7.4 Vehicle Maintenance & Pre-Drive Checks
Key Takeaways
Section 6 of the Land Traffic Act forbids using a vehicle that is not in sound condition or could be dangerous, and requires the equipment the vehicle laws call for.
Check tyre pressure when the tyres are cold, and replace tyres before the tread reaches the wear indicators or when the sidewalls show age cracks.
Keep coolant between the LOW and FULL marks and never fill the reservoir to the top, because coolant expands when hot.
After driving through floodwater, test the brakes first by pressing them gently a few times at low speed.
A red dashboard warning light, such as oil pressure, brake, or coolant temperature, means stop as soon as it is safe.
7.4 Vehicle Maintenance & Pre-Drive Checks
Quick Answer: Section 6 of the Land Traffic Act forbids using a vehicle on the road if it is not in sound condition or could be dangerous, and requires it to have all the equipment the vehicle laws call for. Vehicle maintenance (การบำรุงรักษารถ) is one of the DLT practice-test categories. Before driving, check the tyres, lights, brakes, fluids (oil, coolant, brake fluid), battery, wipers, horn, mirrors, and seat belts. Classic DLT answers include: don't fill the coolant reservoir to the top, because the coolant needs room to expand; test the brakes first after driving through floodwater; cracks around the sidewall mean an old tyre; and never shift into reverse while the car is still moving.
A well-maintained car is less likely to break down in a dangerous place and more likely to stop, steer, and be seen when you need it to.
1. The Legal Basis
- Section 6, Land Traffic Act: No one may use a vehicle that is not sound and secure, that may be dangerous, or that may harm the health of its users or the public. It must have the equipment the Motor Vehicle Act and related laws require.
- Sections 9 and 10 bis: No excessively noisy vehicles, nothing dragging on the road, and no smoke, fumes, or noise above the limits set by notice.
- Tor Ror Or inspection: Private cars aged 7 years or more must pass inspection before annual tax is paid (1.3).
- Modified cars: The 2022 amendment lets police stop the use of an illegally modified car that is unsafe and order it repaired (sections 143 to 143/2).
2. A Simple Pre-Drive Check
Tyres
- Pressure: Check with the tyres cold, using the pressure on the label in the driver's door frame or in the manual. Under-inflation causes overheating, wear on the edges, poor handling, and blowouts. Over-inflation causes wear in the centre and less grip.
- Tread: Replace tyres before the tread reaches the wear indicators, which sit at about 1.6 mm. Below about 3 mm, wet grip drops sharply (7.1).
- Damage and age: Look for cuts, bulges, and embedded objects. Fine cracks around the sidewall are a sign of an old, hardened tyre (a DLT exam point). Replace it even if the tread looks fine. The date code on the sidewall shows the week and year of manufacture.
Lights
Check headlights (high and low), small lamps, tail lamps, brake lamps, indicators, hazard lights, and the plate lamp. Keep lenses clean. Dirty or cloudy lenses reduce how far you can see at night (7.2).
Glass, Mirrors, and Wipers
Wiper blades harden quickly in Thai heat and sun. Replace them before the rainy season, and keep the washer bottle full.
3. Fluids Under the Bonnet
| Item | How to Check | Key Point |
|---|---|---|
| Engine oil | Car on level ground, engine off, wait a few minutes. Pull the dipstick, wipe it, reinsert it, and read it | The level should be between the MIN and MAX marks. Top up with the grade in the manual. A red oil-pressure light means stop the engine safely at once |
| Coolant | Check the reservoir level with the engine cold | Keep it between LOW and FULL. Do not fill it to the top, because coolant expands when hot. Never open a hot radiator cap (7.3) |
| Brake fluid | Check the reservoir level against its marks | A falling level can mean worn pads or a leak. Have it checked promptly |
| Power-steering fluid, if fitted | Check the reservoir marks | Low fluid makes steering heavy and noisy |
| Washer fluid | Check the bottle | Clean water plus washer fluid, not dishwashing liquid |
Battery
Keep the terminals clean and tight. On older batteries that are not maintenance-free, top up with distilled water only, to the marked level. Many batteries have a colour indicator window showing their condition.
4. Dashboard Warning Lights
- Red lights mean stop as soon as it is safe: oil pressure, brake system, high coolant temperature, and charging system. Continuing to drive can destroy the engine or leave you without full braking.
- Amber lights mean check soon: engine management, ABS, tyre pressure, low fuel.
- Every light should come on briefly when you switch on the ignition and then go out. A warning light that never comes on may have failed.
5. Habits That Protect the Car
- Don't shift into reverse while the car is still rolling forward. It damages the gearbox, and it is a standard DLT exam question.
- Don't turn the steering wheel hard while the car is stationary. It wears the tyres and strains the steering.
- Don't rest your foot on the clutch or brake pedal while driving. It wears them out and, with the brake, lights your brake lamps and confuses drivers behind.
- Drive through water slowly, then test the brakes first. Gently press the brake a few times at low speed to dry them (7.1).
- Use engine braking on long descents instead of riding the brakes (2.3).
- Follow the service schedule in the owner's manual for oil changes, filters, brake pads, belts, and coolant.
6. Recognising Trouble Early
| Symptom | Possible Cause | Action |
|---|---|---|
| Car pulls to one side when braking | Uneven brakes or tyre pressure | Check tyre pressure and have the brakes inspected |
| Soft or sinking brake pedal | Air or leak in the brake system, low fluid | Stop using the car and get it repaired |
| Squealing or grinding when braking | Worn brake pads | Replace pads promptly |
| Temperature gauge rising | Low coolant, fan fault, blocked radiator | Pull over safely and switch off. Do not open a hot cap |
| Steering wheel vibrates at speed | Wheel balance or alignment | Have the wheels balanced and aligned |
| Heavy steering and the car leans to one side while driving | A tyre is deflating or has burst | Grip the wheel firmly, ease off the accelerator, and stop safely (7.3) |
DLT Theory Test Traps
- Coolant reservoir: Leave room for expansion. Don't fill it to the brim.
- After floodwater: Test the brakes first.
- Old tyres: Cracks along the sidewall mean the tyre has aged, whatever the tread depth.
- Reversing: Only when the car has fully stopped.
Why should you not fill the coolant reservoir all the way to the top?
Because a full reservoir makes the radiator rust faster
Because extra coolant makes the engine run hotter
Because coolant expands when it gets hot and needs room in the reservoir
Because the engine will not start if the reservoir is full
After driving through a flooded road, which system should you test first?
The starter motor
The brakes
The suspension
The air conditioning
What is the correct way to check the engine oil level?
Check it with the engine running so the oil is circulating
Check it straight after a long drive while the car is parked on a slope
Remove the oil filler cap and look into the engine while it idles
Park on level ground, switch off, wait a few minutes, then read the dipstick between its marks
Sections you finish are checked off in the contents.