Fig. 3.21 The structure of standard fault codes for OBD II

Fig. 3.21 The structure of standard fault codes for OBD II

The malfunction indicator lamp (MIL) should light up when the ignition is first switched on and then go out after about 3 seconds, during which time the ECM is performing a series of 'self checks'. After this, when the engine is running, the MIL should only light up when a malfunction occurs. If the MIL does not light up when the ignition is first switched on, it is an indication that there is a fault in the MIL, or in the ECM itself, assuming that the battery is not flat.

From the repair shop point of view, OBDII provides some features which should produce benefits. Examples of such benefits are: (1) a standardized diagnostic interface and connector (see Fig. 3.20); and (2) standardized fault codes. The fault codes, as presented at the scan tool, comprise five digits, e.g. P0125. Digit 1, at the left-hand end, identifies the vehicle system. Digit 2 identifies the subgroup. Digit 3 identifies the subassembly. Digits 4 and 5 identify the localized system components.

Figure 3.21 shows how a range of fault codes can be constructed by using the recommended standard approach.

The example quoted, i.e., P0125, has the following meaning under this coding system: 'insufficient coolant temperature for closed loop fuel control'. There are many hundreds of codes and full details are given in the SAE J 2012 publication.

3.3 Diagnostic equipment and limitations of DTCs

The preceding sections of this chapter may give the impression that a completely new set of tools and equipment is needed to deal with OBD II and possibly EOBD (European on-board diagnostics). Fortunately this is not necessarily the case because diagnostic equipment, such as the Bosch KTS300 machine that is shown in Fig. 3.22 is capable of dealing with fault code retrieval, analysis and diagnostic work on non-OBD II systems that are equipped with an ISO 9141 serial link and, with the aid of suitable adaptors, OBD II systems also. This and similar equipment is described in greater detail in Chapter 4.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

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