This page is designed to explain how the European On Board Diagnostics 'EOBD' work.
'EOBD' explained.
EOBD, (European On-Board Diagnostic), is a system for warning the vehicle driver that there is a fault which may cause the emission levels to exceed those allowed by the European directive*. It also has the ability to store fault codes and 'Freeze Frame'* Data, concerning an emission related incident.
Basic rule of thumb, as there are variations on vehicle type..
From January 1st 2001 all new petrol engine vehicles should be equipped with EOBD.
From January 1st 2004 all new diesel engine vehicles should be equipped with EOBD.
* 70/220/EEC - Annex XI - Threshold Limits:
- Carbon Monoxide (CO)
- Hydrocarbons (HC)
- Oxides of Nitrogen (NOx)
The following items are monitored by EOBD:
Petrol engines:
- Catalyst
- Oxygen sensor
- Misfire detection
- Mixture regulation
- Evaporative purge system circuit continuity
- Any other component connected to the ECM, which if fails would cause the exhaust emissions to exceed the limits
Diesel engines:
- Catalyst
- Particulate filter
- Electronic fuelling system
- Any other component connected to the ECM, which if fails would cause the exhaust emissions to exceed the limits
- Calculated load*
- Engine speed
- Fuel/Air regulation Fuel Trim (if available)
- Fuel pressure (if available)
- Inlet manifold pressure (if available)
- Vehicle speed (if available)
- Engine coolant temperature
- Fuel contol system status(Open-loop, Closed-loop), (if available)
- The fault code that caused the the freeze frame data
The following signals are available through the diagnostic connector:
- Diagnostic fault / trouble codes (DTC)
- Engine coolant temperature
- Fuel control system status (closed loop, open loop, other)
- Fuel trim (if available)
- Ignition advance
- Intake air temperature
- Manifold air pressure (if available)
- Air flow rate
- Engine speed
- Throttle position sensor
- Secondary air status (upstream, downstream or atmosphere)
- Calculated load value*
- Vehicle speed (if available)
- Fuel pressure (if available)
*Calculated load value.
This value is a reference value only; calculated by the ECM from the current airflow divided by the peak airflow, (with altitude correction). It gives the technician an indication of the proportion of engine capacity being used. (Wide open throttle is 100%).
At the same time the distance travelled by the vehicle since the MIL was activated must be available at any instant through the serial port on the standard link connector, from 2003 if the vehicle is fitted with an electronic speed input. All cars from 2005.
Trouble codes are defined as:
| Single Trip codes | Which result in storing a Permanent code and illuminating the MIL (Malfunction Indicator Lamp) |
| Two Trip codes | Which result in the 1st fault registering as an Intermittent code. The 2nd consecutive fault changing it to a Permanent code and illuminating the MIL |
Note the following:
(DIRECTIVE 98/69/EC -/- 70/220/EEC)
- MIL flashing at 1hz, Catalyst-damaging Misfire detected
- MIL Illuminated (Permanent), a fault is detected
- MIL extinguishes when repaired and memory cleared
- MIL extinguishes if fault is no longer present after 3 consecutive trips, but code and Freeze Frame Data is stored for 40 warm up cycles
Warm up cycles
Warm up cycles are when the engine temperature rises at least 22°C above start up temperature and reaches a minimum of 70°C
Drive cycle
A drive cycle is a sequence of engine start up, followed by a driving condition where an emission related fault would be detected, if present and ending in engine shut off.
EOBD Disabled:
A manufacturer may allow the EOBD system to disable if its ability to monitor is affected by low fuel levels. Disablement must not occur when the fuel tank level is above 20 % of the nominal capacity of the fuel tank.
A manufacturer may allow the EOBD system to disable at ambient engine starting temperatures below -7 °C or at elevations over 2,500 metres above sea level provided the manufacturer submits data and / or an engineering evaluation which adequately demonstrate that monitoring would be unreliable when such conditions exist. A manufacturer may also request disablement of the EOBD system at other ambient engine starting temperatures if it demonstrates to the authority with data and / or an engineering evaluation that misdiagnosis would occur under such conditions.
Parameters within EOBD
Mixture regulation: (Air/Fuel Ratio)
Short Term Fuel Trim and Long Term Fuel Trim (STFT / LTFT)
STFT refers to instantaneous adjustments.
LTFT refers to gradual changes in fuel calibration as a result of engine differences over time.
The parameters are:
-100 % 0 % +100 %
Example:
If the intake manifold on an engine has an air leak, 'STFT' will increase, due to the increase of fuel required, compared to the default programmed values.
e.g.plus 10%.
If the leak is constant, 'LTFT' will increase, to plus 10% and 'STFT' will decrease to 0%
PSA EOBD 16 pin connector Layout, dependent on age and model, not all pins are used in the same way:
Pin 1 - Ignition positive
Pin 2 -
Pin 3 - Data Tacho / MUX network Late generation
Pin 4 - Chassis ground
Pin 5 - Chassis ground
Pin 6 - Data Climate control/CAN High ('IS' Late generation)
Pin 7 - K Line Data Engine management / Automatic gearbox management
Pin 8 - Cooling fan control information / MUX network Late generation
Pin 9 - Charge information
Pin 10 -
Pin 11 - Data BSI
Pin 12 - K Line Data ABS/ESP/GEP/Suspension control
Pin 13 - Data Air Bag/Headlight control
Pin 14 - CAN Low('IS'.. Late generation)
Pin 15 - Data Engine management
Pin 16 - Battery positive
'EOBD' Codes
The 'EOBD' memory chip stores a 5 character code, which may be recalled by a scan tool.
The first character shows which area of the vehicle generated the code.
| P | PowerTrain |
| B | Body |
| C | Chassis |
| U | Network |
The next Character can be a 0 or a 1 or a 2
| 0 | Standard (SAE) OBD code |
| 1 | Manufacturers own code |
| 2 | Manufacturers own code |
If the first character was P (PowerTrain) then the next will be a number, which identifies the specific PowerTrain system concerned.
| 0 | Fuel and Air Metering |
| 1 | Fuel and Air Metering |
| 2 | Fuel and Air Metering, Specifically Injector Circuit |
| 3 | Ignition System and Misfire Detection |
| 4 | Auxilliary Emission Controls |
| 5 | Vehicle Speed control and Idle Control System |
| 6 | Computer Output Circuit |
| 7 | Transmission Related Faults |
| 8 | Transmission Related Faults |
The last two characters are numbers which will identify the specific fault as seen by the onboard systems.
Last Reviewed: 16 April 2021Copyright © 2004...2021 Mark Stammers