25-pin data link connector diagnostics

The purpose of this test is to observe the Diagnostic Trouble Code (DTC) flash codes that are generated at the Mazda 25-pin Data Link Connector (DLC).

Figure 1 - Connection Diagram

Figure 2 - Connecting 'TEN' to 'GND'.

How to perform the test

Please note that this sequence is based on the 25-pin DLC fitted to Mazda variants as listed in the Fault Code Table.

1. Plug one BNC test lead into Channel A of the oscilloscope.
2. Connect a Back-pinning Probe to the colored plug on the BNC test lead, and probe it into the 'B+' terminal on the DLC socket.
3. Connect another Back-pinning Probe to the black plug on the BNC test lead and probe it into the 'FEN' terminal on the DLC Socket as shown in Figure 1.
4. Using a suitable piece of insulated wire, connect one end to the DLC socket into the 'TEN' terminal and connect the other end to the 'GND' terminal.
5. Press the spacebar on the PC to start the scope displaying live data.
6. Switch on the vehicle ignition and follow the instructions below. The codes will then appear on the screen, as shown below.

Accessing fault codes from the PCM memory

Once the scope has been correctly connected to the 25-pin DLC socket:

1. Switch the ignition on.
2. The codes will now be displayed on the screen. Once the first code is repeated, this indicates all codes have been read from the PCM.
3. Switch ignition off.
4. Analyze any codes retrieved from PCM.

Erasing fault codes from the PCM memory

1. Ensure the ignition is switched off.
2. Disconnect the battery earth cable for at least 30 seconds.
3. Depress the brake pedal for at least 20 seconds (total 50 seconds).
4. Reconnect the battery earth cable.
5. Repeat the code reading sequence to ensure all DTCs have been removed from the memory of the PCM.

WARNING: Disconnecting the battery may erase the memory from ancillary devices such as clocks and radios.

Important Note: This sequence is for the Mazda variants listed in the Fault Code Table. For other manufacturers, check if the vehicle is equipped with a 25-pin DLC connector and also check to see if the above test can be carried out. Serious damage to the PCM will occur if the test is carried out on a vehicle that does not support the above test method. Refer to the manufacturer's wiring and technical information for further support.

Example waveforms

Example waveform 1 - No fault codes present (P0000)

Example waveform 2 - Fault codes present (P0110)

Example waveform 3 - Multiple fault codes present (P0100 - P0110 - P0120)

Waveform notes

How to interpret the codes from the example waveforms

Figure 3 - How to read fault codes

The sequence begins with a long pulse, indicating the start of the code-reading sequence from the PCM. The example waveforms show codes in the 4-digit EOBD/OBDII format, but other variants of Mazda give a two-digit code flash sequence. Consult the manufacturer's technical information for two-digit codes.

The 4-digit code in Code Sequence 1 is indicated by 4 groups of signals.

Digit 1

• The first digit is a single, long pulse (indicated by a white arrow)
• As there is no short pulse, the first digit is a 0.

Digit 2

• Digit 2 consists of a long pulse (white arrow) followed by a short pulse (black arrow).
• This pattern indicates that the second digit is a 1.

Digit 3

• Digit 3 is again just a single long pulse, indicated by a white arrow.
• This pattern indicates another 0.

Digit 4

• Digit 4 is also a single long pulse (indicated by another white arrow).
• This digit is another 0.

Putting these four digits together gives Code 0100, which is the EOBD Code for "Mass Air Flow (MAF) sensor/volume air flow (VAF) sensor circuit malfunction".

Example waveform 1

There are no pulses after the start pulse, indicating that all digits are 0. The table below tells us that:

Code P0000 = EOBD Code for No Fault Found.

Therefore there are no fault codes present on the vehicle.

Example waveform 2

There is one code that is repeated as it is the only code stored in the PCM. The table below tells us that:

Code P0110 = EOBD Code for Intake Air Temperature (IAT) Sensor.

Example waveform 3

The PCM is signalling 3 codes. The table below tells us that:

Code P0100 = EOBD Code for Mass Air Flow (MAF) sensor/volume air flow (VAF) sensor circuit malfunction.

