EOBD & OBDII limits


OBD functions:

The engine-management ECU is required to use OBD (on-board diagnostic) functions to monitor all of the systems and components within the vehicle whose failure could lead to substantial increases in pollutant emissions. An error is present once defined diagnostic thresholds (limits) are exceeded.



The OBD regulations defined by the CARB ( California Air Resources Board )  and EPA ( European Pollution Agency ) apply to all passenger vehicles with up to 12 seats as well as small trucks up to 6.35 t. The EOBD stipulations, valid since 01. 01. 2000, apply to all gasoline-engine passenger vehicles and light commercial vehicles of up to 3.5 t and 9 seats. Starting in 2003 EnhancedOBD capabilities will also be mandatory for passenger vehicles and light commercial vehicles with diesel engines.



The CARB OBD (OBD II) concept is based on relative limits. This means that the limits on acceptable pollutant concentrations within the exhaust gases vary according to the emissions category in which each individual vehicle is certified (LEV, ULEV, etc.). The European (EOBD) regulations are based on absolute limits.( see Table 1 below)


Operational requirements:

One of the OBD requirements stipulates monitoring for all electrical wiring leading to the control unit. This means that “comprehensive components” (such as the airmass meter) are monitored for signal plausibility (OBD II) as well as signs of electrical failure (EOBD). Complex OBD functions check the diagnostic system to verify that it is operational. The prescribed response to failures varies according to the problem’s potential consequences.

CARB OBD and EOBD use different criteria ( see Tables 2 and 3 below ). The type of diagnosis is defined by the pollutants concentration which could be expected (empirical data) due to failure of a given component. Simple operational checks (black and white tests) only assess the basic operational status of the system or component (whether the secondary-air injection valve opens and closes, etc.). The qualitative operational check (flow check) provides more precise information on system performance. One example is the catalytic-converter check, where the monitored data are employed to assess ageing. The corresponding data are available for readouts through the diagnostic interface.


Malfunction indicator lamp (warning lamp):

The MIL (malfunction indicator lamp) alerts the driver to operating problems. Systems designed for conformity with CARB and EPA regulations must trigger the MIL to indicate detected errors after no more than two operating cycles.Within the EOBD’s range of application, the MIL must respond to detected problems by lighting up no later than in the third operating cycle (with waivers available for a maximum of up to ten operating cycles). If the error disappears again (problem with an intermittent contact, etc.) the corresponding error code remains stored in the malfunction log throughout the subsequent 40 driving cycles. The MIL goes out again after three driving cycles with no detected malfunctions. The MIL responds to problems that could lead to catalytic-converter damage (combustion miss) by flashing.


Emergency operation:

When the system detects an error it reverts to operation on substitute default data (for

engine temperature, etc.) or to the emergency backup mode (e.g. limitation of engine

output power). These strategies are intended to


Activation conditions:

The diagnostic routines run only after the activation requirements have been fulfilled.

Among these are:


Inhibit conditions:

The system cannot always run the engine management and diagnostic functions simultaneously.

Certain inhibit functions that prevent specific operations from being processed are also present. To cite one example, the fuel tank’s ventilation system (evaporative- emissions control) cannot operate while the catalytic-converter diagnosis function is in progress.


Temporary interruption of diagnostic routines:

Under specific conditions, diagnostic routines can also be suspended to prevent spurious malfunction alerts.

These conditions include:


Readiness code:

Before proceeding to access the malfunction log, it is important for the technician to verify that the diagnostic routines have really been run during the proceeding driving cycle. Corresponding

confirmation is available in the form of readiness codes available through the diagnostic interface. The system registers these codes to confirm that the essential diagnostic routines have been completed. Recalls The government can demand that manufacturers of vehicles that fail to comply with the OBD requirements recall these vehicles at their own expense.


 Table 1

Exhaust-emissions limits

CARB: relative limits

1.5 times limit in each emissions category

EOBD: absolute limits

CO: 3.2 g/km

HC: 0.4 g/km

NOX: 0.6 g/km

New EOBD limits are anticipated for implementation starting 01.01.2005

 Table 2

Diagnostic processes and malfunction response in CARB and EPA

Leads to concentrations of < 1.15 times limit

  • Error status indicated only by service tester

Concentration < 1.5 times limit

  • Operation check (black and white test)
  • Error status indicated by MIL
  • Error status registered by scan tool

Concentration ≥ 1.5 times limit

  • Qualitative operation check
  • Error status indicated by MIL
  • Error status registered by scan tool

Table 3

EOBD diagnosis and malfunction response

Malfunction leads to pollutant concentration < limit

  • Monitoring of electrical wiring and min./max. plausibility checks adequate
  • Error status indicated by MIL
  • Error status registered by scan tool

Pollutant concentration ≥ limit

  • Qualitative operation check
  • Error status indicated by MIL
  • Error status registered by scan tool

    Increasingly stringent emissions controls have led to progressively more complex diagnostic routines. As a result, almost 50 % of the Motronic system’s entire performance potential is devoted to on-board diagnosis processes!

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