Why Your Tyre Won't Hold Air
A tyre that slowly loses pressure is properly annoying. Not flat enough to notice right away, but the TPMS light keeps flicking on. Or the car pulls to one side. Or that one tyre looks a bit low every few days.
Five main reasons it happens. Some are a $10 fix. One might mean a new tyre (tire). Let's get into it.
1. Nail, Screw, or Foreign Object
Most common cause of a slow leak by far. You've driven over a nail or screw, and it's embedded in the tread, letting air bleed out gradually. Sometimes the object actually plugs the hole partway, which is why the tyre doesn't go flat instantly. Just slowly loses air over hours or days.
How to Find It
Inflate the tyre to the correct pressure and inspect the tread carefully. Rotate it slowly and look for anything stuck in there. Nails, screws, glass, wire. Sometimes the object sits flush with the tread surface, easy to miss.
Can't spot anything? Mix dish soap with water in a spray bottle and spray the tread and sidewall. Bubbles form wherever air is escaping.
The Fix
Puncture in the tread area (not the sidewall) and the hole is smaller than 6mm? It can usually be repaired with a plug-patch combo. The plug seals the hole from inside, the patch covers it. A plug-only repair from a roadside kit is temporary. Not a permanent solution.
Cost to fix:
- Professional plug-patch repair: $30 to $60 AUD / $20 to $40 USD
- DIY tyre plug kit (temporary): $15 to $25 AUD / $10 to $20 USD
Important: Puncture in the sidewall or shoulder? Can't be safely repaired. You need a new tyre.
2. Faulty Valve Stem
The valve stem is the small rubber or metal piece where you attach the air pump. Rubber deteriorates over time, cracks, and starts leaking. The valve core inside can also fail or work loose.
How to Check
Apply soapy water around the valve stem and the base where it meets the rim. Bubbles mean that's your leak.
Try pressing the valve core with a small tool or the back of a tyre gauge cap. Air hissing out when you're not pressing, or the core feels loose? Needs replacing.
The Fix
- Loose valve core: Tighten it with a valve core tool. Costs a couple of dollars from any auto parts store.
- Worn valve core: Replace it. Under $2 each.
- Cracked rubber valve stem: Whole stem needs replacing. Requires removing the tyre from the rim, so it's usually done during a tyre change or rotation.
Cost to fix:
- Valve core replacement: $5 to $15 AUD / $3 to $10 USD
- Valve stem replacement: $20 to $40 AUD / $15 to $30 USD (typically done during tyre service)
3. Rim Seal Leak (Bead Leak)
The tyre bead is the inner edge that sits against the wheel rim. For the tyre to hold air, that seal needs to be completely airtight. Corrosion on the rim surface, especially on alloy wheels, can break the seal and cause a slow leak.
Particularly common on:
- Older alloy wheels where the clear coat has broken down
- Cars driven in coastal areas where salt speeds up corrosion
- Wheels that have been kerbed or scraped
How to Check
Soapy water test again. Spray around the bead area where tyre meets rim. Bubbles along the rim edge means bead leak.
The Fix
Tyre comes off the rim. Rim surface gets cleaned, corrosion sanded or wire-brushed away. Bead sealer applied, tyre remounted. Severe cases may need the rim refinished or replaced entirely.
Cost to fix:
- Bead reseal (clean and remount): $40 to $80 AUD / $30 to $60 USD per tyre
- Rim refinishing: $100 to $250 AUD / $80 to $180 USD per wheel
4. Temperature Changes
This one catches people off guard. Air contracts when cold and expands when warm. For every 10 degrees Celsius drop in temperature (about 18 Fahrenheit), tyre pressure drops by roughly 1 to 2 PSI.
So you filled your tyres on a warm afternoon, then the TPMS light comes on the next cold morning? Might just be physics, not a leak.
How to Tell
- Light comes on during cold mornings but switches off after driving (tyres warm up, pressure rises)
- Multiple tyres show low pressure, not just one
- Pressure difference is small, only 1 to 3 PSI below recommended
The Fix
Check and adjust tyre pressures when cold (before driving or after the car has sat for 3+ hours). Set them to the recommended PSI on the placard inside the driver's door jamb. Not the number on the tyre sidewall. That's the maximum, not the target.
Heading into winter? Worth adding an extra 1 to 2 PSI above the recommended cold pressure to account for the temperature drop. Just don't exceed the tyre's maximum rating.
Cost to fix: Free. Just top up the air.
5. Cracked or Damaged Sidewall
The sidewall is the vertical part of the tyre between the tread and the rim. Cracks, cuts, or bulges here can let air escape slowly.
Common causes of sidewall damage:
- Hitting a pothole or curb at speed
- Running on a very under-inflated tyre (the flexing damages internal structure)
- Age. Rubber degrades over time, especially sitting in the sun.
- Overloading the vehicle beyond the tyre's weight rating
How to Check
Visually inspect the sidewall for cuts, cracks, or bubbles. Run your hand along it and feel for anything odd. A bulge in the sidewall means the internal structure is damaged. That tyre can blow out without warning.
The Fix
Sidewall damage cannot be repaired. Cracks, cuts, or a bulge in the sidewall means the tyre needs replacing. Full stop. A sidewall failure at speed is dangerous.
Cost to fix: New tyre, $100 to $400+ AUD / $80 to $300+ USD depending on size and brand.
How to Check Your Correct Tyre Pressure
Your car's recommended pressure is on a sticker (placard) inside the driver's door jamb. Also in the owner's manual.
The number on the tyre itself (e.g., "Max 44 PSI") is the maximum it can safely hold. Your car's recommended pressure will almost always be lower.
Some cars run different pressures front and rear, or different pressures for light versus heavy loads. Check the placard.
When to See a Mechanic
Go now:
- Tyre losing more than 5 PSI overnight
- Visible bulge in the sidewall
- TPMS light stays on even after inflating to the correct pressure
This week:
- Slow leak that needs topping up every few days
- Visible nail or screw in the tread
- Bubbles around the valve stem or bead area
No rush:
- Pressure dips slightly in cold weather but comes back to normal when warm
- Old valve stems. Replace them at your next tyre change.
Ask TorqueBot
Not sure what PSI your car should be running? Wondering whether that nail in the tread can be patched or means a new tyre? TorqueBot knows the recommended pressure, tyre size, and specs for your specific vehicle. Just tell it your year, make, and model and it'll give you the numbers. No digging through owner's manuals required.