Manual

Onboard Diagnostics
OBD2 MONITORS
OBD2 27
into the engine where the vapors are burned. The EVAP Monitor checks
for proper fuel vapor flow to the engine, and pressurizes the system to
test for leaks. The computer runs this Monitor once per trip.
The EVAP Monitor is supported by “spark ignition” vehicles only. The
EVAP Monitor is a “Two-Trip” Monitor. If a fault is found on the first trip,
the computer temporarily saves the fault in its memory as a Pending
Code. The computer does not command the MIL on at this time. If the
fault is sensed again on the second trip, the PCM commands the MIL
“On,” and saves the code in its long-term memory.
Oxygen Sensor Heater Monitor - The Oxygen Sensor Heater
Monitor tests the operation of the oxygen sensor’s heater. There
are two modes of operation on a computer-controlled vehicle: “open-
loop” and “closed-loop.” The vehicle operates in open-loop when the
engine is cold, before it reaches normal operating temperature. The
vehicle also goes to open-loop mode at other times, such as heavy load
and full throttle conditions. When the vehicle is running in open-loop, the
oxygen sensor signal is ignored by the computer for air/fuel mixture
corrections. Engine efficiency during open-loop operation is very low,
and results in the production of more vehicle emissions.
Closed-loop operation is the best condition for both vehicle emissions
and vehicle operation. When the vehicle is operating in closed-loop, the
computer uses the oxygen sensor signal for air/fuel mixture corrections.
In order for the computer to enter closed-loop operation, the oxygen
sensor must reach a temperature of at least 600°F. The oxygen sensor
heater helps the oxygen sensor reach and maintain its minimum
operating temperature (600°F) more quickly, to bring the vehicle into
closed-loop operation as soon as possible.
The Oxygen Sensor Heater Monitor is supported by “spark ignition”
vehicles only. The Oxygen Sensor Heater Monitor is a “Two-Trip”
Monitor. If a fault is found on the first trip, the computer temporarily
saves the fault in its memory as a Pending Code. The computer does
not command the MIL on at this time. If the fault is sensed again on the
second trip, the computer commands the MIL “On,” and saves the code
in its long-term memory.
Oxygen Sensor Monitor - The Oxygen Sensor monitors how
much oxygen is in the vehicle’s exhaust. It generates a varying
voltage of up to one volt, based on how much oxygen is in the exhaust
gas, and sends the signal to the computer. The computer uses this
signal to make corrections to the air/fuel mixture. If the exhaust gas has
a large amount of oxygen (a lean air/fuel mixture), the oxygen sensor
generates a “low” voltage signal. If the exhaust gas has very little
oxygen (a rich mixture condition), the oxygen sensor generates a “high”
voltage signal. A 450mV signal indicates the most efficient, and least
polluting, air/fuel ratio of 14.7 parts of air to one part of fuel.
The oxygen sensor must reach a temperature of at least 600-650°F,
and the engine must reach normal operating temperature, for the
computer to enter into closed-loop operation. The oxygen sensor only
functions when the computer is in closed-loop. A properly operating
oxygen sensor reacts quickly to any change in oxygen content in the