The most important questions and answers on car diagnosis

Reading and correctly interpreting the diagnostics and fault memory of the car in CAREXPERT repair shop

Why car diagnostics and fault memory reading?

Most drivers are only subconsciously aware of the various indicators that flash on the dashboard when the ignition is switched on. If everything is working well, the lights will eventually go out again. It is all the more surprising when an engine check light (also known as a MIL (Malfunction Indicator Light)) stays on or suddenly comes on while driving. The light can either flash briefly (a temporary fault has been detected), stay on steadily (a permanent fault has been recorded) or flash rapidly (a serious fault has been detected). You can ignore sporadic faults for the time being, as sensitive electronics can be disturbed by special circumstances such as the weather or specific driving manoeuvres. Temporary faults are also initially recorded as fluctuating contacts due to corrosion on the connectors or broken cables. If the fault does not recur, it is usually cleared automatically.

If it happens again within a certain period of time, it will be considered as a permanent error, will be recorded in the error memory and the driver will be informed by a lit MIL message. The driver must then either visit a repair centre to read out the error memory, or do it himself using the on-board diagnostics access. If the engine tell-tale flashes rapidly, this is a serious error which can cause damage to other components. If possible, stop immediately and turn off the engine.

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On-board diagnostic structure

On-board diagnostics (OBD) basically consists of a large number of sensors that mainly monitor the proper functioning of the exhaust gas cleaning system. However, thanks to the increasing use of electronics in modern cars, on-board diagnostics have also become very important in everyday workshop life. Systems that used to operate independently of each other are now co-ordinated and checked against characteristic maps. On the one hand, this increases fuel efficiency, but it also makes it more difficult to solve problems when symptoms occur. On-board diagnostics thus include all electronic controls and sensors communicating via a common data network (CAN-bus). The incoming signals are checked and tested for plausibility. For example, an electronic throttle signal indicating 'full throttle' may in itself be perfectly correct. However, if, at the same time, the brake pedal sensor transmits a signal of maximum voltage, a conflict of objectives will occur, which will be interpreted by the on-board Diganose as an error.

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  • Car diagnostics

What is the purpose of error memory?

Not every error is an immediate fault, so any problems encountered will be monitored first for a period of time. This process is called "debounting". If a failure occurs again, it is treated as a fault and recorded in the fault log. If the fault memory is now read out, it is possible to know what the fault is and, with the help of the appropriate software, to obtain additional information, e.g. on the time of occurrence of the fault or the vehicle's operational status. This information will help an experienced mechanic to narrow down the causes and carry out repairs in a targeted way. By the way, only the fault memory is mentioned, although the actual location of the memory usually depends on the corresponding control unit. This distinction is simply not necessary for data handling and interpretation.

How is error memory read out?

Each on-board diagnostic unit has an interface, effectively a socket and a corresponding data protocol that allows it to connect a diagnostic device, laptop or smartphone to the car. Of course, this requires that the voltage supply to the control units is ensured, i.e. the ignition is switched on. The diagnostic test deals with all detected ECUs and displays the recorded faults. This is done either in coded form (OBD-2 standardised format) or directly as descriptive error text. You now have the option of simply deleting the error in the hope that it will not return, or fixing it immediately. Which is better, will require experience. Under no circumstances should you rely blindly on the error memory, as reported errors can have many causes, but more on this later. However, once a faulty component is diagnosed, it is replaced and the fault memory is cleared. Once the problem has been solved, the error no longer occurs.

How much does it cost to read out the error memory?

Reading out the error memory is a small job for which the service centre charges from €15 to €50. The price includes information on the possible cause, recommendations for correcting the error and clearing it. If the repair shop feels that further action is needed, a repair quotation is usually prepared.

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