What is OBD?
Cars contain increasingly complex electronic systems, making access to diagnostic information essential. Manufacturers therefore adopted the standard OBD2 port, now found on vehicles covered by OBD2 or EOBD requirements.
OBD stands for On-Board Diagnostics. Here is what the standard requires, what the check engine light does and what an OBD2 scanner can read.
The essentials in 4 points
- The same port for every car The 16-pin connector is standardised and must be placed inside the passenger compartment.
- A required warning light The MIL must signal any emission-related fault.
- Standardised fault codes Detected faults are stored as codes, identical across all makes.
- 10 diagnostic modes From live sensor readings and freeze frame data to fault code clearing and warning light resets.
Where does the OBD standard come from?
OBD (On-Board Diagnostics) was first introduced by the CARB (California Air Resources Board) to monitor vehicle emissions. Electronic engine control units and their associated sensors helped vehicles reduce those emissions.
The OBD standard states that the vehicle must, throughout its service life, permanently monitor the correct operation of the engine.
OBD / OBDI
Early on-board diagnostics monitored emission-related faults, but connectors and communication protocols were still largely manufacturer-specific.
OBDII 1996
Introduced in the United States to standardise diagnostic connectors, fault codes and communication protocols.
EOBD Europe
European OBD, based on OBDII and specific to European vehicles. This is the standard that applies to cars sold in Europe.
EOBD was introduced in Europe at the same time as the Euro 3 emissions standard. It states that emission-related failures must be signalled by a warning light on the dashboard, and that codes corresponding to the detected failures must be stored by the vehicle.
Standards and dates of application
Since Euro 3 and European directive 98/69/EC, the standard has applied from the following dates, depending on the vehicle:
| Vehicle type | Newly introduced engine types | All new vehicles |
|---|---|---|
| Light petrol vehicle (< 2500 kg) | 2000 | 2001 |
| Light diesel vehicle (< 2500 kg) | 2003 | 2004 |
| Light LPG vehicle (< 2500 kg) | 2003 | 2004 |
First column: applies to newly approved vehicle types. Second column: applies to all new vehicles in the relevant category.
Before those dates? Sometimes compatible anyway
Some vehicles built before the relevant requirement took effect may still be compatible, if the manufacturer adopted the technology early. The simplest approach is to check your model.
Commercial and company vehicles
They are also affected by this directive, but their application dates differ: 2006 / 2007.
The check engine light, or malfunction indicator lamp (MIL)
Euro 3 and the associated directive introduced a dashboard warning light requirement. The MIL, or Malfunction Indicator Lamp, uses an engine-shaped symbol specified in ISO 2575. It is amber or yellow, not red.
It warns of a problem with the vehicle's emission control system. The light may behave in the following ways.
Steadily illuminated
The control unit has detected and confirmed an emission-related fault. Consult the vehicle handbook: whether it is safe to continue driving depends on the fault and any other symptoms or warning lights.
Flashing
A fault that could damage vehicle components has been detected. Stop as soon as it is safe and follow the vehicle handbook's instructions. Reduced-power or limp mode may also be activated.
Intermittent
The warning light comes on and then goes out. An intermittent fault may still be stored in the control unit and should be investigated.
Off
The light is not reporting an active emission-related fault. This does not necessarily mean that no fault is stored or present.
A light that is off does not mean that no fault is present: some have very little effect on the operation of the vehicle, such as a malfunction in the glow plugs - see our OBD2 repair case studies.
The OBD diagnostic connector
The directive states that the connector must be placed inside the passenger compartment. Usually it is located under the steering wheel in the fuse compartment, or under the ashtray near the handbrake. You can then connect a diagnostic scanner to access the information.
Cannot find it? Our OBD port location guide shows where to look, with photos for each model.
