The short answer
Connect the tool, let it identify the car and run the auto scan. Go into each control unit flagged in red, note the codes and check the freeze frame data. Fix the cause and only then clear the codes. Scan again after a road test to make sure they haven't come back.
Reading faults is what you will do most with your tool. It looks simple, and it is, but there is a big difference between "clearing the light" and diagnosing. Here is the method garages use so they don't waste time or money.
First, scan the whole car
Even if only one warning light worries you, scan every control unit. Many faults are linked: an ABS fault can turn on the stability control light, and low battery voltage leaves codes in half a dozen modules at once. Seeing everything together gives you the clue.
- Connect the tool and turn the ignition on without starting. If you are unsure, follow how to connect the tool.
- Identify the car by VIN.
- Choose Auto Scan. The tool works through every control unit one by one.
- When it finishes you will see the list: "Pass" or "OK" for clean units and the number of faults on the rest.
- Save or print the report before you touch anything. You will want it for comparison later.
How to read each code
Every fault has a code and a description. Codes starting with P are engine and gearbox, C chassis (brakes, suspension, steering), B body (airbag, comfort, lights) and U communication between control units.
| What you see | What it means |
|---|---|
| Current or present | The fault is happening right now. This is the one to deal with. |
| Stored or intermittent | It happened at some point and isn't happening now. It could be a loose contact or a one-off event. |
| Pending | The control unit has seen it but not confirmed it yet. If it happens again, it will turn on the warning light. |
| Freeze frame | A snapshot of the sensors at the moment of the fault: revs, temperature, speed, load. Pure gold for working out when it happens. |
The code tells you what the control unit detected, not which part is faulty. A P0171 (system too lean) doesn't mean "replace the lambda sensor": it could be an air leak, a dirty air flow meter or an injector. The freeze frame and live data help you narrow it down.
Tip. Search for the code together with the make and engine. Many faults are well known on a particular model and that can save you hours. We have guides to some of the most common ones, such as P0300 and P0420.
When to clear and when not to
Clear the codes once you have fixed the cause, or when the fault was clearly a one-off (for example, after changing the battery or unplugging a connector). Don't clear "to see if it comes back" without first noting the code and its freeze frame: you would be throwing away the information that helps you most.
- Inside the control unit, choose Clear codes. The auto scan usually has a quick clear-all option too.
- Confirm. Some control units ask you to switch the ignition off and on again.
- Read again. If the code reappears straight away, the fault is still present.
- Take the car for a drive and scan again.
Airbag and brakes. If the airbag or ABS light comes back on after clearing, don't ignore it. These are safety systems: find the cause before you drive.
The engine warning light
The amber engine light has its own guide, covering what it means when it flashes or stays on and what happens at your roadworthiness test: how to clear the engine warning light.
Which tool you need to read every control unit
A generic OBD2 reader only reads the engine and emissions. For ABS, airbag, automatic gearbox, climate control or comfort you need a tool that reaches every system. In the iCarsoft range, every multi-brand tool does, such as the CR Pro, as does every single-brand tool for your make, such as the MB V2.0 for Mercedes. If you also want to test components from the tool, have a look at the CR Pro+. We explain it in more detail in what a diagnostic tool can do.
