On Board Diagnostics
Diagnostic Trouble Codes (DTCs)DTCs are divided into two separate categories; obligatory S.A.E. dedicated codes and voluntary codes added to a specific application. The S.A.E. diagnostic codes, which commence with a number "0", e.g. P 0234, are detailed in numerical order. Voluntary codes, which have been added to the system and commence with a number "1", e.g. P 1234, are included.
A description of the location and operation of the component is followed by the individual codes and their fault definition.
On Board Diagnostics II (OBDII)
OBDII covers any failure of the powertrain system likely to affect exhaust gas quality; this includes fuel, ignition, transmission, anti-lock braking, active suspension, tire pressure monitoring and active differential failures. The emission effect threshold is an increase of 1.5 times the base vehicle standard.
The OBDII document contains clauses covering standard communication protocols, fault codes, vehicle terminology and vehicle interface points. Whereas the original OBD only monitored failed items, OBDII provides failure prediction by observing performance deterioration over a period of time. The four main areas of observation are catalyst, misfire, exhaust gas recirculation and secondary air system.
Catalyst Monitoring
Precise control of the fuel and air mixture to the correct stoichiometric level is essential to the proper function of the three way catalyst, which oxidizes Carbon Monoxide (CO) and Hydrocarbons (HC), while reducing Nitrous Oxide (NOx).
Deterioration of the catalyst conversion efficiency leads to a higher level of emissions. In order to be able to detect a change in the efficiency of the catalyst, the control system must observe both the incoming and the outgoing exhaust gases. To achieve this aim, exhaust gas oxygen sensors are fitted both upstream and downstream of the catalytic converter.
Misfire Monitoring
As engine misfire is the major cause of damaged catalytic converters, CARB requires that control systems must be able to monitor the quality of each individual firing and so detect an engine misfiring. The control system must recognize three areas of engine misfire:
- A misfire which causes instantaneous catalyst damage.
- A misfire which will cause a vehicle to fail a Federal Emissions procedure.
- A misfire which will cause a vehicle to fail an Inspection and Maintenance test.
The misfire detection diagnostic uses the engine speed input, derived from the engine speed sensor, as its primary malfunction detection parameter.
Secondary Air Injection Monitoring
The secondary air injection system pumps extra air into the exhaust system (upstream of the catalysts) for a period immediately after engine start. The purpose of secondary air injection is to reduce catalyst warm up time and thus reduce overall exhaust emission.
Monitoring must indicate when the air flow, from the secondary air injection system, decreases to the extent that an emissions failure level is reached. The system can gauge the air being delivered by recording the drift in oxygen sensor switching levels as secondary air injection is in operation.