The short answer
Tyre pressures and tread, oil level, coolant level, screenwash, and all the lights. Together they take five minutes and prevent most roadside breakdowns.
- Tyre pressures, cold, against the figures in the door shut or the manual, not the number on the tyre itself.
- Tread depth across the whole width. The legal minimum is 1.6mm across the central three quarters, and a 20p coin is a rough gauge: if the outer band is visible, get them checked.
- Oil level on the dipstick, with the car level and the engine cold.
- Coolant level between the marks on the expansion tank, checked cold only.
- Screenwash topped up with proper screenwash, not water.
- Lights, all of them, with someone watching or against a wall or window at night.
What will ruin it
Never open a hot coolant expansion tank. The system is pressurised and the coolant is above its normal boiling point, so removing the cap causes it to flash to steam and it causes serious scalds every year.
Why it works
Nearly all roadside breakdowns come from tyres, battery or fluids, and all three give warning if anybody looks. A monthly five minutes converts a motorway breakdown into a driveway job.
Gas work requires Gas Safe registration by law and most electrical work is notifiable. Where a tip says to call somebody, that is the advice. Full disclaimer.