Ford Transit 2002 Owner's Manual page 1150

misfires caused by the ignition system, fuel system or mechanical engine components. As
each cylinder fires, a characteristic crankshaft acceleration is produced. The monitor detects
irregularities in the acceleration pattern using the Crankshaft Position (CKP) Sensor, thus
detecting the misfire. It can also detect which cylinder has misfired. Combustion misfires
can be categorized as follows:
Type A: These can cause catalytic converter damage due to excessive internal
temperatures. If a certain number of misfires occur over a pre-determined number of
engine revolutions, the MIL will flash to alert the driver of the concern.
Type B: These can lead to an increase in emissions to a point above the EOBD threshold. If
the misfire is detected during a second trip, over a pre-determined number of engine
revolutions, the MIL will flash. If the misfire does not occur over the next three trips, the
MIL will be extinguished
Air/Fuel Ratio (AFR) Monitor (All except vehicles with diesel engine)
The HO2S fitted before the catalytic converter (upstream) measures the oxygen content of
the exhaust gas and the variations in it. This then enables the PCM to adjust the opening
tomes of the fuel injectors to maintain the correct AFR. This is known as Short Term Fuel
Trim (STFT). If the same variation is registered a pre-determined number of times, a
permanent correction factor is applied. This is known as Long Term Fuel Trim (LTFT), which
is stored in the KAM. When the correction factors exceed pre-determined limits a DTC will
be set in the KAM. If a concern is detected in either the STFT or LTFT, and it is still present
on a second trip, the MIL will flash.
Heated Oxygen Sensor (HO2S) Monitor (All except vehicles with diesel engine)
This monitors the operation of the pre (upstream) and post (downstream) catalytic
converter HO2S sensors. It will detect deviations in air/fuel ratios (AFR) and sensor faults.
Upstream HO2S response test: This checks if the upstream HO2S is able to switch fast
enough and whether the voltage output is correct. The operation of the sensor heating
element is also checked. This test is carried out under closed loop control.
Downstream HO2S test: This test is only started when the upstream HO2S test has been
successfully completed. To protect the downstream HO2S, it is only switched on when it has
reached a pre-determined minimum temperature and switched off when it reaches a pre-
determined maximum temperature. The test consists of checking whether the minimum
and maximum output voltages are within certain limits. If they are not, the fuel system is
switched to open loop and controlled using either a rich or lean mixture until they are within
limits.
Catalytic Converter Efficiency Monitor (All except vehicles with diesel engine)
The efficiency of a catalytic converter is measured by its ability to store and later release
oxygen to convert harmful gases. The efficiency is reduced if the converter becomes
contaminated, as it ages, and at high gas flow rates because the exhaust gas does not
remain in the converter long enough to complete the conversion process.
The monitor operates by comparing the number of times the upstream and downstream
HO2S switch between rich to lean cycles. If the converter is operating correctly this ratio
should be near zero. As it approaches one, it indicates inefficient conversion. At this point,
no significant conversion is taking place and the downstream HO2S is being made to switch
at nearly the same rate as the upstream HO2S. Therefore, the lower the number or
switches of the downstream HO2S, the more efficient the conversion process.
Combustion Noise Monitor (Vehicles with common rail fuel injection)
In diesel variants, the Combustion Noise Monitor is used to trim the fuel injection pulse
lengths. Each fuel injector has an associated set of correction data that is determined
during a production end of line test. The Combustion Noise Monitor is used to determine
how the fuel injector charateristic changes from this initial calibration over the life of the
fuel injector.
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