Car Leaks Advice: Can you Do It Yourself?

Gary put this video together for people who are a long way away and wondering whether to attempt the work themselves before making the drive, or whether to try some basic checks first. It is honest about what you can do; it is equally honest about where DIY stops being sensible.

Safety first: electronics in water

Dashboard warning lights after a water leak often mean water has reached an electronic control unit. The airbag control box commonly sits under the front seats, below the carpet line; by the time there is visible water on the carpet surface, that box may already be sitting in it. Gary is clear on this: a safety-related unit that has been in water cannot simply be dried out and put back. You do not know what it will do in a crash. The path for a submerged airbag control unit is to buy a replacement (often available through a dealer parts department), dry everything out, fix the leak, then have a garage fit and reprogram the new unit; half an hour of labour at most. A £30 OBD reader from the internet will not turn the airbag light off; you need a garage with proper diagnostic equipment, typically around £15.

Non-safety electronics -- radios, amplifiers -- are a different matter. Worst case there is a stereo that stops working; you can try drying those out and replace if needed.

Mould: more serious than most people realise

Gary has seen customers who were ill for months without connecting it to the car. One came to the door looking a terrible state; when Gary opened the car the reason was obvious. One of the New Again team ended up with a doctor issuing a warning over mould exposure. The spores travel the same way dust does; bash a cushion in sunlight and you can see what is floating in the air. That is what you are breathing in a mouldy car.

For DIY treatment, Gary recommends Formula 409 by ChemSpec -- an award-winning antimicrobial that is safe to use on all interior surfaces. Read the label: you want the version without bleach, which can be listed as peroxide. Apply it properly; do not try to drive with the windows open and assume that sorts it. If the mould is bad, Gary offers a collection service: a driver with a mask collects the car and the decontamination starts as soon as it arrives.

Three categories of leak

Gary categorises water leaks into three types.

The first is basic blockages -- blocked sunroof drain pipes, debris filling a gully, outlets blocked under the body. These are the ones you might reasonably tackle yourself. For a sunroof drain, stand on the seat with the panel open, pour a jug of water into the recess at each corner and have a friend watch the wheel arch for it to emerge. If it does not, the pipe is blocked. Be careful unblocking: pushing an airline or a probe down the drain can pop the pipe off its connection further down, behind the dashboard. Check the pipe is seated at both ends before declaring the job done.

The second category is inherent model-specific faults. Some cars have known weak points -- pollen filter housings on certain hatches, areas under the pedals on others. These are often documented online and on forums; worth researching your model before starting.

The third is quirky leaks. Gary has seen cars that only leak when parked on a specific slope; water that routes through a speaker housing and drips out nowhere near where it entered; leaks caused by old accident repair welds. These need a professional with the right equipment and the experience to replicate the conditions.

Drying reality

Most people underestimate what is in the foam under the carpet. Modern cars with thick foam can hold four or five gallons of water -- twenty litres -- without it being obvious at the surface. Cars Gary has seen returned eight months after a supposed DIY fix, still wet underneath.

Proper drying means: suction machine to pull out the surface water and the top of the foam; then a three-phase hot-air blower through an eight-inch pipe, directed under the bulkhead, left to run. Even with professional equipment this can take days. A domestic fan heater left running overnight is not the same thing, and Gary would not recommend leaving one unattended.

Four rules if you do it yourself

Gary ends with four points: clean up the debris after unblocking so it cannot reblock; do not wipe away water trails before you have confirmed where the leak is -- that evidence matters; always check for a second leak source before assuming you have fixed it; and lift the carpet to confirm how much water is actually there, because it is almost always more than you expect.

See our car water leak diagnosis page for details on the 28-point check we carry out, and carpet drying service for the extraction and drying side of the work.