EOBD trouble codes are a part of the EOBD standard, more information on EOBD is available here: All about EOBD

What is a Trouble Code?

Trouble codes, error codes or fault codes are normally numbers that the vehicle generates to indicate that one of its systems is not working correctly.

This number can then be referenced to a textual description of the code, either directly through a diagnostic tool (SPi/ACR4) or via automotive technical manuals.

How EOBD trouble codes work

EOBD trouble codes have a fixed format, a letter followed by 4 digits:

P0403

The first digit indicates the system in this case P for Powertrain

• P - Powertrain (Engine)
• B - Body (Alarms/central control units)
• C - Transmission (Gearbox)
• U - Network (Connections between systems/CAN bus)

The second digit indicates the type of error code, 0 for this code

• 0 - Generic (part of the EODB specification)
• 1/2 - Enhanced (manufacturer code)

The third digit indicates the subsystem within the vehicle, 4 in this case.

For P codes (engine) this is the list:

• 1 - Fuel / Air
• 2 - Fuel / Air
• 3 - Ignition / Missfire
• 4 - Emission Control
• 5 - Vehicle or Idle Speed
• 6 - ECU
• 7 - Transmission
• 8 - Transmission
• 9 - Control Modules
• 0 - Reserved

The next 2 digits (03) define the error code within the subsystem, and this is used to look up in either manufacturer specific code lists or EOBD generic ones.

So for P0403 we know that there is a problem in the engine, it's a generic code, and to do with emission control. If we use a SPi or ACR4 or even look the code up in a book we get the trouble code description:

P0403 : Exhaust Gas Recirculation Circuit Malfunction

What about manufacturer specific codes?

Enhanced codes for example: P1401

We know that this is a powertrain code, it is an enhanced code, and is to do with the emissions subsystem, but for it to mean anything we need to know which manufacturer the code is for.

On an Audi the code description is: Exhaust Gas Recirculation Valve Short Circuit to Ground

But on a Ford the code is: Differential Pressure Feedback Electronic Sensor Circuit High Voltage

Both different! So it is important to know the manufacturer to identify the code fully.

What else can I find out?

On certain vehicles you can view freeze frame data, this gives you a component reading of related sensors or outputs at the exact time the fault occurred. This sort of information can guide you even closer to diagnosing the fault, especially if the fault is not present at the current time.

And remember..

Even though you have a description for the fault code now, that doesn't mean that the part is faulty, it could be an issue inside the ECU, the connections on the loom, the loom itself or it could be the part at fault. There have also be instances where the ECU software has been setting faults, because the tolerances of the testing it does were too tight for the component in question, requiring a dealer ECU software upgrade.

So along with trouble codes, check freeze frame information, components and follow a diagnostics process.

To help with diagnosing faults and recording the information that the customer tells you, take a look at the Free Workshop Diagnostics Check List which is available from the main SP Diagnostics site.