What should I put in the screenwash bottle?

Why plain water is a bad idea, how to get the dilution right for the season, and clearing blocked jets without damaging them.

Difficulty: beginner
Time: 10 min
Read: 4 min
Safety: caution

Short answer

Use proper screenwash mixed to the strength on the bottle — a stronger mix in winter for a lower freezing point, a weaker one in summer. Never plain water, never washing-up liquid. The screenwash bottle is the one reservoir under the bonnet you can safely fill to the top.

It is the least glamorous fluid in the car and the one that most often causes a genuinely dangerous moment: a low sun, a salted motorway, and nothing coming out of the jets.

Safety

Screenwash concentrate is toxic if swallowed and must be kept away from children and animals. Never use washing-up liquid: it contains salt, damages paintwork and rubber, and can block the pump and jets. Plain water freezes in winter and can split the bottle, the pump or the pipework, and a frozen washer system is an MOT failure as well as a visibility problem.

What you'll need

Screenwash concentrate or ready-mixed
A funnel
A pin or a fine wire (optional)

Step by step

Find the right cap.The screenwash filler cap carries a windscreen and spray symbol. It is easy to confuse with the coolant tank on some cars, and putting screenwash into the coolant system is an expensive mistake.
Read the dilution table on the concentrate.The bottle gives ratios against a protection temperature. Winter needs a much stronger mix than the summer ratio, typically neat or close to it for hard frost protection.
Mix in a separate container if you can.Pouring concentrate and water separately into the car gives an unknown strength. Mixing in a jug first means you know what is actually in there.
Fill to the top.Unlike oil and coolant, there is no level to respect. A full bottle is exactly what you want, particularly before a long winter journey.
Run the washers before you drive away.It clears the old fluid out of the pipes and confirms both front and rear jets are actually working.
Check where the jets are pointing.They should land in the upper third of the swept area so the wipers carry the fluid down the screen. Many cars have adjustable jets that can be nudged with a pin.
Clear blocked jets carefully.A fine pin or a strand of wire, gently. Some cars have fan jets built into the wiper arm which cannot be poked and simply need flushing through.
Top up in autumn as a routine.Getting the winter strength in before the first frost is the whole point. Doing it in January means doing it on a frozen bottle.
Keep a spare bottle in the boot in winter.Motorway salt spray empties a screenwash bottle astonishingly quickly, and a service station bottle costs several times what a supermarket one does.

Tips

Ready-mixed screenwash is convenient but often only rated to a mild frost. Check the temperature on the label rather than assuming.
Point one jet slightly higher than the other so the spray covers the screen faster at speed.
A smeary screen after washing is usually a dirty screen or tired wiper blades, not weak screenwash.

Common mistakes

Filling with plain water — It freezes, which can split the bottle, seize the pump and burst the pipes, and it does nothing to shift road film and salt. It is also an MOT failure if the washers do not work.
Adding washing-up liquid — It contains salt, which corrodes, and it strips wax from paintwork. It also foams badly and can clog the pump and jets.

If it doesn't work

Pump runs but nothing comes out

Cause: Blocked jets or a disconnected pipe — Fix: Clear the jets with a fine pin, then check the pipe under the bonnet has not popped off its spigot.

No sound from the pump at all

Cause: A blown fuse, or the pump has failed — Fix: Check the washer fuse first — the handbook shows which one. If the fuse is sound, the pump needs replacing.

Washers work at the front but not the rear

Cause: Separate pump or a split pipe running to the tailgate — Fix: The pipe usually splits where it flexes at the hinge. This is a garage job, though a cheap one.

Screen smears badly when washing

Cause: Grease film on the glass or worn blades — Fix: Clean the glass properly with a dedicated glass cleaner and replace the blades if they judder.

Questions people ask

Can I use water in summer?

It is better than nothing but still a poor idea. Water alone will not shift the oily road film that causes glare, and it lets algae grow in the bottle and block the jets.

How strong should the winter mix be?

Look at the temperature ratings on the concentrate and choose one that covers the coldest night you realistically expect. Many concentrates need to be used neat or close to it for hard frost protection.

Is a frozen washer system an MOT failure?

The washers have to work at the time of the test, so a frozen or empty system will fail. It is also one of the most common and most avoidable failures there is.