Diagnostic tools

On OBDII systems, once a Type A or Type B code has been set, the MIL will come on and remain on until the component that failed passes a self-test on three consecutive trips. If the fault involves something like a P0300 random misfire or a fuel balance problem, the light won’t go out until the system passes a self-test under similar operating conditions (within 375 rpm and 10% of load) that originally caused it to fail.

The MIL lamp won’t go out until the emissions problem has been repaired. Clearing the ECM’s diagnostic trouble codes with a scan tool or disconnecting the power supply won’t prevent the lamp coming back on if the problem hasn’t been fixed. It may take several drive cycles to reset the fault code, but eventually the MIL lamp will come back on if the problem hasn’t been fixed.

A single fault entry is automatically cleared from the ECM memory if the same fault fails to re-occur in 40 successive driving cycles in which the same operating conditions are satisfied. In order for a single fault entry to be automatically cleared without the same operating conditions being satisfied as when the fault first occurred, 80 successive driving cycles must be clear for the fault to be cancelled.

Diagnostic tools

Diagnostic tools such as ’Testbook’ are used to interrogate the ECM memory to determine the nature of the fault. Diagnostic trouble codes are read from memory which relate the emissions problem being experienced.

In order to hook up a diagnostic scan tool for fault code checking, vehicles equipped with OBD have a 16–pin J1962 diagnostic connector, which is usually located under the front facia, either driver side or passenger side.

The scan tool contains software that analyzes the signals received from the vehicle and displays text or diagrammed readout of any malfunctions found and suggests possible solutions to the problem. The OBD fault codes are most often accessed in response to MIL lamp illumination or driveability problems experienced with the vehicle. The data provided on the scan tool can often pinpoint the specific component that has malfunctioned, saving substantial diagnosis time and effecting quick and accurate repairs.

Data received by the scan tool includes a “freeze frame” of all sensor readings experienced at the time the fault is recorded, which helps diagnosis in the case of intermittent failures.

Fuel metering

For a satisfactory combustion process, precise fuel injection quantity, timing and dispersion must be ensured. If the air:fuel mixture in the combustion chamber is not thoroughly atomized and dispersed during the combustion stroke, some of the fuel may remain unburnt which will lead to high HC emissions.

The fuel injection system provides accurately metered quantities of fuel to the combustion chambers to ensure the most efficient air to fuel ratio under all operating conditions. A further improvement to combustion is made by measuring the oxygen content of the exhaust gases to enable the quantity of fuel injected to be varied in accordance with the prevailing engine operation and ambient conditions; any unsatisfactory composition of the exhaust gas is then corrected by adjustments made to the fuelling by the ECM.