy.
Inflate to the correct pressure first. A soft tyre does not push enough air out to show anything.
Spray in this order, and watch each area for at least thirty seconds:
- The valve
- Around the bead, on both the outer and inner side
- Across the tread
- The inner rim edge and the back face of the wheel
That thirty second rule is the instruction most guides leave out. A slow leak produces one growing bubble, not foam. Glance for three seconds and you will miss it every time.
Turn the wheel as you go. The leak may be sitting at the six o’clock position where you cannot see it.
If you find nothing, it is very likely the back face of the wheel or a bead seat leak that only reveals itself in a water bath. That is a fitter’s job, with a tank.
Safety. Do not jack a car on soft ground or a slope to do this. Never get under a jacked car. If you are not confident, do not.
What a fitter should do, and what to ask for
A proper slow leak diagnosis looks like this:
- The wheel comes off and the tyre is dismounted. You cannot diagnose most of these with the tyre still on
- Both the inside of the tyre and the whole wheel are inspected
- The wheel goes into a water bath under pressure
- Any corrosion on the bead seat is cleaned back to clean metal and resealed
- A new valve or TPMS service kit is fitted
- The wheel is rebalanced (as part of the same wheel balancing process) and torqued to specification on refitting
And two questions worth asking directly:
- “Was a new valve fitted when the tyre went on?”
- “Was a TPMS service kit used?”
If the answer to either is no, you have very probably found your cause. It should also be put right.
Is it a warranty job?
Fair framing, in both directions.
Likely covered: if the tyre was fitted recently and the leak is at the valve, the sensor seal or the bead, that falls inside the fitter’s own work. A reasonable fitter will re inspect at no charge.
Not a fitting fault: a screw picked up two weeks later is a puncture, not bad workmanship. The good news is that punctures in the repairable tread area are often fixable, which costs far less than a replacement.
Under the Consumer Rights Act 2015, services should be carried out with reasonable care and skill and goods should be of satisfactory quality. That is general information rather than legal advice, but it is a fair basis for a polite conversation.
What not to do
Do not keep topping it up and ignoring it. Running persistently under inflated overheats the sidewall and causes irreversible internal damage. It can turn a cheap repairable puncture into a scrap tyre.
Do not use a sealant kit on a tyre with a known slow leak. It makes diagnosis much harder and can foul the sensor.
Do not fit a new tyre to a corroded wheel without cleaning it. You will be back.
Do not rely on the TPMS light as a leak detector. Many systems only warn at around 25 percent below the recommended pressure. By then the tyre has been running under inflated for a while.
Do not chase it with nitrogen. As manufacturer guidance on tyre inflation gases explains, nitrogen migrates marginally more slowly than air through rubber. It does not fix a leak, and no gas will.
A seasonal pattern we see every year
Every October and November we get the same wave of calls. Four tyres all low, all at once, on a car that was fine in September.
Four simultaneous punctures would be remarkable. A twelve degree drop in overnight temperature is not.
We still check properly, because occasionally one of those four really is leaking. But the honest answer on most of those calls is: set them to spec on a cold morning, write the numbers down, and recheck in a week.
That is not the answer that sells a callout. It is the right one.
FAQs
Is it normal for tyres to lose a bit of pressure? Yes. Air permeates slowly through rubber, and a loss of a couple of PSI a month is normal. Several PSI a week is not.
How much pressure loss is a problem? As a rough guide, more than 2 to 3 PSI a week is worth investigating. A tyre that is noticeably down overnight should not be driven on until it has been checked.
Can a new tyre have a puncture already? It can pick one up on the way home. A brand new tyre is no more resistant to a screw than a worn one — the same way age rather than mileage decides a tyre’s fate, as we cover in how old your tyres really are. A manufacturing fault in the tyre is possible but uncommon.
Why do my tyres lose pressure in cold weather? Air contracts as it cools. Pressure falls by roughly 1 PSI for every 5°C drop. This is why so many tyres read low on the first frosty morning of the year.
Should a new valve be fitted with a new tyre? Yes. On a TPMS equipped car, the sensor should also get a new service kit, and the nut should be torqued to specification. Both are small costs that prevent a very common leak.
Can a TPMS sensor cause a slow leak? Yes. The seal and grommet at the base of the sensor are consumable parts. A reused seal, or a nut that is over or under tightened, will weep.
Can you find a slow puncture without taking the tyre off? Sometimes, with soapy water at the valve, bead and tread. Leaks at the bead seat, the back face of the wheel or inside the barrel usually only show up with the tyre dismounted and the wheel in a water bath.
Is a slow puncture repairable? Often, if the injury is within the repairable central area of the tread and the tyre has not been run flat. Repairs should be done to BS AU 159 with an internal patch plug. Sidewall damage is never repairable.
If a tyre keeps dropping pressure
We can come to the car, dismount the tyre, check the wheel and the bead seat properly, and fit a new valve or sensor kit where it is needed.
At your home or your work, without you losing a morning to it.
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Related: mobile tyre fitting in Glasgow, emergency tyre replacement.
