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 Nissan OBD

Nissan's OBD system has changed couple of time since its first introduction. There are number of types and methods depending on the year, model and specification of the intended country. Here, the main three types of OBD system is discussed. Unfortunately, due to complication and different system being employed for each electronic carburettor models, the procedures for models with electronic carburettor models are not featured in this section.
The brief characteristics of the three main OBD systems are as follows;

TYPE I
TYPE I OBD was used until around 1987, depending on the model. Retrieving and clearing of the codes are done through the ECU box itself.

TYPE II
TYPE II OBD was used from around 1987 to 1995, depending on the model. Retrieving and clearing procedures are similar to that of the TYPE I OBD but comes with multi detection mode for more precise diagnosis.

TYPE III
On some models, TYPE III OBD appeared as early as 1989 onwards. Retrieving and clearing of the codes could be carried out via the in-car service socket or the ECU box itself.

Retrieving and Clearing the fault codes

Procedures for retrieving and clearing the fault codes differs depending on the system being TYPE I OBD, TYPE II OBD or TYPE III OBD.

1. Turn the ignition key to 'ON' position.


2. Gently turn the 'CONTROL SCREW' clockwise with a suitable flat screw driver until it stops (See Diagram above).
   Note:The CONTROL SCREW must be treated gently as forcing it will result in destruction.


3. Start the engine without touching the accelerator pedal.


4. Move the gear lever from Neutral to 1st and back to Neutral. For automatics, from 'N' to 'D' and back to 'N' again.


5. Where fitted, switch on the air-conditioning and switch it back off again.


6. Drive the car with speeds above 10 km/h.


7. Stop the car but with the engine running, check for codes.

1. Turn the ignition key to 'ON' position.


2. Gently turn the 'CONTROL SCREW' on the ECU anti-clockwise with a suitable flat screw driver until it stops.
   Note:The CONTROL SCREW must be treated gently as forcing it will result in destruction.


3. Leave it for over 2 seconds.


4. Start the engine without touching the accelerator pedal.


5. Move the gear lever from Neutral to 1st and back to Neutral. For automatics, from 'N' to 'D' and back to 'N' again.


6. Where fitted, switch on the air-conditioning and switch it back off.


7. Drive the car with speeds above 10 km/h.


8. Now all the fault codes should be cleared. Once all the codes are cleared, it will show code '44' (Normal code) to show that everything is fine.

TYPE II OBD

Fault codes retrieved will be shown by the number of flashes shown by the green and red LED indicator on the ECU box itself. Therefore, it may be necessary to remove some trims and fittings to locate the ECU.
The TYPE II OBD is pretty similar to the TYPE I OBD, but the main difference being that the TYPE II OBD has 5 detection modes;

• MODE 1
  This is the default mode. In 'MODE 1', certain models may switch to 'emergency mode' which enables the car to be driven with pre-set variables even if one of the components such as crankshaft position sensor becomes defective. In such event, the engine warning light will notify the driver and the red LED indicator on the ECU will flash showing the fault code(s).


• MODE 2
  'MODE 2' indicates the fuel mixture feedback. The red and green LED indicator show whether the fuel mixture is optimised by the airflow meter etc.


• MODE 3
  'MODE 3' is the standard diagnosis mode where stored fault code(s) can be retrieved.


• MODE 4
  'MODE 4' is the ON-OFF diagnosis mode where relevant sensor signal can be manually tested.


• MODE 5
  'MODE 5' is the progressive mode where signals from components such as crankshaft position sensor, airflow meter etc. is tested while the vehicle is being driven. The fault code(s) will show up as it occurs however, it does not store the code(s) in the ECU memory unlike the other modes, thus only suitable for tracking fault(s) which occurs intermittently.

1. Turn the ignition key to 'ON' position.


2. Gently turn the 'CONTROL SCREW' clockwise with a suitable flat screw driver until it stops (See Diagram above).
   Note:The CONTROL SCREW must be treated gently as forcing it will result in destruction.


3. The green and red LED indicator will start to flash together. The number of times those two LED indicators flash together is the mode. Flash once = 'MODE 1', flash twice = 'MODE 2'... The modes will constantly change until particular mode is selected.



4. To select the mode, turn the 'CONTROL SCREW' half a turn to the left within a second after the number of flashes of the mode you wish to select. If the you couldn't turn the 'CONTROL SCREW' in time, it will shift to the next mode, so you need to wait until it comes back to the mode you wish to select.

1. Turn the ignition key to 'ON' position.


2. Gently turn the 'CONTROL SCREW' clockwise with a suitable flat screw driver until it stops (See Diagram above).
   Note:The CONTROL SCREW must be treated gently as forcing it will result in destruction.


3. The green and red LED indicator should start flashing simultaneously. Wait until they flash 3 times and soon after, give the 'CONTROL SCREW' about half a turn anti-clockwise. In the event the timing is missed, it will flash 4 times to go onto 'MODE 4' and 'MODE5' subsequently before eventaully coming back to 'MODE 3'.


