Diagnostics
A warning light tells you something is wrong, not what. Snap-on equipment traces the actual fault before anyone quotes you for a part.
Find the fault, not just the symptom.
Every modern car relies on an engine management system that flags problems early, but only if someone reads it properly. We use Snap-on diagnostic equipment to trace fault codes across electrical, engine management and other onboard systems, whether you've got a warning light on or just want an annual check to confirm everything's running at its best.
Far more than the engine light.
A modern car runs dozens of control units, and each one logs its own faults. The Snap-on equipment reads across all of them, not only the engine light.
Start with the colour.
Every warning light comes on when you turn the key — that is the car testing its own bulbs, and it is completely normal. The ones that matter are the ones still lit once the engine is running.
What each light is telling you
The common ones, and what to do about each. Symbols vary a little between makes. If yours isn't here, your handbook will name it, or send us a photo of the dash.
The engine is overheating. Nearly always a lack of coolant or a fault somewhere in the coolant system.
What to do: Stop somewhere safe and let it cool. Carrying on with an overheating engine causes expensive damage, and sometimes damage that cannot be undone at all.
A problem with the oil — too little, or occasionally too much. Too little and parts wear and overheat; too much and they are over-lubricated and drag.
What to do: Check the level on the dipstick when the engine is cold and the car is level. If it is low, top it up with the right grade. If the light stays on with a correct level, stop driving it and call us.
The battery is not charging properly. Usually the alternator, the drive belt, or the power cables rather than the battery itself.
What to do: Head somewhere safe rather than continuing a long journey. As the voltage drops you can lose lights, power steering and ABS with very little warning.
Either the handbrake is still on, or the brake fluid is low, or there is a fault in the braking system.
What to do: Check the handbrake is fully off first. If it stays lit, treat it as urgent and stop — this is the one light never to gamble on.
The power assistance has failed. You will usually feel this before you see it — the steering goes heavy and the car is sluggish to turn, especially parking.
What to do: If you can still steer it safely, come straight to us. If not, pull over and call for assistance.
The airbag system is deactivated. Sometimes that is deliberate, if a rear-facing child seat is fitted. Otherwise it is a fault.
What to do: If you have not turned it off on purpose, get it looked at — the airbag may not deploy in a crash.
The catch-all. The engine management system has logged a fault and often puts the car into a reduced-power mode to protect itself. It covers everything from a loose fuel cap to a failing sensor, which is exactly why the symbol alone tells you nothing.
What to do: Book it in. Drive gently on the way, avoiding hard acceleration or braking. This is the light diagnostics exists for — the code behind it is what actually identifies the fault.
A fault in the anti-lock braking system.
What to do: Your brakes still work, but the anti-lock does not — so a hard stop on a wet or icy road can lock the wheels and you lose steering with them. Worth sorting before winter, not after.
Pressure in at least one tyre has dropped below where it should be. Mandatory on new cars since 2014.
What to do: Check all four against the pressures in your handbook or on the door pillar — they differ front to rear on a lot of cars, and the correct figure is not the one on the tyre wall.
The filter that traps soot in a diesel exhaust is clogging up. Common on cars that only do short, cold, stop-start journeys and never get hot enough to clear themselves.
What to do: Get it looked at rather than ignored. A blocked DPF left long enough stops being a cleaning job and becomes a replacement.
Flashing means the system is actively working — you are losing grip. Staying on solid usually means it has been switched off or has a fault.
What to do: Flashing in bad weather is normal and it is doing its job. Permanently lit in the dry is worth a check.
On a diesel this normally lights while the plugs pre-heat and goes out before you start. If it stays on or flashes afterwards, there is a fault.
What to do: If it flashes while running, book it in. Failing glow plugs show up first as hard starting on cold mornings.
Information, not warnings. Indicators, main beam, cruise control, lights on: a system is switched on and behaving. Nothing needed.
Reading the code is the easy part.
Anyone can plug in a reader and tell you a number. What you're paying for is what happens next.