Connector pinout
| Pin no. | Description |
|---|---|
| 1 | Reserved for manufacturer use |
| 2 | J1850 BUS+ (SAE) |
| 3 | Reserved for manufacturer use |
| 4 | Chassis ground |
| 5 | Signal ground |
| 6 | CAN High |
| 7 | K line (ISO) |
| 8 | Reserved for manufacturer use |
| 9 | Reserved for manufacturer use |
| 10 | J1850 BUS- (SAE) |
| 11 | Reserved for manufacturer use |
| 12 | Reserved for manufacturer use |
| 13 | Reserved for manufacturer use |
| 14 | CAN Low |
| 15 | L line (ISO) |
| 16 | Battery + |
Communication protocols
Understanding the OBD2 standard
Although the connector is standardised, vehicles may use different communication protocols, including ISO 9141-2, ISO 14230, SAE J1850 and ISO 15765. The firmware in ELM adapters handles these protocols. Our software interprets the messages using the diagnostic services defined in SAE J1979.
The protocols based on lines K and L
They all use the same physical connection, but differences in the data sent make them incompatible with each other.
ISO 9141-2
This protocol is used mainly by European manufacturers.
ISO 14230 (KWP2000 ou KW2000)
ISO 14230 is the successor to ISO 9141 and shares its main characteristics. Used mainly by European manufacturers. It contains two "sub-protocols" that differ in initialisation: slow init (5 baud init) and fast init.
KW1281, KW71 et KW82
Specified by SAE J2818, used mainly by German manufacturers (Audi, BMW, Volkswagen, Porsche) before EOBD became mandatory.
Protocols based on SAE J1850
Two variants, largely American.
PWM (SAE J1850)
Used mainly by Ford - not necessarily on Fords sold in Europe, which then use an ISO protocol.
VPW (SAE J1850)
Used mainly by General Motors.
Protocols based on CAN
The present and future of on-board diagnostics.
CAN (ISO 15765)
Widely used for OBD communication over CAN, providing faster, more flexible communication than older OBD protocols.
CAN (SAE J1939)
Used mainly for heavy vehicles: trucks, agricultural machinery, construction equipment.
On heavy vehicles - trucks, agricultural machinery, construction equipment - the diagnostic connector is usually different: it looks like this one.
The 10 OBD diagnostic modes
OBD defines 10 diagnostic modes, regardless of the communication protocol. Not every engine control module (ECM) supports every mode; support varies by vehicle. Our OBD2-compatible vehicle list includes results tested by users.
Mode 1
Current sensor values
This mode returns the current values of certain sensors:
- engine speed
- vehicle speed
- engine temperatures (air, coolant)
- oxygen sensor readings and fuel and air metering information
Each parameter has a PID (Parameter Identifier). For example, engine speed uses PID 12 in decimal, or 0C in hexadecimal. The 2007 version of the standard listed 135 PIDs; individual vehicles support only a subset.
See the PIDs supported per vehicle
Mode 2
Freeze frame data
This mode returns the freeze frame data of a fault: when a fault is detected, the ECU records the sensor data at the precise moment it appears.
Mode 3
Stored fault codes
This mode returns stored emission-related fault codes. The wider diagnostic code system uses 4 categories:
- P0xxx - powertrain faults (engine and transmission)
- C0xxx - chassis faults
- B0xxx - body faults
- U0xxx - network communication faults
Mode 4
Clear fault codes and reset the warning light
This mode clears eligible stored emission-related fault codes and resets the check engine light. It also resets associated diagnostic data and readiness status.
Clearing a code does not repair the fault. If the cause remains, the warning light can return on a subsequent journey.
Mode 5
Oxygen sensor monitor test results
This mode returns oxygen sensor monitor test results, mainly for petrol vehicles. On CAN-based vehicles, these results are accessed through Mode 6 instead.
Mode 6
On-board monitor test results
This mode returns on-board monitor test results, including checks on systems that are not continuously monitored.
Mode 7
Pending fault codes
This mode returns pending fault codes. After a repair, it can help check whether a fault is being detected again before it becomes a confirmed code. It uses the same code definitions as Mode 3.
Mode 8
Control of on-board systems or components
This mode allows a scan tool to request control of certain on-board systems or tests, such as an evaporative emission system test. Vehicle support is limited.
Mode 9
Vehicle information
This mode returns information about the vehicle:
- the VIN (Vehicle Identification Number)
- calibration information
Mode 10 (or Mode A)
Permanent fault codes
This mode returns permanent fault codes. Unlike codes reported in Modes 3 and 7, they cannot be cleared with Mode 4. The vehicle clears them after repairs when its relevant diagnostic checks confirm that the fault is no longer present.