Users Manual Part 2

35
MDMAXLITEA
1. O
2
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 exhaust
stream. A faulty oxygen sensor reacts slowly, or its voltage
signal is weak or missing.
The Oxygen Sensor Monitor is supported by “spark ignition”
vehicles only. The Oxygen Sensor 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.
2. O
2
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,