Faulty sensors

Atmospheric pressure will vary with altitude and have a resulting influence on the calculations performed by the ECM in determining the optimum engine operating conditions to minimise emissions. The following are approximate atmospheric pressures for the corresponding altitudes:

• 0.96 bar at sea level
• 0.70 bar at 2,750 m (9,000 ft.)

As emissions are affected, the OBD system may activate the MIL lamp and the relevant fault code and freeze-frame conditions will be stored in ECM memory.

Faulty sensors

The ECM identifies a faulty signal or faulty sensor by way of three test steps: 1. Signal or component short-circuited to ground 2. Signal or component short-circuited to battery voltage 3. No signal or component missing (open circuit)

A specific trouble code is set for each type of test, this helps the technician to determine the cause of the fault during diagnostic analysis.

Misfire Monitoring

Misfire monitoring is vital to avoid damage to the catalytic converters. The Engine Control Module (ECM) must therefore monitor the firing of each individual cylinder to detect misfire and be capable of recognizing the type of misfire likely to cause catalyst damage or the failure of an emission test. During misfire monitoring, the signal from the Crankshaft Position Sensor (CKP) is used to determine engine speed and position as a reference for engine timing.

The diagnostic checks for crankshaft speed fluctuations to determine the presence of a misfire condition by the variation in speed between each one of the crankshaft sensor trigger points. The strategy is finely tuned so that it can determine the severity of the misfire condition and activate the MIL lamp if the misfire condition is severe enough to cause excess emissions or catalyst damage.

Spark timing

Spark timing is automatically optimised by the EMS, the timing is advanced when engine speed is increased or when the air:fuel ratio (AFR) is weakened. Spark timing advance is decreased when engine load is increased or when the HO₂ sensors detect that the exhaust emission of HC and NO_x is too high.

Cold starting

From year 2000, ECD3 legislation requires passenger car catalytic converters to work within a few seconds of cold engine start up. Catalysts must be capable of working effectively in a wide range of ambient temperature conditions from sub-zero to 50°C. Secondary air injection is often used on vehicles in the US to improve cold start emissions, and similar measures are likely to be introduced for European vehicles in the near future. OBD is used to check the correct operation of the SAI system and dedicated P-codes are allocated for SAI fault diagnosis.