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Mechanics guide

How your car works inside, system by system, explained the way a friend who knows cars would explain it. At the top, what you need to know. Inside each page, if you are curious, the in-depth explanation.

Compact car drawn as an X-ray, in translucent grey, with the engine, exhaust, tanks and brakes visible
Illustration 1. The car used throughout the guide, as an X-ray: the engine at the front, the exhaust line underneath, the tanks under the rear seats and the brakes at all four wheels.

This guide comes from something that happens to me almost every day: someone plugs in an adapter, gets a code or an odd reading and has no idea whether it is serious, whether they can keep driving or whether the garage is about to give them a nasty bill. To understand what your car is telling you, you need to know, at least roughly, how the things under the bonnet work.

So here it is, system by system. Each page starts with what you need to know, without rambling, and below it you have the Open the bonnet button with the in-depth explanation for anyone who wants to understand the why of it all. The drawings are ours: always the same car and the same engine, in grey, with the part in question in colour, so you always know where you are looking.

For each system I also tell you about the faults we see most often and what you can check yourself with Final OBD, which is exactly why we made it.

The systems, one by one

  1. 01How an engine worksPetrol and diesel, and how hybrids, plug-ins and electric cars share and recover energy.
  2. 02Knock and pre-ignitionWhat knock is, the difference between detonation and pre-ignition and how the engine control unit corrects it cylinder by cylinder.
  3. 03Misfires, catalyst and lambda sensorsWhy a cylinder misfires, what a flashing engine light means and how the lambda sensors keep an eye on the catalyst.
  4. 04EGR and particulate filterWhy they fill up with carbon and soot, what a regeneration is and why removing them costs you in the end.
  5. 05AdBlueThe tank, the pump, the injector and the NOx sensors: how it works, why it crystallises and what happens if it runs out.
  6. 06Turbo and intercoolerHow the turbo works, what a big intercooler really gains, the blow-off valve and the oil cooler.
  7. 07CoolingWhere the coolant goes and what it cools, the thermostat, the pump, the radiator and air-cooled engines.
  8. 08Clutch and dual-mass flywheelWhy a clutch slips or judders, what a dual-mass flywheel is and which habits kill it early.
  9. 09Manual gearboxThe lever, the selector cable and the shift cable, and what happens inside the box when you select each gear.
  10. 10Automatic gearboxTorque converter, dual clutch and CVT: how each one works, the selector and their faults.
  11. 11AcceleratorCable or electronic: the two pedal sensors, the motorised throttle and limp mode.
  12. 12BrakesHow much pressure the pedal makes, discs and pads, the ABS and why the handbrake slackens.
  13. 13Air conditioningThe compressor and the gas circuit, why it stops cooling and how to look after it in winter too.
  14. 14BatteriesThe 12 V battery every car has, the hybrid one and the EV one: what they should read and how their health is measured.
  15. 15LightingHalogen, xenon and LED; which replacement LEDs pass the MOT and why an LED triggers a bulb failure warning.
  16. 16FuelThe tank pump, the high pressure pump, the filter and the injectors, on petrol and diesel.
  17. 17Oil and lubricationThe pump, wet and dry sumps, pressure, the filter and why good oil changed on time makes an engine last.
  18. 18Timing belt and chainBelt, chain and wet belt: when each one is replaced, why the chain stretches and what happens if it snaps.