Fault codes: how to identify and fix them at your shop
The check engine light came on and the customer arrived worried. From there, the speed and accuracy of the diagnosis shape their experience with the shop. Fault codes are the starting point of that diagnosis, not the final answer. Knowing how to interpret them, confirm the cause and record what was done is what separates an accurate repair from swapping parts blindly.
What fault codes are
Modern cars have electronic modules that monitor the engine, transmission, brakes, body and other systems. When a sensor reads a value outside the expected range, the module stores a diagnostic trouble code (DTC). These codes are read through the on-board diagnostics (OBD) port with an automotive scanner, and they help mechanics know where to start.
How to read an OBD code
In the OBD standard, a code has one letter and four digits. The letter indicates the system: P for powertrain, B for body, C for chassis and U for the network connecting the modules. The first digit shows whether the code is generic (0) or manufacturer-specific (1), and the rest identify the circuit or component involved.
How to fix fault codes efficiently
- listen to the customer: ask when the problem started, when it happens and whether there was any recent service
- read and record the codes before clearing anything, including freeze frame data when available
- confirm the cause: the code points to a symptom, not necessarily the faulty part. Test sensors, connectors and wiring before replacing components
- check the manufacturer's technical information for reference values and procedures
- make the repair and clear the codes
- road-test the vehicle and scan the modules again to make sure the fault has not returned.
A sound diagnosis cuts vehicle downtime, avoids rework and increases customer satisfaction.
Record the diagnosis to work more efficiently
Writing down the codes found, the tests performed and the fix applied on the work order creates a valuable history. If the car comes back, the team knows exactly what has already been checked. Over time, those records also reveal the most frequent faults by model and help guide estimates and inventory.
The Onmotor system is a Brazilian repair shop management system that stores each vehicle's service history, organizes work orders and speeds up the team's routine. It does not replace the scanner, but it makes sure the diagnosis is not lost.
Organize your shop with Onmotor
Want a more organized routine for diagnostics and repairs? Get to know Onmotor, browse more content on our blog or talk to our team.