Introduction to On Board Diagnostics

Number of components of modern motor vehicles are now controlled by computer electronics and this has led to significant improvement of performance and reliability.
However, once things go wrong, it can be a real nightmare. Some symptom can be intermittent making diagnosis very difficult. Moreover, some symptom can sometime clear itself during diagnosis process, in which case the cause will be unknown until it strikes again, which could be couple of hours, days or even weeks.
Of course, the use of oscilloscope or special diagnostic equipment may come in handy for this sort of task but at the same time not all mechanics can have access to such equipment or facility.

Thus the engine management computer, commonly referred to as the ECU (Engine Control Unit) comes with a function which monitors the system network and when a fault is detected, it may switch to so-called 'safety mode' to guarantee safety as well as inform the driver as necessary and records the associated problem areas by means of 'fault codes'.

This is the concept of OBD (On Board Diagnostic) system, the self-diagnosis feature of the ECU. The stored 'fault codes' of the problem areas can be retrieved by carrying out a particular procedure.

Fault codes are displayed by either flashing a warning light on the dash board in certain intervals or by providing different voltage from the service socket etc.

This may seem like a wonderful system which gives solutions to all problems, however in reality this is not the case. It should be noted that the OBD is never perfect. The OBD may show that the system is running in perfect order by showing 'normal' code despite the fault is still present and visually evident. Furthermore, the later OBD systems has come a long way which is capable of detecting many areas which the earlier systems could not monitor, it still cannot detect every single fault on the car.
Therefore, it is vital that the OBD should be treated as an aid to diagnosis, not a solution.

Introduction to pre OBD II Toyota OBD system

Toyota's OBD system is one of the most user-friendly and no major changes has been made since the it was first introduced.

The design of service connectors may differ from model to model as well as year to year but the main procedures are mostly identical.
Early service sockets were round shaped and voltage meter had to be connected in order to retrieve the codes. The codes were interpreted by counting the number of times the voltage changes from 0 volts to 5 volts in given intervals.

However, later service sockets are mostly square shaped and do not require a voltage meter. Instead it blinks the 'Check Engine' warning light on the dash board.

Initially, fault codes were displayed in 1 digit codes, however as the OBD became capable of tracking more areas, it was no longer possible to cope with just 1 digit codes thus later models display codes in 2 digits. Furthermore, some models are equipped with dual mode detection, which in addition to the primary standard diagnostic function, the system has a secondary enhanced function which is capable of detecting intermittent faults.

Retrieving fault codes

2 different shape of service sockets

1. Turn the ignition key to 'ON' position and make sure the 'Check Engine' warning light on the dash board comes on.


2. Turn the ignition key to 'OFF' position.


3. Open the cover of the service socket located in the engine compartment.


4. Link the terminal marked TE1 and E1 together with a piece of wire.
Note: Terminals are often filled with grease to prevent corrosion and this sometimes creates difficulties seeing each terminals, hence care should be taken to ensure the right terminals are linked.


5. The 'Check Engine' warning light on the dash board should start to flash. The codes can be interpreted by counting the number of flashes.
   If the 'Check Engine' warning light does not flash, the vehicle may be fitted with one of the early OBD systems which require a voltage meter to retrieve the codes. In such case, '+' probe of the voltage meter should be connected to terminal marked VF and '-' probe to E1 terminal and count the number of times the voltage moves between 0 to 5 volts.


6. The code differs from model to model but if the system is normal, fault code no. '1' will shown where the 'Check Engine' warning light blinks continuously. In case of a fault(s), there will be a pause between the blinks. The first set of blinks show the first digit of the code and second set of blinks after the pause indicates the second digit of the code. For instance, code '41' will blink 4 times in a row and a pause, followed by a single blink. If there were more than one codes being recorded, it displays from the lower number codes and go back to the beginning after displaying all the relevant codes.

1. First of all, it is vital to check everything is fine under 'STANDARD MODE' diagnosis. Should there be a fault, that should be rectified before proceeding.


2. Any fault codes previously stored under 'STANDARD MODE' must be cleared.


3. With the ignition key in 'OFF' position, link the terminal marked TE2 and E1 together with a piece of wire .


4. Turn the ignition key to 'ON' position and ensure the 'Check Engine' warning light on the dash board starts to flash.
   Note: It is important that from this point onwards, the ignition key must never be switched back to 'OFF' position until the diagnosis is complete.


5. Turn the ignition key and start the engine. During this process, the starter motor signal is checked. If the car was started other than using the ignition key (i.e. push start), fault code indicating 'Starter signal error' will be displayed.


6. Take the car to a test run. During the test drive, the speed must exceed 5 km/h otherwise 'speed sensor' error will be recorded by the system.


7. After the test run, stop the car but even at this point, the ignition key must never be switched back to 'OFF' position quite yet.


8. Link the terminal marked TE1 and E1 together using another piece of wire.


9. The codes will be shown in the manner described in the previous section.

Clearing the fault code

Any fault codes stored in the ECU will remain until they are cleared. Whenever a fault is rectified or a routine service has been completed, it is vital to clear the fault codes.

For most vehicles, removing the EFI fuse for at least 10 seconds will clear the codes stored in the ECU. For certain cars with diesel engines, it may be necessary to remove the fuse named ECD or AM2 instead.
However, there may be a few exceptions where codes cannot be cleared in this manner. In such circumstance, there should be a fuse named ECU which should be removed to clear the code. Alternatively, disconnecting the battery or ECU connector will undoubtedly clear the code(s).

After clearing the code(s), run the tests again to ensure they are actually cleared. If the code(s) still remains, either the clearing procedure is incomplete or the fault area(s) has not been rectified.