# OBD diagnostics: what it is and what a fault code shows

_Updated: 8 November 2026_

**Short answer:** OBD diagnostics means reading the faults the car has stored, through the OBD port under the steering wheel. A fault code tells you which part of the car has the problem, not which part to replace, and a cheap phone adapter mostly reads engine faults.

> **The service we recommend: Diagnostics** We read faults from every computer in the car and tell you what is urgent. We tell you the price when you tell us the model. [More about the service](https://autoindustrija.rs/en/services/car-diagnostics)

## What OBD diagnostics is

OBD diagnostics reads the faults the computers in the car have recorded. The computers constantly check the engine and the parts that clean the exhaust gases. When something is not working as it should, they record a fault and turn on the engine light on the dashboard.

The OBD port is where those faults are read. It has 16 contacts (pins) and the same shape on every car with OBD, so the same device plugs into any of them. It is most often under the steering wheel, on the driver’s side. [More about the OBD port](https://autoindustrija.rs/en/advice/where-is-the-obd-port)

OBD2 is the name of today’s OBD system, so the same thing is also called OBD2 diagnostics. In Europe it is called EOBD. On cars that have it, the Serbian technical inspection checks the system that reports faults in the exhaust emission equipment through the OBD port. That is the engine light. [More about OBD at the technical inspection](https://autoindustrija.rs/en/advice/relocated-obd-port-inspection)

**Whether a car has OBD2 depends on fuel and year**

- **Petrol:** from 2001.
- **Diesel:** from 2004.
- **Electric cars:** not required, so most do not answer the adapter.

## What a cheap ELM327 adapter does not see

ELM327 is an adapter that plugs into the OBD port and sends data to an app on your phone over Bluetooth or Wi-Fi. A cheap ELM327 adapter reads what the OBD2 standard requires: faults and data related to exhaust emissions. The other computers in the car follow the manufacturer’s rules, and they need a tool that works with your make.

That is why a phone adapter usually does not see this:

- **ABS, airbags, air conditioning and keys:** The ABS or airbag light is on, and the app says there are no faults.
- **The meaning of codes added by the car manufacturer:** The manufacturer decides what they mean, so a tool for your make explains them.

An adapter labelled “ELM327 v1.5” is a copy. The chip maker never made that version. The copy works like an older version, with its flaws.

An adapter left in the port draws power even when the car is off, so the battery can go flat when the car stands for days.

## What OBD diagnostics shows

The fault code is only the start. From the car’s computers we also read this:

- **Stored faults:** Faults the computer has seen and recorded.
- **Pending faults:** A fault the computer noticed during the last drive. After a repair, this shows whether the fault is coming back.
- **Engine conditions at the moment of the fault:** With the fault, the computer stores the revs, engine temperature and speed. This shows us under what conditions the fault appears.
- **Live engine data:** While the engine runs, we watch the revs, temperature and sensor values. This helps when the fault code alone does not say enough.
- **Whether the car has finished its own checks:** After the faults are cleared or the battery is disconnected, the car has to be driven for a while before it checks all systems again. Unfinished checks show that the faults were cleared recently or the battery was recently disconnected.
- **Distance since the faults were cleared:** Some cars remember how far they have driven since the faults were cleared.

## What a fault code means

A fault code has a letter and 4 numbers, for example P0300. The letter tells you which part of the car reports the fault:

| Letter | Part of the car |
| --- | --- |
| P | Engine and gearbox |
| C | Brakes, ABS, steering and suspension |
| B | Airbags, air conditioning and the rest of the cabin |
| U | The link between the computers in the car |

The first number after the letter tells you who defined the code. A code with 0 is the same on every car. A code with 1 is a car manufacturer code, so only a tool for your make reads it exactly.

For engine faults, with the letter P, the second number tells you which system reports the fault:

| Code starts with | What reports the fault |
| --- | --- |
| P01, P02 | Fuel and air |
| P03 | Ignition and engine misfire |
| P04 | Exhaust emission equipment |
| P05 | Speed and idle |
| P06 | Engine computer |
| P07, P08 | Gearbox |

The last two numbers tell you which fault it is within that system.

