A tyre that keeps going flat overnight usually has a leaking valve, a corroded rim where the tyre seals against the wheel, or a puncture that only opens under load. Before chasing any of those, rule out temperature, because a 10°C drop overnight lowers pressure by roughly 2 to 4 PSI without any leak at all.
Five causes account for almost all overnight deflation:
- Overnight temperature drop
- Valve stem or valve core
- Rim corrosion and bead seal failure
- A puncture that seals when stationary
- TPMS sensor seal
Rule Out Temperature First
Air contracts as it cools. A car parked outside through a cold night loses pressure that returns once the tyre warms, and the TPMS light frequently comes on during the morning commute before clearing later.
Test it in one step. Check the pressure cold in the morning, then check it again after the car has sat in the afternoon without being driven.
- Pressure recovers on its own: temperature, not a leak
- Pressure stays low or keeps falling: a genuine leak
Compare the suspect tyre against the other three on the same morning. All four dropping together points to weather. One dropping alone points to a fault.
Valve Stem & Valve Core
The valve is the most common single cause of overnight loss, and the cheapest to fix.
Two separate parts fail:
- The valve core, the small spring-loaded insert inside the stem, which can loosen or fail to seat
- The rubber stem itself, which perishes, cracks at the base, or splits where it passes through the wheel
Rubber stems age and should generally be replaced whenever a tyre is changed. A stem on its second or third tyre is a likely culprit.
Test it by brushing soapy water over the valve and watching for bubbles. A loose core sometimes tightens with a valve tool, though replacement is more reliable.
Rim Corrosion & Bead Seal
The bead is where the tyre’s edge clamps against the wheel rim. A leak here produces exactly the overnight pattern, since it is slow and continuous.
Corrosion is the usual cause, particularly on alloy wheels and on cars driven through winter road salt. Rust and oxidation build a rough surface that the tyre cannot seal against.
Kerb impacts also bend the rim edge enough to break the seal without visibly damaging anything.
A garage fixes this by removing the tyre, cleaning the rim back to clean metal, and refitting with sealant. Pumping the tyre up repeatedly will not resolve it.
A Puncture That Seals Under Weight
Some punctures leak only in certain positions. An object sitting in the tread can seal the hole while the wheel rests on it and open as the wheel rotates, or the reverse.
This explains a tyre that holds pressure during a short test but empties overnight, and it is why a garage sometimes finds nothing on a quick inspection.
Inspect the whole tread surface, rotating the wheel to see the section resting on the ground. Nails and screws frequently sit flush and are easy to miss.
TPMS Sensor Seal
Cars built since 2014 carry a tyre pressure sensor inside each wheel, held in place by a rubber grommet that degrades over time.
Sensor seals typically last five to seven years. A failing one leaks from inside the wheel where no external inspection will find it.
Suspect this where the tyre is otherwise sound, the valve tests clean, and the car is more than five years old.
Finding the Leak at Home
Work through this in order:
- Inflate to the pressure on the driver’s door jamb, not the figure moulded into the tyre sidewall
- Mix washing-up liquid with water in a spray bottle
- Spray the valve first, then the full tread, then both sidewalls, then the rim edge all the way round
- Watch for bubbles forming, which may take 30 seconds to appear on a slow leak
- Rotate the wheel and repeat on the section that was touching the ground
Recheck pressure 24 hours later if no bubbles appear. A drop of more than 2 to 3 PSI confirms a leak too slow to see.
What a Garage Does Differently
Two methods find leaks that soapy water misses.
Water submersion. The wheel comes off the car and goes into a tank, which reveals bubbles from anywhere including the inner sidewall and the back of the rim.
Pressure decay testing. The tyre is pressurised and monitored electronically, which identifies very slow leaks and confirms whether a repair has worked.
Both require the wheel off the vehicle, which is why a roadside inspection sometimes finds nothing.
What Not to Do
- Do not keep topping it up indefinitely. Running persistently under-inflated damages the sidewall structure and causes irregular tread wear.
- Do not use aerosol tyre sealant as a permanent fix. It can prevent a proper repair and some types damage TPMS sensors.
- Do not ignore it because the tyre still looks fine. Overnight loss indicates an active fault that worsens.
Frequently Asked Questions
How much pressure loss overnight is normal?
Tyres lose roughly 1 to 2 PSI per month naturally. Losing that much in a single night indicates a leak rather than normal permeation.
Why does only one tyre go flat overnight?
A single affected tyre points to a specific fault rather than temperature, since weather affects all four equally. Check its valve and rim first.
Can a tyre leak only when the car is parked?
Yes. Punctures that seal under rotation but open under static load produce this pattern, which is why garages sometimes find nothing on a brief inspection.
Is it cheaper to fix the valve or replace the tyre?
Valve replacement typically costs a fraction of a new tyre and takes minutes. Diagnose the cause before agreeing to a replacement tyre.
Does cold weather alone trigger the TPMS light?
Yes. A sufficient temperature drop can push pressure below the warning threshold without any leak, and the light often clears once the tyres warm up.
