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On-Board-Diagnosis 2

1Lambda oxygen sensor pre-catalyst17Fuel pump relay
2Heating Lambda I18Canister-purge valve
3Lambda oxygen sensor after-catalyst19Camshaft adjustment
4Heating Lambda II20Injector 1
5Temperature coolant21Injector 2
6Temperature intake air22Injector 3
7Switch clutch pedal23Injector 4
8Switch brake pedal24Throttle valve servo motor
9Potentiometer exhaust gas recirculation25Valve exhaust gas recirculation
10Fuel supply pressure26Governor fuel pressure
11Knock sensor27Output stage/ignition coil
cylinder 1
12Hall-effect pulse generator camshaft28Output stage/ignition coil
cylinder 2
13Speed sensor crankshaft29Output stage/ignition coil
cylinder 3
14Sensor for inlet manifold pressure30Output stage/ignition coil
cylinder 4
15Accelerator pedal  
16Angle transducer throttle valve  


Since 1998 there is an EU regulation which prescribes a continuous examination and fault indication or storage during operation of all functional elements relevant for the exhaust gas. In addition, emergency run qualities should help to avoid complete breakdowns and further damages. It is in force since 2000 for petrol engines and 2004 for diesel engines. Everything is standardised:- the check light,- the plug-in connector,- the data transmission. Even the number of miles will be saved, completed after the first occurrence of an emission-related defect and the consequent illumination of the Mal function Indication Lamp.

How it works

The wiring diagram shows the most important components for an OBD-supervised petrol engine system. This starts with the insertion of the broadband lambda oxygen sensor in front of the catalytic converter. After it has been implemented, the Lambda value can be determined and its alterations can be observed within milliseconds. With it a diagnosis of the combustion per cylinder is possible. The sensors in the flywheel also allow to draw conclusions on the circularity of the engine and the performance of single cylinders. However, it is important that beforehand a learning process has been completed, meaning enough time for the control device to rule out all additional influences as for example jerking during the initial drive.

Controlling the exhaust gas recirculation, the activated carbon system and the secondary air blow is made more exact by the improved observation data of the Lambda sensor. Moreover the processes are similar. In certain time intervals the valves in question are opened and the alteration of the Lambda value is observed. If the expected change does not come about, the process is repeated after some time. Repeated deviations from the preset control device values leads to an error message with further consequences.

From the above it should be obvious why a simple error message read out is not sufficient as exhaust emission test for vehicles featuring OBD. Since these could have been deleted shortly before. Therefore, all systems must be looked at, without giving any attention to old problems. These test runs are programmed by the manufacturer in such a way that they do not take too much time in the workshop. It is not necessary to actually take the car for a ride.