4. Once 'MODE 3' is selected, the green and red LED indicator will flash independently to show the fault code(s) stored.

1. Turn the ignition key to 'ON' position.


2. Gently turn the 'CONTROL SCREW' on the ECU anti-clockwise with a suitable flat screw driver until it stops.
   Note:The CONTROL SCREW must be treated gently as forcing it will result in destruction.


3. Wait until the green and the red LED indicator flash together for 4 times.


4. Once it gets passed 'MODE 4', the codes should be cleared.


5. Run the test again under 'MODE 3' described in the previous section to verify whether the fault code(s) are cleared successfully.


6. Once all fault codes are successfully cleared, either code '44' or '55' will show up.

1. Turn the ignition key to 'ON' position.


2. Gently turn the 'CONTROL SCREW' clockwise with a suitable flat screw driver until it stops (See Diagram above).
   Note:The CONTROL SCREW must be treated gently as forcing it will result in destruction.


3. The green and red LED indicator will start to flash together. Wait until it flashes 4 times and soon after, give the 'CONTROL SCREW' about half a turn anti-clockwise. In the event the timing is missed, it will flash 5 times to go onto 'MODE 5' before eventaully coming back to 'MODE 4'.


4. Once 'MODE 4' is selected, the green and red LED will stop flashing where the system is now ready for testing.

- Ignition switch testing -
5. Turn the ignition key to 'START' position and start the engine. When the ignition key is in 'START' position, the red LED indicator should come light up. If not, the ECU is not receiving the 'START' signal, therefore related areas should be inspected.

- Idle switch testing -
6. Depress the accelerator pedal to see check whether the red LED indicator lights up. If not, the ECU is not receiving the throttle position sensor signal, therefore related areas should be inspected.

- Speed Sensor testing -
7. Drive the car with speeds over 20 km/h. Once speeds over 20 km/h is reached, the green LED inidicator should light up. If not, the ECU is not receiving the speed sensor signal, therefore related areas should be inspected.

1. Turn the ignition key to 'ON' position.


2. Gently turn the 'CONTROL SCREW' clockwise with a suitable flat screw driver until it stops (See Diagram above).
   Note:The CONTROL SCREW must be treated gently as forcing it will result in destruction.


3. The green and red LED indicator should start flashing simultaneously. Wait until they flash 5 times and soon after, give the 'CONTROL SCREW' about half a turn anti-clockwise. In the event the timing is missed, it will flash once and goes onto 'MODE 1' and 'MODE2' subsequently before eventaully coming back to 'MODE 5'.
4. Once 'MODE 5' is selected, the green and red LED indicator will stop flashing.


5. Test run can now be carried out to check for possible faults. Should there be any faults, the fault code will immediately up by the red and green LED indicators in the manner described previously.


6. The fault code(s) will be repeated 3 times. If the code(s) remains after 3 times, it means the fault is permanent.

1. Turn the ignition key to 'ON' position and ensure the 'Check Engine' warning light comes on.


2. Link the terminals 'CHK' and 'IGN' on the service socket together with a suitable piece of wire for more than 2 seconds and release the wire.


3. The 'Check Engine' warning light starts to flash, and the system is now in detection mode.


4. Check the fault codes.

1. Once again, link the terminals 'CHK' and 'IGN' together with a suitable piece of wire for more than 2 seconds and release the wire. This switches the ECU to default mode.


2. The 'Check Engine' warning light stops to blink, and the code(s) should be cleared.
3. Once again, link the terminals 'CHK' and 'IGN' together and switch the system to detection mode.


4. The 'Check Engine' warning light will start to blink, and the code '55' should show up to indicated that the clearing was successful.

1. Turn the ignition key to 'ON' position and ensure the red LED indicator on the ECU and the 'Check Engine' warning light or 'EXH Temp' warning light comes on.


2. Gently turn the 'CONTROL SCREW' clockwise with a suitable flat screw driver until it stops (See Diagram above).
   Note:The CONTROL SCREW must be treated gently as forcing it will result in destruction.


3. Leave it for more than 2 seconds and turn it back anti-clockwise until it stops.


4. The red LED indicator ('Check Engine' or 'EXH Temp' warning light) will start to flash and the code is ready to be read.


5. The method of interpreting the code is the same as described previously.

1. Gently turn the 'CONTROL SCREW' clockwise with a suitable flat screw driver until it stops.
   


2. Leave it for more than 2 seconds and turn it back anti-clockwise until it stops.


3. The red LED indicator ('Check Engine' or 'EXH Temp' warning light) should stop flashing.


4. Once again turn the 'CONTROL SCREW' clockwise with a suitable flat screw driver until it stops.


5. Leave it for more than 2 seconds and turn it back anti-clockwise until it stops.


6. Once the clearing is successful, the default code '55' will be shown.