What does a car diagnostic check actually involve?

By RPM Autocare, Newton Abbot · Published 26 October 2026

"We'll just plug it in and see" is something many drivers hear when a warning light comes on. Reading the fault codes is certainly part of the job, but it is only the starting point. This guide explains what a proper diagnostic check involves, so you know what to expect, what questions to ask and what you should get at the end.

Why a code is not an answer

Modern cars have a network of computers and sensors that monitor the engine, gearbox, brakes, airbags and more. When one of them sees something outside its expected range, it stores a fault code and may turn on a warning light.

The code tells you where the car noticed a problem, not necessarily what caused it. For example, a code pointing at a sensor could mean the sensor is faulty, but it could equally mean a wiring fault, a vacuum leak or a problem elsewhere that is making the sensor read oddly. If a garage simply replaces whatever the code names, you can end up paying for parts you did not need while the fault stays put.

As we put it on our diagnostics page: a warning light on its own does not tell you much. The useful part is working out why the code is there.

Step 1: talking to you

A good diagnosis starts with a conversation. The details you can give are often more valuable than anything on the screen:

• When does it happen? Cold start, when warm, at motorway speed, when turning left.
• How often? Every time, or now and then. Faults that come and go are among the hardest to trace.
• What else changed? Recent work, a new fuel, a fuel-up, an impact or a jump start.
• What lights are on? Steady, flashing, or one that came on and went off.

Writing a few notes or taking a photo of the dash before the light goes out can help.

Step 2: a visual check

Before connecting any equipment, a technician will usually look over the obvious: fluid levels, leaks, damaged wiring or hoses, loose connectors and anything that looks out of place. Many faults turn out to be simple once someone looks.

Step 3: reading the codes

Advanced diagnostic equipment is connected to the car and used to read the stored fault codes from the different control units. The technician notes what is stored, what is current and what is historic. On some cars, there are codes in modules you would not expect, which can point to the root cause.

This is also where the equipment matters. Different makes need different software to read everything properly. At RPM Autocare, Russ uses advanced diagnostic equipment, and his background as a VW and Ford main dealer trained technician helps on VW, Audi, SEAT, Skoda and Ford, where many faults are familiar. The same careful approach applies on every make.

Step 4: live data and testing

Next, the technician looks at what the car is doing in real time: sensor readings, fuel and air figures, temperatures, pressures. Comparing live readings against what they should be shows which part is not behaving. Depending on the fault, it can also involve testing circuits, checking components and carrying out a road test to reproduce the problem.

Step 5: finding the cause

This is the part that takes experience. The aim is to work out what is really wrong, so that the repair fixes the cause and not only the symptom. Sometimes the answer is a single part. Sometimes it is a fault that is triggering other faults, such as a failing sensor stopping a diesel particulate filter from regenerating.

Step 6: the report and the quote

At the end, you should get a clear explanation of:

• What was found.
• What it will cost to fix.
• How urgent it is, including what can safely wait.

That is how we work at RPM Autocare: we tell you what we found, what it will cost to fix and how urgent it is. If it can wait, we say so. You get a price before any repair work starts, and nothing is done without your agreement.

Common reasons people book a diagnostic check

• Engine management light on, steady or flashing.
• Limp mode or sudden loss of power.
• ABS, airbag or glow plug warning lights.
• Rough running, misfires or poor starting.
• Faults that come and go.
• DPF warnings on a diesel; see our DPF guide.

If you are not sure what a particular light means, our guide to warning lights explains the colours and the most common symbols.

Will you just clear the code?

Only if you ask us to. Clearing a code does not fix the problem, and it can make the next diagnosis harder because the evidence has gone. We find the cause first.

Can I keep driving?

It depends on the light and on how the car feels. If the engine light is flashing, the car is in limp mode or it is driving badly, stop and call. If it is on steady and the car feels normal, get it checked soon rather than leaving it. Red lights such as oil pressure, brake or temperature mean stop safely.

Stuck away from home?

Our work is done at the unit in Newton Abbot, but we have been known to come out to people who are stuck. One customer's car went into limp mode on her way to a week's holiday. We went out to her around 9.30pm, cleared the fault temporarily so she could get the car to us the next day, and found and fixed the fault that same day. We cannot promise a callout every time, so give Russ a ring and we will see what we can do.

Next steps

If you have a warning light, a fault that will not go away or a car that is not driving right, book in or contact us and describe what is happening. We look after cars from Newton Abbot, Kingsteignton, Torquay, Paignton and the wider Torbay area, and the more detail you give us, the faster we can get to the cause.

Related services: Diagnostics · Repairs · DPF regeneration · VW-group & Ford specialist