7. Buying / selling a second hand car – Under the Bonnet Safety Check
Under the Bonnet: Fluids & Battery
A quick look under the bonnet reveals how well a car has been maintained. Checking your levels regularly can prevent a costly roadside breakdown.
Visit RAC Maintenance GuideSimple 3-Step Health Check
1
Check the Oil: Pull the dipstick out, wipe it clean, re-insert it, and check the level. It should be between the min and max marks. If it’s jet black and thick in a petrol car, a service is overdue.
2
Inspect the Coolant: Check the level in the expansion tank (never open it when hot!). The liquid should be clear pink, blue, or green. If it looks like muddy brown water, the cooling system is corroded.
3
The "Mayo" Check: Open the oil filler cap and look at the underside. A thick, white creamy substance suggests water is leaking into the oil—often a sign of head gasket failure.
Smart Thinking: The 12V Battery in EVs
Many local EV drivers don't realise their cars still have a traditional 12V lead-acid battery under the bonnet to run the lights and screens. If this small battery dies, the whole car won't start, even if the main drive battery is 100% full. Check for terminal corrosion just like a petrol car.
Fluid Condition Indicators
| Fluid Type | What to Look For |
|---|---|
| Brake Fluid | Should be a pale golden colour. If it’s dark brown, it has absorbed moisture and needs changing to ensure full stopping power. |
| Battery Terminals | Look for white, powdery corrosion. Clean terminals ensure the car will start reliably on cold Surrey mornings. |
| Power Steering | If the level is low, there is likely a leak in the rack. This is a common MOT advisory and can be an expensive repair if ignored. |
Smart Thinking: Be Wary of "Steam Cleaned" Engines
If an engine bay is sparkling clean but the rest of the car is dusty, the seller may have steam-cleaned it to hide active oil leaks. A slightly dusty engine is often more "honest" as it shows the car's true condition under normal use.