What does a car diagnostic check?

A car diagnostic starts by plugging a scan tool into your car's computer to read the fault codes it has saved, then goes on to test the parts those codes point to, so we find the actual cause rather than a guess. The code tells us where to look. The testing tells us what's wrong.

That second part is the bit that matters, and it's the difference between a proper diagnostic and a quick code read.

What is a fault code?

Modern cars have computers that watch the engine, gearbox, brakes, airbags and more. When a reading goes outside its normal range, the computer saves a fault code and often turns on a warning light on the dashboard.

A code is a starting point, not an answer. A code for a misfire, for example, could be a spark plug, an ignition coil, an injector, a wiring problem or something else. Replace the part named in a quick reading and you might fix it, or you might just spend money on the wrong part and still have the light on next week.

What does a diagnostic actually check?

It depends on the problem, but a diagnostic usually covers:

1. Reading the codes from the engine computer, and from other systems where the car and the scan tool allow it, such as ABS or airbags.
2. Live data. We watch what the sensors are reporting while the engine runs: temperatures, fuel mixture, sensor voltages and so on. A sensor that reads wrong shows up here.
3. Freeze frame. Many cars save a snapshot of the conditions at the moment the fault happened, such as speed and engine temperature. That helps with faults that come and go.
4. Testing the suspect parts. Checking wiring, connections, sensors and components the codes point to, so we confirm the cause before suggesting a fix.
5. Looking and listening. Not every problem sets a code. Noises, vibrations, smells and leaks need a mechanic, not just a scanner.
6. A road test, where it helps, to see the fault happen the way you describe it.

Then you get a plain answer: what we found, what it needs, and the price to fix it.

What can't a diagnostic do?

Honest answer: some faults are hard to catch. A problem that only shows up once a week, on a hot day, on a hill, may not happen while the car is with us. Telling us exactly when it happens helps a lot. Nothing extra is done without your OK in writing.

A diagnostic also isn't a full check of the whole car. If you want everything looked over, that's what a service is for.

When should I get a diagnostic?

Book one when:

• the engine light is on, steady or flashing (flashing means ease off and get it checked straight away; our article on driving with the engine light on explains why)
• another warning light has come on and you don't know what it means; see our guide to dashboard warning lights
• the car is running rough, losing power, stalling or using more fuel than usual
• there's a new noise, vibration or smell you can't place
• the car has gone into limp mode, where it suddenly won't rev or go above a low speed

Red warning lights, especially oil pressure or temperature, mean stop driving and call us first.

Can't I just get the codes read at home?

You can buy a basic code reader, and some people find it useful. It will give you a code. It won't test the wiring, watch live data properly, or tell you which of the possible causes it is. Clearing the code also clears the light, which can hide a problem rather than fix it, and some faults need the stored information to track down.

Quick questions

Car diagnostics in Albany and the North Shore

Midas Albany is at 3C Saturn Place, Albany. Family owned since 2004, MTA Assured. Our $89 diagnostic finds the fault, and it's credited against the repair if we do it. The repair price is agreed before any work starts.

See our car diagnostics and the warning light guide.

Call Robin on 09 415 6595, book online, or request a callback. Monday to Friday 7:30am to 4:30pm, Saturday 8am to 12pm.

Written by the Midas Albany team.