Code P0110 = EOBD Code for Intake Air Temperature (IAT) Sensor.

Code P0120 = EOBD Code for Thottle Position (TP) sensor A/Accelerator pedal position (APP) sensor A circuit malfunction.

Notes

• A long pulse followed by 2 short pulses indicates a '2', as shown in example waveform 3. Higher digits are obtained by adding more short pulses.
• Each 4-digit code is separated from the next by an 8-second delay to allow you to tell them apart.
• If only one code is stored in the PCM, then this code will be continuously repeated as a single fault code.
• If more than one code is stored in the PCM, all codes will be read and then repeated untill the sequence is stopped.

Figure 4 - Engine management control

Technical information

The engine management system is constantly monitored by the PCM as the engine is running under different loads and conditions. All of the input and the output signals are monitored at all times. This is done so that the PCM can keep control of the engine and maintain optimum running conditions at all times. The PCM checks the values from the sensors and compares them with the information stored in the ROM (Read Only Memory) of the PCM. Based on the information stored, the PCM changes the commands to the actuators to alter any parameters to keep optimum running.

While the PCM is reading from ROM and reading all the signals from the sensors and actuators, thus monitoring the operating parameters of the engine, it is also monitoring the tolerances and ranges of each sensor and actuator to see if they are working correctly. When the PCM sees that a component or tolerance is breached then it will initiate a DTC (Diagnostic Trouble Code) which is stored in the RAM section of the PCM, often known as KAM (Keep Alive Memory), or in the EEPROM (Electronically Eraseable Programmable Read Only Memory). The DTC is then stored until action is taken: the fault is rectified on a self-check system, or an instruction is sent to the PCM to clear the DTC and the system is rechecked to ensure that it is operating correctly again.

There are a number of different methods for accessing fault codes depending on the system used on the vehicle - EOBD, OBD I or OBD II:

• Using the MIL Light on the Dashboard to flash the code in a given sequence.
• Connecting a LED, test lamp or scope to a DLC connector and observing the long and short flashes (pulses).
• Connecting a scan tool to the DLC connector and communicating with the PCM directly.

OBD-I (On-Board Diagnostics I)

This legislation requires that vehicles manufactured from 1988 onwards be equipped with a system that is controlled electronically by a computer and is capable of monitoring itself during operation. Any malfunction in the system that leads to an effect in the exhaust emissions has to illuminate a warning light on the dashboard and present a stored DTC. The DTC must accessible and readable with the use of on-board facilities such as Flash Code, LED, Test Lamp, Oscilloscope.

OBD-II

The legislation that supports OBD-II applies to vehicles that are manufactured from 1994 onwards (Spark Ignition Engines) and 1996 onwards (Compression Ignition Engines). The main features fitted and that have to be monitored constantly are as follows:

• Internal Combustion
• Catalytic Converter
• Levels of Oxygen (Oxygen Sensors)
• Secondary Air System
• Fuel Evaporative Control
• EGR (Exhaust Gas Recirculation)

The key features for DTCs are as follows:

1. The Malfunction Indicator Lamp (MIL) has to be provided for warning the driver that a fault is present with an additional flashing function.
2. The DTCs must be read from the PCM using a scan tool through the 16-Pin DLC Connector
3. All emission-related components have to be monitored by the PCM for the operation of the engine as well as monitoring the components for defects.
4. When a fault occurs, the system has to be capable of logging perfomance data in the form of a freeze frame.
5. The codes must be formatted to a universal format so that all scan tools and diagnostic equipment can read them and the user can easily interpret them. See Figure 5.

Figure 5 - Structure of Standard Fault Codes

Additional information

The Fault Code Table below is specific to the vehicles listed.

Important Note: This sequence is for the Mazda variants listed in the Fault Code Table above. For other manufacturers, check if the vehicle is equipped with a 25-Pin DLC connector and also check to see if the above test can be carried out. Serious damage to the PCM will occur if the test is carried out on a vehicle that does not support the above test method. Refer to manufacturer's wiring and technical information for further information.

Fault code information reproduced by kind permission of Autodata.

GT173-EN