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What Shows Up on a Diagnostic Test?

Your car flashes a warning light. Something doesn’t feel right. You head to the garage and hear the words: “We’ll run a diagnostic test.”

But what does that really mean? Does it tell you exactly which part has failed, or is it more like a health report your technician interprets? Let’s take a closer look.

What a diagnostic test actually does

A diagnostic test plugs into your car’s OBD-II port, the gateway to its network of ECUs (electronic control units). The system records data whenever something runs outside normal limits.

From this, a diagnostic tool can access:

Fault codes (Diagnostic Trouble Codes, or DTCs).
Pending codes that haven’t yet lit up the dashboard.
Historical data showing previous issues.
Live sensor readings such as boost pressure, coolant temp, and fuel trims.

This gives a technician a snapshot of how your car is behaving, both now and in the recent past.

What shows up in a scan

The information revealed can cover a wide range of systems. For example:

Engine and emissions

Misfires, injector faults, oxygen sensor failures, blocked DPFs, or catalytic converter efficiency issues.

Electrical and electronic systems

Battery charging status, communication faults between ECUs, and irregular voltages picked up by sensors.

Safety features

ABS sensor readings, airbag module warnings, and stability control errors can all appear.

Drivetrain and transmission

Automatic gearbox solenoid faults, pressure control errors, or 4WD system irregularities.

Comfort and advanced modules

Climate control glitches, body control issues, and even ADAS features like parking sensors or lane assist can be flagged.

What it won’t show

Here’s where it’s easy to misunderstand diagnostics: the test doesn’t hand over a part number for replacement.

A code might say “cylinder misfire,” but the real cause could be a spark plug, a coil pack, an injector, or even a wiring fault. Likewise, a slipping clutch or worn suspension bushes won’t trigger fault codes at all, those issues are mechanical, not electronic.

And then there are intermittent problems: if they don’t happen during the scan, they may not show up.

Why live data is powerful

Beyond the codes, live data tells the bigger story. By monitoring readings in real time, technicians can:

Compare actual sensor outputs against factory specifications.
Spot unusual fuel trim behaviour or airflow irregularities.
Watch how systems respond during a test drive, not just in the workshop.

This is often the key to moving from “possible fault” to a clear, accurate diagnosis.

Why professional interpretation matters

Anyone can buy a cheap code reader online. But these only give a small slice of the picture, usually engine-related codes only.

At Technical Vehicle Solutions, we take diagnostics several steps further by combining:

OEM-level software with access to every system.
Advanced tools like oscilloscopes and smoke testers.
Years of experience in interpreting data across different makes and models.

That’s how we turn raw information into a genuine fix, not guesswork.

So, what shows up on a diagnostic test?

In summary:

Error codes – active, pending, or stored.
Real-time sensor data from multiple systems.
Communication health between ECUs.
A clear picture of where the problem lies, even if it doesn’t tell you the exact part to replace.

Diagnostics is the starting point of fault-finding, not the end of it.

Book a diagnostic test with the experts

Got a warning light, strange running issue, or intermittent fault? A diagnostic test is the first step to uncovering the cause. At Technical Vehicle Solutions, our advanced diagnostics go deeper than a simple code read, saving you time, money, and frustration.

📞 Call us today on 01763 230411 to book your diagnostic appointment.