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• Engine Controller
• Brake Switch
• Evaporative Vacuum Leak Detection (EVLD)
• P/N Switch
• Transmission Controls
Output Functionality - PCM outputs are tested for functionality in addition to testing for opens and shorts. When
the PCM provides a voltage to an output component, it can verify that the command was carried out by monitoring
specific input signals for expected changes. For example, when the PCM commands the Electronic Throttle Control
(ETC) Motor to a specific position under certain operating conditions, it expects to see a specific (target) idle speed
(rpm). If it does not, it stores a DTC.
PCM outputs monitored for functionality include:
• Fuel Injectors
• Air Pump Switchover Solenoid
• Short Runner Valve Solenoid
• Ignition Coils
• Throttle Body (Electronic Throttle Control/Throttle Position Sensor)
• Purge Solenoid
• EGR Solenoid
• Radiator Fan Control
• Transmission Controls
OXYGEN SENSOR (O2S) MONITOR
DESCRIPTION - Effective control of exhaust emissions is achieved by an oxygen feedback system. The most
important element of the feedback system is the O2S. The O2S is located in the exhaust path. Once it reaches
operating temperature 300° to 350°C (572° to 662°F), the sensor generates a voltage that is inversely proportional
to the amount of oxygen in the exhaust. When there is a large amount of oxygen in the exhaust caused by a lean
condition, misfire or exhaust leak, the sensor produces a low voltage, below 450mV. When the oxygen content is
lower, caused by a rich condition, the sensor produces a higher voltage, above 450mV.
The information obtained by the sensor is used to calculate the fuel injector pulse width. The PCM is programmed
to maintain the optimum air/fuel ratio. At this mixture ratio, the catalyst works best to remove hydrocarbons (HC),
carbon monoxide (CO) and nitrous oxide (NOx) from the exhaust.
The O2S is also the main sensing element for the EGR, Purge System, and Catalyst and Fuel Monitors.
The O2S may fail in any or all of the following manners:
• Slow response rate (Big Slope)
• Reduced output voltage (Half Cycle)
• Heater Performance
• Dynamic shift
• Shorted or open circuits
Slow Response Rate (Big Slope) - Response rate is the time required for the sensor to switch from lean to rich
signal output once it is exposed to a richer than optimum air/fuel mixture or vice versa. As the PCM adjusts the
air/fuel ratio, the sensor must be able to rapidly detect the change. As the sensor ages, it could take longer to detect
the changes in the oxygen content of the exhaust gas. The rate of change that an oxygen sensor experiences is
called 'Big Slope'. The PCM checks the oxygen sensor voltage in increments of a few milliseconds.
Reduced Output Voltage (Half Cycle) - The output voltage of the O2S ranges from 0 to 1 volt. A good sensor can
easily generate any output voltage in this range as it is exposed to different concentrations of oxygen. To detect a
shift in the air/fuel mixture (lean or rich), the output voltage has to change beyond a threshold value. A malfunc-
tioning sensor could have difficulty changing beyond the threshold value. Many times, the condition is only tempo-
rary and the sensor will recover. Under normal conditions, the voltage signal surpasses the threshold and a counter
is incremented by one. This is called the Half Cycle Counter.
OPERATION - As the Oxygen Sensor signal switches, the PCM monitors the half cycle and big slope signals from
the oxygen sensor. If during the test neither counter reaches a predetermined value, a malfunction is entered and a
Freeze Frame is stored. Only one counter reaching its predetermined value is needed for the monitor to pass.
EMISSIONS CONTROL 25 - 3
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