## The code tells us where to look, not which part to replace

The same fault can have several causes. P0171, for example, means the engine is getting too much air and too little fuel. The cause can be air getting in through a crack in a hose, a dirty sensor that measures the air, low fuel pressure or a bad lambda sensor, the sensor that measures oxygen in the exhaust gases.

That is why live engine data is checked before any part is replaced.

## Clearing a fault does not fix it

Clearing only turns off the light, and the fault stays. If the fault is not repaired, the fault and the light come back.

Clearing also deletes the record of engine conditions at the moment of the fault. Without that record, we cannot see under what conditions the fault appeared. That is why we clear faults after the repair, not before it. After clearing, the car starts its checks from the beginning.

## When to get OBD diagnostics

Get OBD diagnostics when a light comes on on the dashboard, because the car has then noticed a fault. These are some of those lights:

- **The engine light is on:** You find out what the fault is and whether you can drive on. [More about dashboard warning lights](https://autoindustrija.rs/en/advice/dashboard-warning-lights)
- **The light came on, then went out:** The fault stays recorded even when the light goes out by itself. The computer may erase it only when the same fault does not come back in at least 40 drives, so we read it even when the light is no longer on.
- **The ABS, airbag or another light is on:** We read the computer that turned the light on, not just the engine. [More about the ABS light](https://autoindustrija.rs/en/advice/abs-warning-light)
- **You are buying a used car:** Before you pay, you see the faults and whether they were cleared recently.
- **The technical inspection is in a few days:** You see in time the fault that the inspection checks.
- **The car has just been repaired:** We check whether the fault is really fixed, then clear the faults.

The engine light is flashing? The fault can damage the catalytic converter, an expensive part. Get in touch straight away. [More about the engine light](https://autoindustrija.rs/en/advice/check-engine-light)

## Through the same port a thief makes a key

Through the OBD port a mechanic reads faults and makes new keys, and a thief can buy the same device. OBD protection stops this theft: port relocation, an electronic blocker or a lock. [More about theft through OBD](https://autoindustrija.rs/en/advice/obd-car-theft)

## Frequently asked questions

### How much does OBD diagnostics cost?

We’ll tell you the price when you send the make, model and year, before any work starts. The price depends on whether only the engine or the whole car is read and whether your make needs a tool that works with it specifically. [More about the price of diagnostics](https://autoindustrija.rs/en/advice/car-computer-diagnostics)

### Do I get the fault explained in plain words?

Yes. You get a list of faults in words, without codes: what is urgent, what can wait and what is an old fault that has already been resolved.

### Can diagnostics damage the car?

No. Reading faults changes nothing in the car. Clearing faults is a separate step, and changing settings is a separate job. [More about coding](https://autoindustrija.rs/en/services/car-coding)

### Does diagnostics work if the car has OBD protection?

Yes. Before a service, do what your protection needs. With relocation, plug the adapter into the hidden socket. Switch off the blocker the way it was fitted: with an adapter with a code, with a fob or through the alarm. Unlock the OBD lock with its key. The diagnostic tool then connects to the OBD port, as before. [More about OBD protection](https://autoindustrija.rs/en/obd-protection)

Have a question about your car? Message us on WhatsApp or call 067 774 1606.

## Read also

- [Car dashboard warning lights: meaning and when to stop](https://autoindustrija.rs/en/advice/dashboard-warning-lights)
- [Check engine light (engine warning light): what it means and what to do](https://autoindustrija.rs/en/advice/check-engine-light)
- [Car won't start: what to check first](https://autoindustrija.rs/en/advice/car-wont-start)

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Page on the website: https://autoindustrija.rs/en/advice/obd-diagnostics-explained
auto-industrija, Belgrade. Phone and WhatsApp: +381677741606. E-mail: kontakt@autoindustrija.rs.