Service Section Technical Pages

VEHICLE DIAGNOSTICS : Fault Codes or Error Codes

Many of the mechanical systems of modern vehicles are controlled by computer. The electronic control unit (ECU) computer managing the operation of a vehicle system may be connected to many input circuits from different sensors and also many output circuits of different actuators. A substantial amount of electrical circuits may be involved meaning any fault which occurs could be difficult to locate .

The vehicle computer system is programmed to monitor inputs from sensors, process information and correct system conditions by outputs to actuators.

Should a fault occur , the electronic control unit will in theory respond by logging a fault or  error code in computer memory . If the fault code is sufficiently important the control unit can activate illumination of a  dashboard warning light to alert the driver .

Fault codes only indicate an error as seen by an electronic control unit they do not tell you exactly what is wrong or which part to repair or replace. A fault code may only log after several driving cycles have taken place.

Fault Code Read using a Scan tool

The vehicle technician is  able to connect a diagnostic computer (scan tool) to the vehicle control unit via the vehicle diagnostic plug and read any stored fault codes which have been logged . The fault code logged should give a clue to the technician as to which system input or output presents a problem.

Some older vehicles (usually Japanese)  have a self diagnosis function allowing the the fault code to be read by counting the flashes of the check engine light and looking up the value in a reference table.

Fault with vehicle but no fault code logged

On some occasions electronic faults can happen faster than the vehicles electronic control unit can see the fault. Further investigation of a particular system may be necessary using signal test methods with an oscilloscope.

Historical fault codes
Intermittent faults which occur during driving may be logged as an error fault code
Random disconnecting of electrical connectors in the engine bay may also cause  fault codes to be logged.

Fault codes may be logged and stay in memory for a considerable time

Serial Data

 Most Scan tools can communicate with vehicle systems in real time allowing data to be read from sensors and actuators, though this function is limited by the processing speed of both scan tool and vehicle computer.

Data Logging and Freeze Frame Data

Data is often logged when a particular fault has occured and the parameters at which the fault has happened are recorded providing a snapshot of data as seen by the vehicle computer.

EOBD Fault codes

The term EOBD refers to European On Board Diagnostics and derived from OBD2. OBD2 trouble codes consist of a five digit alphanumeric identifier. The first letter identifies the control unit type setting the code:

  • P = Powertrain
  • B = Body
  • C = Chassis
  • U = Network

EOBD or European On Board Diagnostics.  Provides basic data relating to engine management systems on Post year 2000 vehicles in the european market.

EOBD standards apply to:

  • Petrol vehicles from May 2001 >
  • Diesel vehicles from 2003>
  • EOBD Fault codes , sometimes referred to as P....  codes are generally logged for factors which affect vehicle exhaust emissions. EOBD monitoring has various modes including Freeze frame Data, which can be useful to help diagnostics.Vehicle Manufacturer specific fault codes

    Manufacturer Specific Fault codes

    Specific fault codes relating to in depth component functions may not covered by EOBD

    Manufacturer specific fault codes provide more in depth information than the generic eobd fault codes.

    TSB's or Tech Service Bulletins

    Certain fault codes may be listed in manufacturers technical service bulletins as they may occur in a common sequence indicating known failure of a particular component.

    Fault Code Letter Prefix

    P =  Powertrain

    B = Body

    C = Chassis

    U = Network

    Second digit of fault code refers to a generic type

    O = Generic EOBD fault code

    1 = Manufacturer Specific

    Third digit / Letter of Powertrain codes

    0 = Fuel Air or Emissions

    1 = Fuel system or Air intake system

    2 = Fuel system or Air intake system

    3 = Ignition or Misfire

    4 = Exhaust emissions

    5 = Speed signals or Auxilliary inputs

    6 = Computer Control unit or Output circuit

    7 = Transmission

    8 = Transmission

    9 = Reserved code for later definition by SAE (Society of Automotive Engineers)

    A = Hybrid

    B through to F are for future allocation

    Third digit of code refers to sub system

    Fourth and Fifth Digits of Code :4th and 5th digit identify specific sub system

    Setting variable related to particular code

    Typical Fault Code


    Investigating PO400 further,  the code is set due to an exhaust gas recirculation system flow problem which could be caused by a blockage or flow restriction due to stuck egr valve, wiring issue, or blocked exhaust silencer etc.

    Clearing Fault Codes

    Fault codes may be cleared from control unit memory using the scan tool .

    This function is carried out after a repair has been completed