Car diagnostics: when you need them and what they tell you

When you need a diagnostic check#

• A warning light has come on and stays on: engine, ABS, airbag, gearbox, brakes.
• The car behaves differently: less power, a rough idle, hesitation, higher fuel use or hard starting.
• The automatic gearbox shifts harshly or late.
• An electronic system has stopped working: parking sensors, air conditioning, windows, central locking.
• Before you buy a used car, to see which faults it has stored.
• Before the ITP (Romania's periodic roadworthiness test), if a warning light is on: better to find out in good time what's wrong.

Dashboard warning lights: what the colours tell you#

Every manufacturer has its own warning lights, and the exact meaning is in the owner's manual. As a general rule, the colour tells you how urgent it is:

How a diagnostic check is done#

Modern cars have several computers: for the engine, gearbox, brakes, airbags and comfort systems. When a sensor reports a value outside its limits, the computer stores a code and, if the problem matters, lights a warning lamp.

1. Reading the codes: the mechanic plugs the scan tool into the diagnostic socket, usually under the dashboard near the steering wheel, and reads the faults from every computer, not just the engine's.
2. Live data: they watch the sensor readings with the engine running or on a test drive.
3. Testing: based on the codes and the data, they test the suspect parts, connectors and wiring, sometimes with separate measuring equipment.
4. The verdict: they tell you what they found, what needs fixing and in what order.

Professional scan tools and access to the manufacturer's information make the difference on more complex systems. Ask the garage whether it works on your make and whether its tool can see all of the car's computers.

Why a fault code isn't a diagnosis#

A code tells you what the computer noticed, not why. A code about a sensor giving odd readings may mean a faulty sensor, but also a broken wire, a corroded connector or a real problem in the system the sensor measures. Replace only the part named in the code and you may pay for a good part and still have the fault.

• Several codes can share one cause: a weak battery or a poor earth can trigger faults in several systems at once.
• Old codes stay stored: some are historic, from a problem already fixed.
• Clearing codes fixes nothing: the light may go out for a while and come back if the fault is still there.
• Not every fault leaves a code: worn brake pads, shock absorbers or a worn clutch show up in a mechanical inspection.

Don't just ask for the codes to be cleared. Ask for the cause: a warning light switched off by force can hide a problem that is getting worse.

What to tell the mechanic#

The mechanic wasn't in the car with you. The more precisely you describe what happened, the faster and cheaper the search.

1. What you noticed: the warning light, the noise, the smell, how the car pulls or shifts.
2. When it happens: cold or warm, at start-up, when accelerating, at a certain speed, in the rain.
3. Since when: it appeared suddenly or got worse over time.
4. Recent work on the car: a repair, a replaced part, a new battery.
5. Whether the light flashes or stays on; take a photo of the dashboard if you can, once safely stopped.

Ask up front what the diagnostic check costs and whether it comes off the repair bill if the same garage does the repair.

A diagnostic check before you buy a used car#

A check at a garage you choose, not the seller, shows which faults the computers have stored, including ones that don't light a warning lamp. Combine it with a mechanical inspection on a lift and with the service history: the invoices, the mileage at each job and, if the car is on CariX, its car passport. If you find codes cleared very recently and a seller in a hurry, ask questions. Also check whether the car has an open recall campaign.

How CariX helps#

CariX doesn't read the car's fault codes: the garage does the diagnostic check with its own tool. In the free driver account we help with the rest.

• Find the garage: search by service and city and see opening hours, services with price ranges and reviews, which come only from customers after a finished job. A booking request needs only a name and phone number.
• "Is the price fair?" shows an indicative price range for a diagnostic check in your city, from the prices listed on CariX; the real price depends on the car and what is tested.
• Warranties: if the garage records a warranty on the job, you see it under "Active warranties" on the car and get an email 30 days before it ends (for a warranty in months), so you can go back to the same garage if the fault returns.
• The car passport keeps the jobs recorded by garages, with the mileage they confirmed, useful both when you buy and when you sell.

Getting the car ready for its inspection? Read what is checked at the ITP too.